Workshop Report on “Applications of Terahertz-To-Infrared Probes in Molecular and Materials Sciences”

Workshop Report on “Applications of Terahertz-to-Infrared Probes in Molecular and Materials Sciences”

Summary

The two-day workshop took place on April 14th and 15th, 2011, at the Westin hotel in Arlington, VA. Experts were invited who could contribute to the dialog from various perspectives. A number of NSF Program Directors were also in attendance. The workshop program and list of attendees can be found on the workshop web site, http://biologyphysics.blogspot.com/, and at the end of this report. The primary objective was to bring together leading theorists and spectroscopists working on a broad range of problems that are of interest to the NSF, with a view to: (a) identifying the important fundamental questions that can only be answered via interrogation with electromagnetic radiation spanning the terahertz-to-infrared (THIR) regime; (b) assessing the potential impact that development of advanced THIR light sources would have on these problems; and (c) gauging the broader impacts of such a development. In addition to summarizing all presentations and discussions, this report makes several recommendations that are succinctly summarized in the following six bullets:

• Compelling scientific arguments were presented highlighting the crucial importance of spectroscopy in the THz-to-infrared regime in terms of addressing many of the gaps in our current understanding of the physics and chemistry of materials and living systems. Filling these knowledge gaps would be transformative, with the potential for remarkable societal impacts.

• Considerable effort/resources should be directed towards the development of the next- generation of tunable, high-intensity sources of THIR radiation that meet the requirements for performing sophisticated non-linear and/or time-resolved pump-probe spectroscopies where, in principle, both the pump and the probe could be in the THIR regime.

• A strong case was made for combining high-intensity, tunable light sources with high magnetic fields.

• A number of intense THIR FEL sources already exist at different locations around the globe. Collaborative experimental efforts must be encouraged to further assess whether such sources are, indeed, capable of addressing the most challenging science described in this report.

• A variety of compact (non-FEL) radiation sources either already exist or show tremendous potential for advanced spectroscopy in the THIR regime. Detailed studies should be undertaken to determine how much of the science described in this report can be achieved using this technology.

• A detailed fact-finding mission and subsequent workshop are needed to assess the state-of- the-art in THz-to-infrared technologies – both sources and associated spectrometers – and to chart the most efficient path towards achieving the scientific goals described in this report.

This report is intended to serve as a resource for researchers in the condensed matter physics, materials science, biochemical and biophysical research communities, informing them of future prospects involving spectroscopy in the THIR regime; it also offers recommendations to light source designers based on the needs of these research communities.

Table of Contents

Acknowledgements 1

I. Introduction 2

II. Workshop Outcomes and Conclusions 3

III. Sub-committee reports 8 A. Grand Challenges for Bio-Molecular Studies 8 B. Applications of Strong Terahertz Probes in Condensed Matter and 10 Materials Physics C. Realizing the full potential of EPR 15 D. Far infrared gas-phase spectroscopy pertaining to catalysis, astro- 17 chemistry and bio-chemistry E. Terahertz (THz) Microscopy 18

IV. Presentation Summaries 20

V. References 34

Appendix A. Workshop Program 46

Appendix B. Workshop Participants 50

Appendix C. Nijmegen EPR Workshop Program 54

Appendix D. Report on Terahertz (THz) Microscopy Workshop 56

Appendix E. White Papers 58

Acknowledgements

The workshop was funded by the National Science Foundation through the Division of Physics (award number 1045354). The following NSF programs contributed financially to the workshop:

Physics of Living Systems Chemical Instrumentation National Facilities Molecular Biophysics

The organizers would particularly like to thank Program Directors Krastan Blagoev, Kelsey Cook, Guebre Tessema and Kamal Shukla for their support of the workshop.

Workshop Chair Stephen Hill

Detailed note takers John Singleton Gregory Boebinger

Report writing Stephen Hill Martin Gruebele Andrea Markelz Peter Armitage Mark Sherwin Nick Polfer Gwyn Williams

Web site (http://biologyphysics.blogspot.com/) Sara Bradley

Administrative support (Florida State University) Karol Bickett Amber Cornell

The Chair/PI would like to thank Martin Gruebele for providing considerable assistance in the preparation of the original proposal submitted to the NSF.

Finally, the organizers are extremely indebted to Sara Bradley for taking care of essentially all logistical arrangements for the workshop, including reserving the venue, coordinating meals and coffee breaks, assisting participants with travel arrangements and reimbursement, and maintaining the workshop website.

I. Introduction

The terahertz-to-infrared (THIR) region of the electromagnetic spectrum, spanning frequencies from 1011 to 1014 Hz, shows extraordinary promise for interrogating biomolecules and solid state phenomena [1-4]. For this reason, the development of advanced light sources and spectroscopic techniques that cover this frequency range will be of tremendous importance in terms of furthering our understanding of the physics of condensed matter and living systems. In spite of this, experimental data in the THIR regime is scant. There are two primary reasons: (1) there has been a limited availability of powerful radiation sources spanning the notorious ‘terahertz-gap’ between the gigahertz (GHz) electronics and infrared (IR) optics regions of the electromagnetic spectrum [2,5]; and (2) THIR excitations are usually highly damped (broad) due to comparable mode frequencies and relaxation rates. Recent studies make a compelling case for development of advanced light sources operating in the THIR regime. This development could transform the way spectroscopy is performed, providing unsurpassed brightness, time-resolution, coherence and continuous tunability. It is envisioned that such sources would enable experiments on biomolecules and solids that are presently impossible, including nonlinear and multidimensional (pump-probe) studies, and sub-micron imaging, e.g., of the large amplitude motions of biomolecules induced by intense terahertz (THz) or IR fields. The primary objective of the workshop was be to bring together leading theorists and spectroscopists working on a broad spectrum of problems which are of interest to the National Science Foundation (NSF), with a view to (a) identifying the important fundamental questions that can only be answered via interrogation with electromagnetic radiation spanning the THIR regime, (b) assessing the potential impact that development of advanced THIR light sources would have on these problems, and (c) gauging the broader impacts of such a development. The two day workshop took place on April 14th and 15th, 2011, at the Westin hotel in Arlington, VA. Experts were invited who could contribute to the dialog from various perspectives. The workshop program can be found in Appendix A, and at the workshop web site – http://biologyphysics.blogspot.com/; a list of workshop participants with brief descriptions of their research interests is given in Appendix B. These research interests span from molecular biophysics (both experimental and theoretical), to astro-chemistry, to condensed matter physics. Nearly all of the participants have a very strong background in instrumentation – particularly in terms of THIR light sources. The workshop included a mix of presentations and discussion. Presenters were asked to avoid excessive discussion of their own latest results and instead focus on the broader visions/objectives of the workshop. Talks were punctuated by several 1-2 hour discussion sessions focused on the three issues identified above. This report is organized as follows: summaries of the main outcomes and conclusions of the workshop are presented in Section II; this is followed in Section III by a series of self- contained reports generated among sub-groups of workshop participants (these were submitted to the project PI some time after the workshop); short summaries of each presentation made at the workshop are provided in Section IV; and an extensive list of references is given in Section V. Following the main report are a series of Appendices: A. Workshop Program; B. Workshop Participants and their Expertise; C. Program for subsequent EPR Workshop held at the University of Nijmegen; D. Program for subsequent THz Microscopy Workshop held at Jefferson Lab; and E. White Papers submitted by some of the participants in advance of the workshop. Once approved by the NSF, this report will be made available via the National High Magnetic Field Laboratory web-site (http://www.magnet.fsu.edu/).

2 II. Workshop Outcomes and Conclusions

This section of the report summarizes the main outcomes of the workshop in the form of succinct bulleted conclusions and recommendations, followed by relatively brief supporting arguments. More extensive supporting documentation can be found in Sections III and IV.

• Compelling scientific arguments were presented highlighting the crucial importance of spectroscopy in the THz-to-infrared (THIR) regime in terms of addressing many of the gaps in our current understanding of the physics and chemistry of materials and living systems. Filling these knowledge gaps would be transformative, with the potential for remarkable societal impacts.

The importance of the THIR regime for molecular and materials research is perhaps best summarized by means of Figs. 1 and 2. Unlike small organic molecules, whose fingerprints lie exclusively in the near-IR, large proteins, lipids and nucleic acids have fingerprints that extend well into the THz and sub-THz regimes [1-3]. These excitations extend well beyond single chemical bonds, and even beyond the molecule itself when it exists in its native solvent environment. Most importantly, these low frequency modes contain the non-linear (anharmonic), large amplitude motions that drive biomolecular function – see Fig. 1 [1,6].

Fig. 1. Cartoon of the frequency response of DNA (L.L. Van Zandt, Purdue University). At sub-THz frequencies, the response is due to relative motion of large molecular components; at hundreds of THz, the modes correspond to bonds between individual atoms. Molecular function emerges as a result of the motions occurring in this frequency range (the THIR regime), spanning the whole gamut of electronic excitations, vibrations and their couplings. This applies not only to DNA, but to essentially any large (bio-) molecule.

Meanwhile, spectroscopy in the THz-to-infrared spectral range is also crucial for understanding the properties of technologically important materials, where many of the fundamental energy/time scales fall into the THIR regime (see Fig. 2), e.g., magnetic and other collective electronic excitations, lattice vibrations, electronic band gaps and bandwidths, and the energy gaps associated with many ordered states. Consequently, future advances in spectroscopy in this challenging frequency range will spur advances in materials discovery.

• Considerable effort/resources should be directed towards the development of the next-generation of tunable, high-intensity sources of THIR radiation that meet the requirements for performing sophisticated non-linear and/or time-resolved pump-probe spectroscopies where, in principle, both the pump and the probe could be in the THIR regime.

Fig. 2. Schematic matching the THz-to-infrared regime to the energy scales that are tunable using the highest available quasi-static laboratory magnetic fields. Such a combination allows access to a wide range of physical phenomena, a small selection of which are shown as vertical bars indicating frequency regions of interest; the field positions of the bars are merely suggestive. The use of the magnetic field to provide tunable energy scales is illustrated by the two diagonal lines: the lower purple line indicates the Zeeman (spin) splitting for g = 2, applicable to electrons in free radicals (biological/chemical process) and to many transition metal ions; the purple line also corresponds to the cyclotron (orbital) frequency for an effective mass ~me, which is typical for many metals and correlated electron systems; the upper green line represents the cyclotron frequency for an effective mass of 0.05me, typical of many semiconductor systems—both as a conduction-band effective mass and as an excitonic reduced mass. The availability of high fields will allow these energies to be tuned through many regions of interest. The region above about 10 THz covers general electronic excitations (see Fig. 1), a broad class of phenomena that encompasses chemical/biological reaction initiation, interband excitations in semiconductors and oxides, and the optical initiation of transient structural and electronic phase transitions. A key element of much of the science discussed at the workshop involved the ability to pump at high intensity in this region, and then probe the field-tuned phenomena at lower energies in a time-resolved fashion.

Source intensity, or lack thereof, probably represents the biggest challenge in terms of realizing much of the science discussed at the workshop, although tunability is challenging as well. Plenty of technologies capable of generating coherent radiation in the THIR regime already exist, but their powers are typically very low (often µW to mW), especially within the terahertz-gap from ~0.1 to 10 THz. Most of the workshop presentations spoke to the need for much higher intensity sources because of severe damping in biologically relevant samples, and the desire to probe inherently non-linear processes in both molecular and condensed matter systems. Even the most basic linear spectroscopies currently suffer from severe sensitivity limitations in portions of the THIR regime, especially in cases where magnetic dipole active resonances are sought, e.g., in the case of electron paramagnetic resonance at frequencies above about 0.5 THz. Meanwhile, most of the truly transformative experiments proposed at the workshop require multiple color, time- synchronized pulses of intense radiation, with the ability to easily tune across different portions of the THIR regime. Relatively little discussion focused on possible future THIR light source technologies, because the emphasis of the workshop was placed on the science. However, a few

4 speakers suggested that current state-of-the-art capabilities in the mid- and near-IR are very close to reaching the specifications desired by many of the workshop participants; unfortunately, the situation in the terahertz and far-IR lags significantly behind. Several speakers also expressed the opinion that next-generation Free-Electron Laser (FEL) sources would be capable of meeting the demands of many of the proposed experiments. This issue is discussed in more detail below, where a number of recommendations are made.

• A strong case was made for combining high-intensity, tunable light sources with high magnetic fields.

Aside from the obvious necessity for strong magnetic fields in the case of magnetic resonance spectroscopy (@ ~35 T/THz for g = 2), many of the non-EPR workshop participants also spoke about the importance of combining high magnetic fields with high-intensity, tunable light sources. A magnetic field is a non-invasive and reversible thermodynamic probe that directly couples to the spin and orbital degrees of freedom of atomic, molecular and/or band electrons in solids, producing non-negligible energy shifts. Importantly, the electronic energy shifts produced in most molecules and materials using the highest attainable quasi-static magnetic fields exactly match the THIR regime, as illustrated in Fig. 2. It should be noted that similar recommendations concerning the possible colocation of a high-intensity THIR light source at a high magnetic field facility was made in a recent National Research Council report entitled “High Magnetic Field Science and its Application in the United States: Current Status and Future Directions” [7]. Moreover, two European labs are currently pursuing such an approach: the FELIX FEL facility in Rijnhausen was recently moved adjacent to the High Magnetic Field Laboratory at the University of Nijmegen; and the Dresden-Rossendorf High Magnetic Field Laboratory is situated adjacent to the ELBE FEL; both facilities have programs focused on FEL-based spectroscopy at high-fields. Wim van der Zande, Head of the Molecular and Laser Physics group in Nijmegen (formerly at FELIX), was one of the participants at this workshop.

The workshop brought together experts from rather diverse research fields. The relatively short duration of the meeting provided just enough time to share important knowledge and to initiate several lines of discussion that continued for many months beyond the actual workshop itself, i.e., fact-finding and report writing activities continued well into 2012. Some of the additional fact-finding activities that occurred right the workshop are summarized below:

• A meeting was held in Nijmegen on July 4, 2011, entitled “EPR Spectroscopy with THz Radiation in Very High Fields: Roadmap and Applications”. The meeting was coordinated by Wim van der Zande, who also extended an invitation to Hans van Tol. Details of the meeting are included in Appendix C.

• A UK/US funded Workshop to Initiate Collaboration for Terahertz Microscopy was held at Thomas Jefferson National Accelerator Facility (TJNAF) on July 11, 2011. The meeting was attended by several of the NSF workshop attendees: Gwyn Williams, Larry Carr, Peter Weightman and Andrea Markelz. Details of the workshop are included in Appendix D, and a brief report is included in Section III of this report. A follow-up meeting was convened at TJNAF on August 11, 2011, involving Stephen Hill, who was unable to attend the July workshop. Discussion again focused on possible ways to initiate US/UK collaboration in the area of THz spectroscopy.

5 • Stephen Hill visited Mark Sherwin at the University of California at Santa Barbara (UCSB) on August 2nd and 3rd, 2011, where he participated in pulsed EPR experiments using their FEL. This meeting resulted in the document entitled “Realizing the full potential of EPR” found in the following section of this report.

• Stephen Hill also visited the Nijmegen Centre for Advanced Spectroscopy and the High Field Magnet Laboratory at the University of Nijmegen on June 7-8, 2012, where he met with Wim van der Zande and was given a tour of the new FEL facility. It should again be emphasized that the NSF workshop did not explicitly address the issue as to what kind of light source would best serve the needs of the various stakeholders. However, on the basis of the reports submitted by the workshop participants (Section III), together with the knowledge gained from subsequent fact-finding activities at three of the four THIR FEL facilities represented at the workshop (Nijmegen, TJNAF and UCSB; Daresbury being the fourth), a number of clear recommendations emerge.

Many workshop participants expressed the opinion that an appropriately designed FEL is the answer. A very bright, 4th generation FEL source is operational at TJNAF, but it has not been characterized in the depth or detail needed for broad, high-quality science. This could be achieved through relatively modest investment, but would require collaborative efforts on the part of both spectroscopists and FEL scientists. Meanwhile, significant progress has been made at UCSB [8], Dresden [9] and Nijmegen [10] in terms of demonstrating the feasibility of using an FEL for high-field magnetic resonance spectroscopy. However, much work remains in order to demonstrate superiority over existing non-FEL methodologies.

From the low frequency side, there have been remarkable advances in solid-state sources based on low-frequency (~10 GHz) oscillators that are then multiplied and amplified. Linear spectroscopies are currently possible at frequencies up to 1 THz using this approach, and with exceptional dynamic range. High-power non-linear spectroscopies have been demonstrated at frequencies up to ~0.15 THz, and the prospects for extending this approach to higher frequencies are promising. These technologies are potentially much cheaper than FELs and offer considerably more control, i.e., one can essentially generate arbitrary waveforms, such as chirped pulses, and sophisticated NMR-type sequences of pulses with arbitrary phases and amplitudes (see summary of talk by David Plusquellic in Section IV). From the high-frequency side, a wide variety of solid-state laser devices exist in the mid- and near-IR. Importantly, these sources are also capable of producing quite significant powers. The frequency range from 1 to 10 THz remains problematic. Nevertheless, remarkable advances have been achieved in the area of broadband THz time-domain spectroscopy, whereby single-cycle THz pulses are generated via the optical rectification of intense femtosecond laser pulses. Electric field strengths of up to 1 MV/cm have been achieved via this approach [11,12], albeit over a significant bandwidth.

6 • A detailed fact-finding mission and subsequent workshop are needed to assess the state-of-the-art in THz-to-infrared technologies – both sources and associated spectrometers – and to chart the most efficient path towards achieving the scientific goals described in this report.

During the past decade, a number of US workshops and reports have focused on scientific applications and source technologies spanning the THz-to-infrared regime. Of particular note is the DOE-NSF-NIH Workshop on Opportunities in THz Science that took place in 2004 [3]. This was followed by an NSF-funded study (DMR0520481) for a Concept and Engineering Design of a Free Electron Laser Light Source for High Magnetic Field Research that resulted in a further five “BigLight” workshops organized by the US National High Magnetic Field Laboratory (NHMFL) [13]. However, the landscape continues to evolve, both in terms of a growing scientific need for intense radiation sources in the THIR regime, and in terms of advances in source technologies that have occurred during the corresponding time frame. A workshop held in October 2005 concluded that a high repetition rate, 4th generation light source [14], employing superconducting RF cavities with energy recycling capabilities, would provide the best solution in terms of cost and the needs of the scientific community at the time. A subsequent conceptual design was developed, modeled on the FEL at TJNAF. The construction of such an FEL would represent a major undertaking, requiring significant investment. A detailed fact-finding exercise is required to compare the TJNAF design with alternative approaches that could combine warm (non-superconducting) accelerator-based FEL technologies operating in the THz and far-IR, such as those at UCSB [8] and Nijmegen [10], with a suite of mid- and near-IR tabletop sources. This fact-finding exercise would need to consider both cost and the pros and cons of each approach in terms of the science that would be enabled. Funding would be required to secure the services of a team of carefully selected experts to carry out this important study. Once completed, the findings would be shared and discussed among the broader community at the proposed workshop.

7 III. Sub-committee reports

III-A. Grand Challenges for Bio-Molecular Studies (Gruebele/Markelz)

During the breakout discussion sessions, this sub-committee developed the following bulleted list of grand challenges:

• Detection of correlated motions in biomolecules. o Measurements Required: The frequency range of the correlated motion is the TERAHERTZ range. It is therefore necessary to directly probe at these frequencies. Heterogeneity effects that result in a complex and broad spectrum represent a known challenge. To address this challenge both single molecule THz spectroscopy and multidimensional THz spectroscopy measurements on bulk samples are needed. These techniques will require high power for single molecule near field THz microscopy and 2D IR.

• Determination of the role of surface heterogeneity in water dynamics. o Both measurements and theory are required: systematic studies as a function of the scale of hydrophobic/hydrophilic heterogeneity, as a function of perturbation of the water dynamics, as a function of distance from the surface.

• Determination of the interplay between transport and phonons for organic crystals. o Measurements Required: 2D THz/IR, both THz-THz and IR-THz, to determine the higher order anharmonic energy transfer.

• Determination of the coupling within multiple protein complexes.

• Determination of vibrational energy flow through biomolecules: energy flow complements ‘structural flow’ (i.e. allostery or structural rearrangements to accommodate substrates).

• Determination of the role of correlated motions in protein function o Measurements Required: 2D THz spectroscopy.

• Terahertz polarimetry of complex systems. o Measurements Required: High sensitivity determination of rotation and ellipticity for liquid phase samples. This will require precise and rapid polarization control (linear and circular) and a high power source for optically dense samples.

The grand challenge is to move from biomolecular structure towards functional dynamics. The sizes of typical biomolecules dictate that functional dynamics occurs in the sub-THz to THz frequency range. X-ray crystallography (via Debye-Waller factors) and NMR (via relaxation times and residual dipolar couplings) can provide dynamical information, but THz has continuous coverage over a wider range of time scales (few ps to sub-ps) and directly monitors phenomena associated with charge motion in biomolecules (transition dipole). An issue that is often seen as an impediment in terms of the spectroscopy of biological systems in the THz-to-IR (THIR) regime is the strong absorption due to collective motions of the hydrogen-bonded solvent (water) network; this response covers the frequency range from GHz to THz [15]. However, this absorption can also be used to one’s advantage, e.g., to discriminate

8 between materials with different water content such as muscle and fat. Recent work has found that the solvation water shells surrounding a biomolecule absorb very differently from bulk water [16]. What is more, these solvation shells have been shown to extend several nanometers from the surface of a biomolecule [17], i.e., much further than the highly perturbed surface water layers detectable by crystallography or NMR. Thus, a single biomolecule can affect the dynamics of 1000s of surrounding water molecules (and vice-versa), and THz spectroscopy can provide information on the coupling between the large amplitude molecular modes and the solvent motion [1,2,16-19]. This is obviously important, because this coupling plays an integral role in biomolecular dynamics and function. For example, rearrangements of water are responsible for a substantial fraction (~50%) of the free energy changes during folding or binding reactions of proteins. In this sense, it would be fairer to say that the protein and water fold together. This represents an area where spectroscopy in the THIR regime can make a huge impact, particularly imaging. For example, if a molecule is excited to saturation by a powerful THz source tuned to a collective mode frequency, it will undergo a large amplitude motion which can be imaged with atomic resolution (by modulating the illumination and detecting via STM [20]), yielding dynamical structural information. Although initial efforts along these lines have been reported, saturation of large amplitude motions is not presently possible with available sources. As another grand challenge, THz spectroscopy can be applied in living cells. For example, changes in the dynamics of in-cell solvation water can be monitored on average by putting cells in a hypotonic environment to change their water:biomolecule ratio. Such experiments could provide insights into the ratio of biological to bulk water inside cells. Currently, even averaged results accessible with diffractive optics are unavailable. Wide wavelength tuning would allow the effects of water and biomolecules to be disentangled to some extent. Biomolecules in vitro and in vivo have large amplitude motions in the 5 – 100 cm−1 (0.2 – 3 THz) range that are related to their folding, as well as functional fluctuations of their structure. A key biological example is the “induced fit” explanation for enzyme activity: the energy landscape of a biomolecule is modulated by binding to a partner (e.g. a substrate), such that proximity of a binding partner may deepen a local free energy minimum, allowing a cooperative fit. On the largest structural scale, one refers to allostery, where motions are transduced through the biomolecule over distances that can exceed nanometers. Both small-scale (induced fit in enzymes) and large-scale (allostery) motions occur in the THz range. Many of the experiments to probe such motions require substantial power: some require saturation of the THz transition to produce a net structural change that can be detected by scanning microscopy; others are THz-IR or even THz-THz analogs of the current generation of 2D-IR experiments. Two-dimensional spectroscopy has substantially increased net resolution in the mid-IR, and will be even more important in the far-IR, where line shapes can be very broad due to overdamped vibrations. These experiments have so far been limited to the IR regime using table-top laser systems that cover energies of the order of a few 1000 cm-1 [21-24]. This has restricted studies to the pumping of individual chemical bonds and intramolecular dynamics which function on ps time-scales. A tunable source operating at lower frequencies would potentially allow similar studies to be extended down to molecular modes and intermolecular transfer of energy⎯processes that are more applicable to the function of living organisms [25]. One can also envision multiple, overlapping, time-synchronized sources covering regions where there are no suitable sources at present, and which is likely to be important in terms of the dynamics of, e.g., biomolecule conformation. Pulses in excess of 1 microJoule are typically required for 2D experiments in the

9 mid-IR, and similar intensities will be needed in the far-IR (greater damping vs. slightly larger transition dipole moments). As a bonus, it is possible that THz 2D pump-probe experiments could turn out to be easier than the current IR experiments, assuming a proper source is available: the optical requirements for coherent experiments are not as exacting at longer wavelengths, and the transition dipoles for large amplitude motions of polar biomolecules (e.g. helix dipole moments) are quite large. Energy flow through biomolecules is closely related to structural reorganization. It has been shown by experiments and theory that electron flow, or energy flow, can occupy well- defined paths within biomolecules, and that these paths can be controlled by mutations or binding to other molecules. THz radiation can directly excite the low frequency modes most involved in energy transport, rather than the very localized modes excited in the mid-IR. Once excited by a THz pulse, energy flow can be followed by a variety of probes, including fluorescence. For such experiments, as well as absorption experiments where biomolecule and water structure are to be distinguished, wide tunability is also an indispensable asset.

III-B. Applications of Strong Terahertz Probes in Condensed Matter and Materials Physics (Armitage)

Picosecond (10−12 sec) timescales are one of the most ubiquitous in condensed matter material systems. For example, the resonant period of electrons in semiconductor nanostructures, the scattering times of electrons in metals, vibrational frequencies of molecular crystals, the lifetime of biologically important collective vibrations of proteins, and − now − even the time an electron in Intel's terahertz (THz) transistor takes to pass under the gate − these are all phenomena in the picosecond range. This ubiquity means that experimental probes employing THz electromagnetic radiation are potentially quite useful. It is unfortunate then that this spectral range lies in the so- called “Terahertz Gap” − above the capabilities of traditional electronics, but below that of optical generators and detectors (photonics). In recent years however, there have been a number of dramatic technical advances that allow measurements spanning this gap. As such, THz spectroscopy has become a tremendous growth field [3], finding potential uses in a multitude of areas including materials characterization for solid-state devices [26,27], optimization of the electromagnetic response of novel coatings [28], probes of superconductor properties [29,30], security applications for explosives and biohazard detection [31], detecting conformational changes of proteins [32], and non-invasive structural and medical imaging [33-35], to name just a few. Although there have been tremendous advances in recent years, there are still places where novel sources of THz radiation could fill important gaps in measurement possibilities for materials systems. For example, there is a lack of highly intense THz light sources that would allow both equilibrium and non-equilibrium (pumped) measurements of materials systems, picosecond time-resolved experiments, nonlinear absorption, and multi-photon techniques. Many of these spectroscopies would also benefit from the orbital, spin and nuclear quantization provided by a magnetic field. Thus, co-location of intense THz sources at high magnetic field facilities is highly desirable. “Strongly correlated” material systems exhibit a multitude of particularly novel properties as a result of interactions and coherent quantum effects. The complexity and the resultant effects of 1023 particles acting in quantum mechanical unison can give rise to a host of beautiful and striking phenomena like superconductivity and magnetism. Like waves on the sea, their behavior

10 is intrinsically collective and is not easily reduced to the properties of individual particles, i.e., they are emergent. Such systems typically have characteristic energies much lower than the eV scale of the isolated atoms and chemical bonds that compose them. One can estimate the relevant frequencies (ν) and energies (E) of these systems from their important temperature scales using the relation E = hν = kBT, where kB and h are the Boltzmann and Planck constants, respectively. One such temperature scale may be the transition temperature Tc to an ordered state like superconductivity. For many interesting materials with strong interactions, Tc ~ 10 K, which corresponds to 0.2 THz and an energy of 0.9 meV. Another important temperature scale is the sample temperature Ts itself. By using frequencies such that kBTs < hν, with hν still well below the energy of any relevant ordering temperature, kBTc, one can access the T = 0 or so-called quantum limit and characterize intrinsic ground state properties [36]. Since the lowest sample temperatures in ‘optical’ spectroscopy are limited to the range of approximately 1 – 10 K, these considerations mean that experimental probes employing THz radiation should be optimal for the investigation of strongly interacting systems. The workshop identified a number of compelling needs for the investigation of “strongly interacting electron systems” with THz spectroscopies. These include:

• Intense FEL sources for near-field nanoscopy Applications of infrared (IR) spectro-microscopy to such diverse areas as physics, chemistry, life sciences and biology, materials science and engineering, forensics and national security. The interest here is that, in addition to having natural frequency scales in the THz, many material systems also exhibit spatial structures on length scales than can approach nanometers. This is particularly true in strongly correlated systems as their many competing ordered phases can lead to locally nucleated states and phase separation. Conventional and synchrotron-based IR microscopy enables characterization of inhomogeneous substances with the diffraction-limited spatial resolution of 10-50 µm. The impact of IR microscopy in both the life and materials/physical sciences strongly motivates the development of experimental approaches suitable for an infrared probe of matter at the nanoscale. Recently, significant progress in infrared nanoscopy has been achieved by several research groups through an innovative combination of atomic force microscopy and IR lasers. Novel scanning near field IR instrumentation facilitates both spectroscopy and imaging with a spatial resolution down to 10 nanometers or better. In this technique the tip of an atomic force microscope (AFM) is illuminated with radiation from a tunable IR gas laser. The technique allows one to achieve imaging with the resolution of the order of 10 nm irrespective of the frequency of the IR source. These functionalities of near field IR nanoscopy have already enabled nano-scale exploration of previously unattainable characteristics of a variety of materials including semiconductors, polymers [37], as well as single viruses [38]. A combination of broadband ellipsometry and near- field nano-imaging has facilitated significant advances in the understanding of the electronic correlations in transition metal oxides [39-41]. Near-field nanoscopy is likely to enable breakthrough results in studies of imhomogeneous and phase-separated systems. Such inhomogeneities are known to occur in many systems of high current interest in the context of both energy and information technologies including but not limited to: high-Tc superconductors, plastic solar cells, electrochromics, materials employed for hydrogen storage, etc. In systems where multiple phases coexist on the nanometer scale, the dynamical properties of these individual electronic phases remain unexplored because methods appropriate to study charge dynamics (transport, infrared/optical, and many other spectroscopies)

11 lack the required spatial resolution. Scanning near-field infrared nanoscopy can circumvent this long- standing limitation. For that reason one can anticipate a major impact of the near-field nanoscopy experiments in a variety of subfields of condensed matter physics and materials science. Because the near field tip-sample interaction is weak, nanoscopy experiments critically rely on the availability of powerful infrared (IR) lasers delivering 1 – 10 mW. This condition is relatively easy to realize only in the 10 mkm and 5 mkm ranges owing to the well developed technologies of CO2 and CO lasers, respectively. Outside of these ranges, tunable radiation with sufficient power can be readily provided only by Free Electron Lasers (FELs). In order to expand this technique’s range of applicability down to the THz frequency range, intense THz FELs should be developed. Sources do not currently exist in which nanoscopy can be conducted in this range. It would also be very useful to be able to couple THz range nanoscopy to high magnetic fields.

• Two color pump - probe experiments Novel electronic phenomena, such as high-Tc superconductivity, often emerge at the boundary between two competing electronic phases. Measurements that probe the interactions between these phases are therefore of interest, e.g., two-color pump-probe spectroscopy using light pulses to drive the system toward or across a phase boundary while probing with THz pulses tuned to an energy for sensing the competing phase(s) (or to limit the excitation range in a continuum system) [42-44]. The pump could be conventional photo-excitation to create mobile carriers from an insulating phase (photo-doping), breaking Cooper pairs [45], or by tuning to a relevant excitation such as the electromagnon in multiferroics [46-48], or a phonon. In molecular materials, one may wish to pump a specific phonon that is known to be connected with the magnetic state. The targeted phonon, for instance in copper halide coordination polymers, may be the out-of-plane pyrazine bending mode, which was recently shown to influence local structure and control the antiferromagnetic exchange interaction [49]. Here one wants to have freedom to tune both the pump beam to coherently excite some particular mode of the system, as well as to probe over a range of frequencies that are natural to the system. A goal of such studies is to tune or even block the magnetic quantum critical transition via light-induced local structure changes. It is of course not routinely possible to have wide spectral control at THz frequencies with sufficient intensity to drive such systems out of equilibrium. However, it could be possible with an FEL. It would also be very useful to couple two-color (THz/IR) pump-probe experiments to high magnetic fields, which could be used to tune the ground state closer to an instability or other competing state. It was also proposed during the workshop to use two color experiments to probe relaxation in electronic glasses. In analogy with structural glasses, electronic glasses can be defined as systems with a random distribution of localized charges [50-52]. Here, long relaxation times and glassy phenomena derive from a combination of disorder and a long-range unscreened Coulomb interaction. The motion of any one charge manifestly necessitates a many-particle rearrangement of the other electron occupations to reach lower energy. Such many-particle processes are inherently slow and inefficient at finding the true ground-state configuration. Electronic glasses or ‘Coulomb glasses’ may be realized in granular metals and amorphous and lightly doped semiconductors, which are all expected to exhibit certain similar qualitative behavior. One of the problems with previous studies using ultrafast techniques to study relaxation in these systems, however, was that optical pulses were used to pump the sample. The assumption is that although

12 charges are excited well above the impurity band, they decay back extremely quickly to excited impurity band states. This fast decay should take place on the picosecond scale and not influence the physics markedly. However, it is much cleaner to excite the manifold of impurity band states directly using THz radiation, i.e., it would be most useful to perform experiments where one pumps in the mid-IR and probes with broadband THz. In general, it is difficult to make low energy pulses that pump only the impurity states using tabletop sources. However, such experiments might be feasible with a FEL.

• Fractionalized excitations in the solid state: Magnetic monopoles The pyrochlore structure generally describes materials of the type A2B2O6 and A2B2O7 where A and B are typically rare-earth or transition-metal elements. Materials with these lattices have been the subject of interest for many years as they frequently host novel magnetic states because of strong geometrical frustration. In Ho2Ti2O7 and Dy2Ti2O7, rare-earth ions have large magnetic moments of order 10µB living on a lattice of corner-sharing tetrahedra [53]. The crystal field induces a strong anisotropy aligning the magnetic moment with a local 〈111〉 axis, Fig. 3. In the ground state, two spins point in and two point out of each tetrahedron. This is analogous to the Bernal-Fowler rules [54] formulated for positions of protons in water ice (two protons next to an O2− and two farther away). It has recently been proposed that the low-energy excitations of the spin-ice state are magnetic monopoles [55]. Reversing a single spin in the ground state of spin ice violates the ice rules on two adjacent tetrahedra that now carry magnetic charges of opposite signs. The monopoles can be further separated (fractionalized) by additional spin flips with no additional violations of the ice rules (see Fig. 3). At low temperatures, spin ice behaves as a gas of magnetic monopoles. This picture is consistent with various experiments, including neutron scattering, thermodynamics, dynamical magnetic susceptibility, and µSR measurements [56,57]. Although the existing experiments and theory are compelling, a quantitative understanding of the dynamics of magnetic monopoles is lacking. It was proposed at the workshop that one may be able to study monopole transport in an ac magnetic field using intense far-IR (FIR) fields. Typically one expects that magnetic dipole transitions are much weaker than electrical ones. Since these are insulating materials they should not be electrically active below their electronic band gap. Therefore, one expects that driven monopole motion will be evidenced by a magnetic dipole active absorption at energies below the spin-flip gap. Although magnetic dipole excitations should be active, they should still be weak and one would need the strong magnetic fields of an intense FIR source to see them with sufficient signal-to-noise. It is anticipate that the spectral weight of any absorption will follow the expectation for the known magnetic field and temperature dependence of the monopole density using the magnetic dipole sum rule. One may also expect that if a monopole description of the low temperature state of the elementary degrees of freedom is valid then one may be able to describe their motion and absorption through some sort of a Boltzmann transport model. In this case, at low frequencies, the monopole transport will be overdamped, but at high enough frequencies the transport may be dominated by the inertia of the monopoles. Effects of inertia have been clearly observed in the motion of domain walls in ferromagnetic nanowires [58]. A Boltzmann model description of the transport of these objects would be a beautiful confirmation of the relevance of the monopole description of the excitations in these systems. One may expect to extract the monopole mass, charge, scattering times and density from optical measurements.

Fig. 3. Spin ice in a rare-earth pyrochlore. (A) Magnetic moments in the ground state. (B) A misaligned magnetic moment adds two elementary excitations with opposite magnetic charges. (C) Magnetic charges are not confined: they can be pulled far apart without incurring a large energy cost.

• THz measurements under pressure A common feature of systems with strong electron-electron correlations is that on changing of a tuning parameter such as pressure or field, a spatially ordered state becomes suppressed and a superconducting state emerges in the vicinity of the phase diagram where the ordered state’s Tc would be expected to go to zero. The inference is that superconductivity may originate from the fluctuations of the ordered state. The tuning parameter is different for different materials. For example, it is doping in cuprates or pressure in pnictides and organic conductors. An example of this kind of physics is recent optical work on tuning of the ground state in BEDT-TTF-based quasi-two-dimensional organic conductors by so-called “chemical pressure”. Such quasi-two-dimensional organic conductors are formed by layers of BEDT-TTF molecules, sandwiched between the layers of anions that serve as charge reservoirs. Optical reactance methods were used in a wide frequency range, going as low as 10 cm−1, to investigate the response of conduction electrons in these materials as well as to register the superconducting gap in the β″-(BEDT-TTF)2SF5CH2CF2SO3 below a Tc of 5.2 K [59]. Signatures of charge-order fluctuations were found in the in-plane optical conductivity of the organic superconductor β″- (BEDT-TTF)2SF5CH2CF2SO3, and compared to a related metallic compound β″-(BEDT- TTF)2SO3CHFSF5, which shows no signatures of the ordered state. It is believed that the differences in these materials can be viewed as a consequence of different chemical pressures. An important extension of these studies would be to use hydrostatic pressure to suppress both superconductivity and charge order, to directly confirm the unconventional origin of superconductivity. However conventional optical equipment does not have enough source intensity to perform low-frequency measurements under pressure. With the use of a high intensity THz source, it should be possible to couple enough radiation into and out of a pressure cell in order to perform these measurements.

• High Magnetic Fields coupled to THz spectroscopy The ability to couple high magnetic fields of the sort created at the US National High Magnetic Field Laboratory (NHMFL) with THz radiation would be very powerful. An important example is provided by high temperature cuprate superconductivity. The nature of the underdoped pseudogap regime of the high-temperature superconductors has been a matter of long-term debate. On quite general grounds, one expects that due to their low superfluid densities and short

14 correlation lengths, superconducting fluctuations will be very significant for transport and thermodynamic properties in this part of the phase diagram. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap, and their significance for understanding high-temperature superconductivity. Recent work using THz time-domain spectroscopy (TTDS) has probed the temporal fluctuations of superconductivity above Tc in La2-xSrxCuO4 (LSCO) thin films over a doping range that spans almost the entire superconducting dome (x = 0.06 to 0.25) [60]. Dynamical measurements in the THz range are a sensitive probe of the onset of superconductivity and measure its temporal correlations on the time scales of interest. These measurements show that superconducting correlations do not make an appreciable contribution to the transport anomalies of the pseudogap in LSCO at temperatures well above Tc. This is interesting because, in LSCO, a region of enhanced diamagnetism extends almost 100 K above Tc [61], while the THz fluctuation conductivity has an extent limited to 10 – 20 K above Tc [60]. This is surprising as one might expect a close correspondence between these quantities [62]. Similarly, it has been argued from Nernst and diamagnetism measurements that Hc2 may be as high as 150 T [61], while the resistive transition is essentially complete in optimally and underdoped LSCO by 30 T [63,64]. Similar behavior has been found recently in YBCO in which a H1/2 contribution to the heat capacity (indicative of nodal d-wave superconductivity) has been found up to fields well in excess of the resistive transition [65]. The high magnetic fields available at the NHMFL would allow spectroscopic measurements in the THz frequency ranges of interest up to fields well past the resistive transitions of these materials. The supposition from measurements other than charge transport is that vortices exist in this range, but that they are so fast that the superconducting response is shorted out from normal electrons. Although one doesn’t expect to see any sharp features at these high fields, it may be that high magnetic fields will show that the electronic background has a particular field sensitivity, indicating a superconducting origin. Even if no superconducting response is seen, it will still be interesting to do THz spectroscopy in this field range where quantum oscillations have been imagined. The inference has been that the high field state of the cuprates may be a Fermi liquid. It will be very interesting to probe quasi-particle coherence and transport in this spectral range. If these systems are truly Fermi liquids at high fields then it will show up as a huge difference in the quasi-particle scattering rates as extracted from THz measurements at high fields, as compared to the normal state at low fields. The location of broadband THz sources at high magnetic field user facilities will be particularly attractive in the context of the experiments described here.

III-C. Realizing the full potential of EPR (Sherwin/Hill)

An electron’s spin is an exquisite probe of its local environment in condensed matter, including solids, liquids, biological molecules and devices. Using electron paramagnetic resonance (EPR), one can interrogate electron spins to extract unique information about the local structure and dynamics in their neighborhood. For example, EPR has been used to determine the structure of protein complexes that have resisted all other techniques in structural biology. EPR is an important tool for understanding the conversion of light into electricity in organic photovoltaics [66], and the mechanisms for decoherence in condensed matter [67-69].

15 Like nuclear magnetic resonance (NMR), EPR becomes vastly more powerful at high magnetic fields and frequencies, and in a pulsed rather than continuous wave (cw) modality. The major bottleneck for high-field, high-frequency pulsed EPR has been the absence of electromagnetic sources capable of high frequency (>100 GHz), high power (>1 kW), high long- term frequency-stability (coherence), and pulse-programmability. The current state-of-the-art in high-field, high-frequency pulsed EPR with full pulse programmability are the instruments developed at Cornell [70] and St. Andrews University (STA) [71]. These instruments operate at a frequency of ~95 GHz (3.5 T) with kW peak power and ns spin manipulation times. The state of the art in static magnetic fields invites one to consider high-power pulsed EPR at frequencies an order of magnitude higher than can be achieved with the current generation of high-frequency pulsed EPR spectrometers (two orders of magnitude higher frequency than conventional X-band spectrometers). Strong scientific and technological drivers are pushing such development. For example, one of the grand challenge of biology and medicine is to understand the functional dynamics of complexes of biological macromolecules (e.g., proteins, drugs, and nucleic acids) on length scales in the 3-15 nm range and time scales in the ns-microsecond range. EPR is one of the few techniques that is, in principle, suited to studying structure and dynamics on these length and time scales under physiological conditions. However, the state-of-the-art is limited to structural studies at temperatures below 100 K and length scales below 8 nm. At such low temperatures, there is no significant dynamics and the protein is frozen into conformations that may not be biologically relevant. Pushing high-power pulsed EPR to much higher fields and frequencies promises transformative improvements in the length scales, temperatures, and time scales that can be probed with this special technique. A UCSB/NHMFL collaboration on FEL-based pulsed EPR has increased the frequency at which high-power pulsed EPR can be performed from 95 GHz (3.5 T) to 240 GHz (8.5 T) [8]. Present capabilities of FEL-EPR include the ability to rotate spins by Pi/2 in 6 ns, 50 times faster than can be achieved with solid state sources operating at 240 GHz, and the ability to measure spin memory times (T2) for spin ½ systems as short as 60 ns, roughly 10 times shorter than can be achieved with solid state sources at 240 GHz; it should be noted, however, that neither of these benchmarks currently surpass the state-of-the-art at 95 GHz. Very soon, 240 GHz FEL- based T1 measurements are anticipated, as are schemes to reduce dead times, implement phase- cycling, and begin to explore sequences with more than two pulses. There are two complementary paths to realizing high-power pulsed EPR at frequencies above 100 GHz and fields above 3.5 T. The STA/Cornell approach is to precisely define the phase and amplitude of a pulse sequence at microwave frequencies; multiply the frequency of the microwave sequence to achieve the desired pulse sequences at a much higher frequency but with low power; and then amplify with a high-gain, high-power, high-bandwidth amplifier. This approach, which has so far been demonstrated up to 94 GHz using Extended Interaction Klystron (EIK) amplifiers, permits highly sophisticated sequences of pulses with arbitrary phases, amplitudes and even frequencies (limited only by the bandwidth of the amplifier). The prospects for extending this approach towards 0.5 THz are good, through further development of high- power microwave amplifiers. However, the path to 1 THz is not so clear. The UCSB approach is to start with a quasi-cw beam at the desired frequency and power level, and modulate that beam directly to define the desired pulse sequence. The UCSB approach has been demonstrated at 240 GHz using a free-electron laser as a source, and light-activated Si shutters as modulators. Both the STA/Cornell approach and the UCSB approach should be vigorously pursued in a co-ordinated fashion. The approaches are complementary, but significant

16 cross-fertilization will enable much more rapid advances, both scientific and technological, than can be achieved by pushing just one of these approaches. The main challenge for the STA/Cornell approach is the development of high-power, high-bandwidth, high-frequency amplifiers. The main candidates are EIK and Gyrotron amplifiers. Both EIKs and Gyrotrons are relatively mature technologies. Pushing amplifiers based on these technologies to higher frequencies, and then incorporating them into an EPR platform, will happen but it will take time. The highest achievable frequency is likely to be above 400 GHz, but significant challenges remain, and the achievable power levels are also uncertain. The UCSB FELs already generate kW-level pulses from 0.1 to 4.7 THz. The main challenge is the development of modulators with sufficient agility to enable the wished-for complex pulse sequences. The light-activated switches that are currently in use at 240 GHz have been shown to work even above 10 THz. In order to develop more complex pulse sequences, electrically-driven modulators will be developed that can handle high power at high frequency. Any modulators developed for use with FEL-EPR can also be used with Gyrotrons and with EIK amplifiers. Parameters such as T1 and T2 vary significantly with frequency. Simply measuring T1 and T2 at both 94 and 240 GHz under similar conditions would elucidate the mechanisms of spin relaxation in a wide variety of systems [67-69]. In the case of spin labels and other biological free radicals, this is not only worthwhile for its own sake, but critical to the development of EPR methodologies for studying biological molecules under physiologically relevant conditions. The STA/Cornell spectrometers can do a lot more than simply measure T1 and T2 at 95 GHz—for example, Double Electron Electron Resonance (DEER) measurements are possible that provide distance and relative orientation information for two spin labels on a biological molecule [72]. At UCSB, it is expected to be able to perform some 2-D EPR experiments at 240 GHz in the future. EPR is poised for explosive growth over the next decade, as THz technology is developed to take advantage of high magnetic fields that are already available. There is now an international effort to push high-power pulsed EPR to the highest possible fields and frequencies. The effort is driven by the likelihood of transformative breakthroughs in condensed matter physics, chemistry, molecular biology, materials science, and medicine. An expanded collaboration within the US can make the world’s most advanced EPR spectrometers available to a wide range of users, and position the nation for leadership in the developments that will realize the full potential of EPR.

III-D. Far infrared gas-phase spectroscopy pertaining to catalysis, astro-chemistry and biochemistry (Polfer)

The majority of experiments conducted at the free electron laser user facilities FELIX (in the Netherlands) and CLIO (in France) involve infrared (IR) multiple photon dissociation (IRMPD) spectroscopy of gas-phase complexes in the mid-IR range (500-2000 cm-1) [ 73 ]. Such experiments are much more challenging at longer wavelengths (<500 cm-1), due to the lower photon flux for those FEL designs, and the lower energy per photon. The usefulness of extending the wavelength region to the far-IR has already been demonstrated for van der Waals-tagged metal clusters, as the metal cage vibrations are diagnostic for the symmetry of the cluster [74]. In combination with quantum-chemical calculations, experiments on size-selected complexes can give insights into the growth mechanism of these clusters, and can rationalize how structure is related to their catalytic properties; the latter is relevant to properties of nanomaterials.

Fig. 4. Far-IR spectroscopic data from polyaromatic hydrocarbons.

In astrochemical research, on-going missions, such as the Herschel Space Observatory (SOHO - launched in May 2009) and the Stratospheric Observatory for Infrared Astronomy (SOFIA), are aimed at gathering far-IR spectroscopic data from interstellar clouds in order to confirm their chemical composition. Some of the suspected molecules are polyaromatic hydrocarbons (PAHs), which are thought to exhibit characteristic “drumbeat” modes in the far IR region (see Fig. 4, left) [75]. Laboratory spectra of these anharmonic vibrations are required to confirm their spectroscopic value. Many studies at FEL facilities have involved molecules of biological interest [73]. The mid-IR spectrum of an entire protein showed that the amide I (C=O stretch) and amide II (N-H bend) modes were consistent with a protein largely composed of α-helical motifs [76]. Gas- phase IR spectroscopy is in principle well suited to the fundamental study of protein folding diseases (linked to Alzheimer’s or Parkinson’s disease), which are known to progress via β-sheet formation. The combination of mass selectivity with laser spectroscopy allows IR spectra of size- selected aggregate states of the protein to be obtained. This is expected to yield information on the early stages of the aggregation process, which remains poorly understood. These gas-phase experiments also constitute a useful benchmark for measurements in the condensed phase, as the inherent large-amplitude “hinge” motions of proteins can be studied in the absence of a hydration shell, devoid of water coupling modes.

III-E. Terahertz (THz) Microscopy (Williams)

Research progress is often tied to progress in the means for investigating research materials. The impact of a century of X-Ray science affords an example. Now researchers are beginning to access the region of the electromagnetic spectrum between microwaves and infrared light: the terahertz (THz) regime. Contrast mechanisms for THz are associated with low energy molecular motions, uniquely those associated with biological processes. Accordingly THz attracts the attention of many leading researchers, including several attending this NSF Workshop: Peter Weightman, Andrea Markelz, Gwyn Williams and Carol Hirschmugl. The low power of present laboratory THz sources substantially limits research to spectroscopic studies of large samples, interesting though these are. The availability at Thomas Jefferson National Accelerator Facility (TJNAF) of a THz source several orders of magnitude

18 more powerful enables the development of power and brightness-hungry THz microscopes. Such microscopes would enable rapid large-scale imaging of skin for disease detection, for example, an area being developed at Cambridge University in the UK in conjunction with the company Teraview, also located in Cambridge, and hampered at present by slow scanning technology. Since the needed accelerator investments are already in place, doing the microscope development at TJNAF offers the most rapid and affordable path. Attaining an operating instrument requires further development. Fortunately the combined capabilities of UK and US research teams are appropriate, but resources are needed for them to be applied toward this goal. Teraview has an instrument located at a laboratory in the UK with an arrangement for access by Teraview collaborators. Work there could produce some scientific results, indicate the potential of commercial instruments for doing science, identify the potential of, and need for, instruments that could be develop from them, indicate the value of commercial style instruments for research use. Terahertz sources, e.g., the FEL at TJNAF and ALICE at Daresbury Lab, are operational but have not been characterized in the depth or detail needed for broad, high-quality science. Working together could define what characterizations are needed and how they are best obtained. Doing so as a shared project among the source labs would foster valuable relationships at the working level. Studies using IR spectroscopy rely on well developed FTIR instruments for both spectroscopy and microscopy. Equivalent instruments do not now exist for THz, but are needed both for the science and to characterize the beams. The zeroth-level vision is a large aperture step-scan asymmetric interferometer with a wire-grid beamsplitter. The original notion of an instrument for spectroscopic microscopy is still appealing, but more foundation is first needed.

19 IV. Presentation Summaries

Martin Gruebele – THz motions from proteins to functional molecules

Gruebele talked about functional motions and folding of proteins, with particular emphasis on the coupling between water and protein dynamics, i.e., how does water mediate protein dynamics and how do proteins couple back to affect the water dynamics. Examples included: large amplitude hinge-motions that are important for the catalytic activity of the phosphoglycerate kinase enzyme; ubiquitin folding kinetics, particularly in terms of the timescale with which the solvation shell responds to protein folding; the differences between various amino acids in terms of their effects on the solvation shell; and antifreeze protein function. The talk outlined various technologies currently used to perform spectroscopies in the 100 GHz to 3 THz range, including low-power (mW) solid state sources, molecular gas and p-Ge lasers, and broadband THz generated by femtosecond IR pulses. Protein-water interactions are studied by measuring changes in absorption as a function of protein concentration (up to 0.2 mMol), hence controlling the amount of ‘hydration water’ that typically extends several nanometers from the protein surface. Frequency/time-resolved measurements can provide information about the way in which the solvation shell adapts to macroscopic folding. Gruebele emphasized the need for tunable high-power radiation sources spanning the THz gap. These are needed (i) to penetrate water so that one can differentially probe the water and protein regions in a sample, (ii) for pump/probe experiments in which one would pump functional modes of proteins or even initiate chemical reaction dynamics, and (iii) for hole- burning (spectral dynamics/diffusion) and other non-linear spectroscopies. Single-molecule pump-probe experiments are currently possible in the IR using STM to probe large amplitude motions. Such experiments should in principle be easier in the THz if a tunable high-power source were available. A desire for a high-stability source was also expressed in order to improve dynamic range over current technology. Finally, picosecond time resolution is needed for studies of spectral dynamics. References [1,2,77-81].

David Leitner – Biomolecules in solution: Computational studies

Leitner also discussed water-biomolecular interactions, though from the theory perspective (albeit with close connection to experiment). He emphasized that it is only at frequencies below ~3 THz that motions involve the entire protein. Ab initio molecular dynamics methods are used to dissect the THz spectrum of the various kinds of water in the solvation shell, from bound to bulk. Differences from the hydration shell to bulk water include: diffusion constants, rotational dynamics (on picosecond timescales), and hydrogen bond rearrangement times that are a factor of two longer for bulk water. Five ‘great challenges’ were then discussed. (1) The first involved the study of simple saccharides, where there is general agreement between THz experiments and simulations. (2) The present frontier involves the study of simple proteins, for which limited, barely qualitative agreement has been obtained between theory and experiment. (3) One of the grand challenges involves more complex interactions between proteins and water, where almost nothing is known yet. A particularly interesting problem involves the antifreeze protein produced by certain organisms that permit their survival in subzero environments. This protein displays a highly heterogeneous interaction with water (some surfaces interact strongly and others weakly), which can change dramatically upon mutation, illustrating a correlation between biological function and hydration dynamics. Challenges (4) and (5) involve the study of membrane proteins and the dynamics in living cells. NSF program manager, Krastan Blagoev, then made a general

20 challenge to the workshop to address the issue of whether one could look at protein dynamics within a living cell. It was suggested that, due to crowding, there might not be much bulk water within a cell. This could actually simplify THz experiment. References [17,25,82-89].

Glenn Edwards – “Forward looking” speculation as to how to apply THz/IR sources to a frontier problem in biology

The first part of Edwards’ talk focused on pattern formation during the early stages of developmental biology. He showed several confocal stained micrograph videos of the early stages of insect embryo formation (within the first 3 hours of fertilization), including polymer dynamics during the nuclear cleavage cycles, the formation of lipid patterns/membranes, adhesion/linkage of cell membranes, leading all the way to the formation of germ cell layers. Edwards then conjectured that one could learn a lot by perturbing such living systems with THz photons. At these frequencies, photon-material interactions would be highly selective, in contrast to generic heating, i.e., selective photo-perturbation instead as opposed to uniformly heating everything. Edwards concluded this part of his talk by suggesting that THz/IR offers an intriguing tool for in vivo perturbation of polymer dynamics during the early stages of Drosophila development. He also stressed the importance of carefully considering the operating parameters of the THz/IR source to specify experimental protocols and the associated promise for advancing biophysical and biological understanding of the dynamics of pattern formation during development. The second part of the talk focused on medical/surgical applications of THz/IR FELs, demonstrating the effects of different wavelengths on tissue ablation. The O-H stretching mode occurs at 3 µm. Irradiation at this wavelength heats only the water, leaving the protein intact. Thus, ablation results from thermal diffusion and chemical kinetics due to heating by the FEL. Irradiation at longer wavelengths (3 µm) will excite both the water and the protein, causing cell rupture and tissue ablation with clean margins. Such a capability has been successfully demonstrated in human neurosurgery. What is needed is the development of desk-top size light sources capable of generating suitable THz/IR pulsed radiation.

Andrea Markelz – Conformation and Dynamics of Proteins: Correlated motions

Markelz’ talk focused on the studies of correlated, or collective protein-water dynamics. Note that not all modes of a protein couple strongly to the water. In addition to the THz, experiments are currently performed by inelastic X-ray scattering and neutron spin-echo techniques. THz experiments are complicated by the highly damped relaxational dynamics of water. A solution to this problem involves partially hydrating the protein; functionality for many proteins emerges at just 30% hydration, i.e., at substantially less than the first hydration shell. A typical experiment involves looking at collective mode changes upon oxidation and hydration. For example, oxidation of the heme group within cytochrome C produces changes in the 0 to 80 cm-1 spectrum. Appropriate theory (dipole-dipole correlation analysis) captures the correct oxidation and hydration dependence of the spectrum over the entire measurement range, suggesting that collective modes are present in the THz range. Another approach described in the talk involves freezing of the bulk water. However, the water near the protein remains liquid. Dielectric loss measurements are then used to probe the collective motions of water near the protein surface. Arrhenius plots provide information on the energies of correlated motions associated with small clusters of water near the protein surface. Large changes in permittivity are observed depending on whether the water is bound or unbound

21 to the protein. The example given involved lysozyme. Markelz concluded with a discussion of the need for a THz source with high dynamic range and an ability to rapidly (100 kHz) modulate polarization. References [90-94].

David Plusquellic – State-resolved THz spectroscopy: Phase coherent broadband methods for high sensitivity at sub microsecond scan speeds

Plusquellic extensively discussed results obtained with Chirped Pulse Fourier-Transform GHz/THz spectrometers developed within his group. These instruments operate at low powers. However, they employ entirely solid-state components, thus enabling rapid phase coherent broadband measurements with exceptional sensitivity, akin to what is possible with an NMR spectrometer, albeit at much higher (~THz) frequencies. Gas phase measurements were presented for low-pressure mixtures containing five components (e.g., ethanol, formaldehyde, methanol, acetone, CO), demonstrating the ability to resolve the sharp resonances (long T2’s) associated with the individual rotational spectra. Condensed phase studies were also discussed. Examples included the alpha helix, beta sheets, hydrophobic nanotubes, crystalline peptide-water systems. The main drawback of the current instrumentation is low power: few milliwatts at microwave frequencies and just 2 microwatts at 1 THz. Plusquellic would like to be able to do hole-burning experiments (to reveal underlying lineshapes), anharmonic mode studies (non- linear resonance), and multidimensional microwave/THz spectroscopy. Such studies will require substantially greater powers. One solution could be an FEL. However, it is also possible that high-bandwidth amplifiers operating at several hundred GHz could become available during the next decade, offering an alternative to this type of high-fidelity study (see also discussion related to pulsed EPR elsewhere in this report). References [95-98].

Peter Weightman – Are there terahertz solutions to problems in understanding the physics of life?

Weightman began his talk with some numbers: ALICE (at Daresbury) can produce 70 kW pulses of 0.6 ps duration, but the average power is only 24 mW (7 months to boil a kettle of water); 1 mm of water attenuates THz radiation by 1018. He then discussed a crucial problem in biological science: the structure and function of the extracellular matrix (ECM). The ECM consists of molecular assemblies of proteins and polysaccharides (glycosaminoglycans) located on the outside of cells. There are an enormous number of different species, some in very low concentrations, that self assemble and interact on length scales of nm to µm and over timescales of 10-15 to 103 seconds. They are key regulators of cell function, and hence organ and organism function. The central problem is how does the structure of glycosaminoglycans drive their functional interactions with other molecules of the ECM and the cell surface to regulate cell activity? This question is relevant to medical research on cancer, neurodegeneration, inflammation, congenital disorders and pathogens. High intensity light sources are needed, because these problems need to be studied on fast timescales and at very low concentrations. Weightman then discussed several other areas in biology where an advanced THz light source could have a major impact. One example was enzymatic catalysis and photosynthesis, where quantum mechanical coherence is thought to play an important role. For instance, recent experiments on marine algae indicate that when you excite one, you excite them all. Central to this excitonic mechanism is a THz mode. Thus, THz spectroscopy is needed to gain deeper insights into this phenomenon. Another area of discussion was the use of rapid Reflection Anisotropy Spectroscopy (RAS) as a probe of molecular conformational changes driven by a tunable, high-power THz pump. Weightman has proposed a 4th Generation Light Source with

22 such applications in mind. Finally, the use of a THz FEL to desorb proteins from surfaces without damage was discussed. References [99-109].

Peter Armitage – Probing correlated electrons in the THz at high magnetic fields and intensities

Armitage discussed three different topics: (1) relaxation in electronic glasses; (2) fluctuations in high-temperature superconductors; and (3) fractionalized magnetic excitations (monopoles) in spin ice. In the first of these problems, glassy phenomena derive from a combination of disorder and a long-range unscreened Coulomb interaction, where the motion of one charge manifestly necessitates the many-particle rearrangement of the other electron occupations to reach low energy. Such many-particle relaxation processes are inherently slow. Up to now, studies have involved optical excitation out of the impurity band, followed by fast relaxation back to an excited impurity configuration; direct spectroscopy within the impurity band has been lacking. Ideally, one would pump in the MIR and probe in the THz. However, such studies are not possible with currently available tabletop sources. In the second example, Armitage discussed THz conductivity measurements in the cuprates, including time domain studies of temporal superconducting fluctuations above the critical temperature in LSCO. It would be of interest to make similar measurements of the fluctuation timescales directly at high-fields. The final example involved the Dumbell model of Castelnovo, Sondhi and Moessner, whereby the physics of magnetic excitations in spin ice maps onto that of a two component plasma where the individual components interact only by a magnetic Coulomb’s law. What one would ideally like to do is measure the monopole transport directly, which should be Boltzmann-like at low energies. However, this driven monopole motion would be magnetic dipole active, i.e., much weaker than electrical transport – hence the need for strong AC magnetic fields and an intense THz source. References [55,60,110-113].

Natalia Drichko – Interplay of Charge Order and Superconductivity in BEDT-TTF-based Materials with ¼-filled Conductance Band

Drichko’s talk focused on the possibility of superconductivity emerging at the charge-ordered- insulator to metal transition in organic charge-transfer salts with quarter filled bands. The charge ordering arises in these compounds as a result of nearest-neighbor Coulomb repulsion. One can use molecular vibrational spectroscopy as a local probe of the charge order: different charge densities at different molecular sites in the lattice give rise to splittings of molecular vibrational modes. To do this, Drichko uses a combination of Backward-Wave-Oscillators (2 – 40 cm-1), Bruker FTIR spectrometers (30 – 10,000 cm-1) and IR microscopes (30 – 15,000 cm-1). Up to now, tuning through the phase diagram has been achieved by studying different BEDT-TTF compounds (salts – e.g., by varying the anion). It is also possible to tune the system with pressure. However, it has so far not been possible to learn anything about charge ordering under pressure, since such methods are not very amenable to optical spectroscopy, though a pressure cell is available at Brookhaven National Labs for such purposes. Another thermodynamic probe that could be used to induce new phases is a magnetic field. Consequently, Drichko advocated for combining tunable THz/IR sources with high magnetic fields. References [59,114-120].

David Tanner – Photons, magnetic fields, Cooper pairs, and quasparticles: The use of synchrotron sources to study pairbreaking in superconductors

Tanner started by describing the infrared beamline, U12IR, at the National Synchrotron Light Source (NSLS), which can be used for broadband spectroscopy in both quasi-continuous and

23 pulsed (100s of ps) modes in combination with a 10 T split-coil magnet. The first part of the talk focused on broadband measurements of the complex optical conductivity of thin -1 superconducting films (MoGe and Ni0.5Ti0.5N) in the 10 to 50 cm range, which is coincident with the superconducting gaps (2Δ) in these materials. It was then shown how simultaneous fits to both components of the frequency-dependent conductivity, σ1(ω) and σ2(ω), can be used to determine the magnetic field (parallel to film) dependence of the spectroscopic superconducting gap (order-parameter) and the pair-breaking parameter. The 2nd part of the presentation dealt with time-resolved pump-probe spectroscopy, using a conventional pulsed laser to excite quasiparticles (and excess phonons), followed by 300 ps broadband infrared probe pulses (synchronized with the laser). Relaxation of the excess quasiparticles involves multiple steps with a range of timescales. The rate-determining step involves a bottleneck effect whereby the recombination of excess quasiparticles is inhibited by the trapping of low-energy (2Δ) phonons that give rise to pair-breaking (Rothwarf and Taylor); the recombination occurs on nanosecond timescales, which can be probed using the aforementioned pump-probe technique. It is found that the application of a magnetic field slows the recombination, contrary to simple expectations. It is thought that it is the gap suppression that affects the recombination and pair-breaking rates (Kaplan et al.), leading to the observed dependence of the relaxation rate on applied magnetic field. References [121,122].

Dmitry Smirnov – Prospects of broadband IR spectroscopy with split-helix magnet

Smirnov talked about IR/THz magneto-spectroscopy at the NHMFL with a particular focus on advantages offered by the availability of a bright broad-band, or tunable, THz/IR source. First, the talk presented a brief overview of broad-band magneto-optical spectroscopy applications to probe low-energy excitations in solids, in particular in correlated electrons systems. The approach relies on performing a Kramers-Kronig transformation of experimentally obtained reflectivity spectra over a wide spectral range, covering several orders of magnitude (from ~meV through several eV). Another important aspect is the ability to probe small, µm-size samples. This was illustrated by presenting an IR magneto-optical spectroscopy study of cyclotron resonance in graphene, a one-atom thick layer of carbon. Smirnov discussed new possibilities offered by the availability of the newest NHMFL 25 T split-magnet. It was emphasized that, when coupled with a high-brightness, broad-band THz/IR source, it could allow (i) significant enhancements of available experimental geometries (Faraday, Voight, angular dependence), (ii) measurements of (sub) µm-size samples, and (iii) implementation of new optical techniques, such as THz/IR ellipsometry. References [123-126].

Nick Polfer – Far infrared spectroscopy of mass-selected complexes pertaining to catalysis, astrochemistry and biochemistry

Polfer first discussed several astronomical missions focused on observations in the far-infrared and millimeter-wave spectral range (50-670 µm). These included the Herschel Space Observatory, which was already half way through its three-year mission, and the airborne Stratospheric Observatory for IR Astronomy (SOPHIA, http://www.nasa.gov/mission_pages/SOFIA/). The goal of these missions is to provide information on early star/galaxy formation and to trace the paths by which potentially life-forming (small) molecules form. Unidentified infrared bands from interstellar clouds are thought to originate from mixtures of neutral and cationic polyaromatic hydrocarbons, but it is not presently clear which ones. Experiments in the far-

24 infrared should provide signatures which are more unique for each molecule. Laboratory spectra of these anharmonic vibrations are required to confirm their spectroscopic value. Polfer then switched directions, pointing out that many studies at IR FEL facilities have involved molecules of biological interest. Gas-phase IR spectroscopy is in principle well suited to the fundamental study of protein folding diseases (linked to Alzheimer’s or Parkinson’s Disease), which are known to progress via β-sheet formation. The combination of mass selectivity with laser spectroscopy allows IR spectra of size-selected aggregate states of the protein to be obtained. This is expected to yield information on the early stages of the aggregation process, which remain poorly understood. These gas-phase experiments also constitute a useful benchmark for measurements in the condensed phase, as the inherent large- amplitude “hinge” motions of proteins can be studied in the absence of a hydration shell, devoid of water coupling modes. Finally, Polfer discussed IR multiple photon dissociation (IRMPD) spectroscopy of gas- phase complexes, which represents one of the main applications of the free electron laser user facilities FELIX (in the Netherlands) and CLIO (in France). The usefulness of extending the wavelength region to the far IR has already been demonstrated for van der Waals-tagged metal clusters, as the metal cage vibrations are diagnostic for the symmetry of the cluster. Experiments on size-selected complexes can give insights into the growth mechanism of these clusters, and can rationalize how structure is related to their catalytic properties; the latter is relevant with respect to properties of nanomaterials. References [75,76,127-131].

Wim van der Zande – Three Short Aspects: Non-linear THz excitation, THz as a low frequency source, and Serendipity

Van der Zande began by describing the Nijmegen Centre for Advanced Spectroscopy (NCAS). This program involves the installation of a 45 T hybrid magnet system as part of the already existing High-Field Magnet Laboratory (HFML), as well as a narrow-band THz free-electron laser (FEL) source. The FEL instrument is called FLARE - Free-electron Laser for Advanced spectroscopy and high-Resolution Experiments. FLARE is a novel light source generating powerful pulsed light in the THz part of the spectrum between 200 GHz (λ = 1.5 mm) and 3 THz (λ =100 µm). Although several FELs in the THz (mm-range) regime are operational, the Nijmegen FEL will be unique as it will generate light in two distinct operating modes: a spectroscopic mode providing narrow bandwidth radiation (spectral resolution Δλ/λ < 10-5) in the form of long pulses of 6 to 10 µs duration with an estimated power of 100 W; and a high intensity pump-probe mode providing trains of micro pulses each of a length of 10 to 100 ps (depending on the wavelength) with a repetition rate of 3 GHz or 20 MHz. Each burst of micro pulses forms a macropulse of 10 µs duration and a 10 Hz repetition rate. The peak power in the pump-probe mode is 2 × 105 W. The FELIX facility described by the previous speaker is also moving to NCAS, making this the most advanced center for THz/IR spectroscopy in the world. FLARE, which occupies a relatively modest footprint (<10 m × 5 m), is housed in an underground, shielded vault, with optical transport systems connecting the light directly to the HFML facility. The cost of FLARE, which employs a commercial normal conducting RF LINAC (10 – 15 MeV beam energy), is relatively modest in comparison to superconducting facilities such as the Jefferson Lab FEL. The first photons were delivered to the HFML in November 2012, enabling an initial proof of principles experiment. Currently, the only other facility combining a THz FEL and (pulsed) high magnetic fields is the Helmholtz Zentrum Dresden in Rossendorf, Germany. Some of the science drivers envisioned for the FLARE/HFML

25 facility include: Spectroscopy and dynamics of solid state materials in high magnetic fields using pulse-echo's techniques, e.g., following electron spin excitations, Landau resonances or cyclotron resonances; Dynamic nuclear polarization (DNP) experiments, potentially enhancing the sensitivity of NMR significantly; Molecular spectroscopy of bio-organic molecules, biomimetics (analogues of biomolecules), and smart organic molecules, providing information on the structure of these molecules and on slow intra- and intermolecular motions related to their functionality; Material science at high (30-45 T) magnetic fields, and applications, amongst others, in the field of biomolecular spectroscopy; Research in electron and spin transport in devices and systems of submicron dimensions. Van der Zande pointed out that he expects novel instruments such as FLARE to have major impacts on the international scientific community not because of the success of reproducing plans such as those laid out in white papers, but because of the new fields of science that become accessible, which were initially unexpected but later on seem totally logical (“after the fact”). He described several such serendipitous occurrences at the Rijnhuizen IR facility. References [132,133].

Robert Peale – Anecdotes from THz science and about THz sources

Peale first discussed the potential use of grating-gated HEMT devices as tunable narrow-band THz detectors based on plasmon resonances in the associated 2D electron gas (2DEG). The grating gate couples THz to the plasmons, as well as defining the plasmon wavevector and the charge density in the 2DEG. Plasmon resonances have been observed in transmission by FTIR for a variety of materials systems. To observe and optimize a resonant electrical photo-response requires a tunable THz source with high spectral density. The pros and cons of various THz sources were discussed, including: p-Ge-, quantum cascade-, and free-electron-lasers, as well as backward wave oscillators. Peale then discussed recent efforts in the area of THz spectral sensing of vapors, including his own experimental survey of characteristic features from the UV to mm-wave regimes for one particular vapor. This work suggests that the THz is the worst possible range for vapor sensing, independent of the state of THz technology and the atmospheric transmission problem, because water lines make it very hard to interpret the underlying vapor spectrum. The presentation concluded by noting that astrophysics continues to be the application where THz (here known as far-infrared or sub-mm wave) reigns supreme. With the Herschel Space Observatory now operating and with the Stratospheric Observatory for Infrared-red Astronomy (SOPHIA) beginning to obtain results, there is a need for far-IR laboratory (ground-based) spectroscopy of gases and minerals. Many minerals look identical in the 1000 cm-1 range, while they become much easier to distinguish in the far-IR (although few features are observed below 100 cm-1). Peale went on to describe his own work in collecting reference spectra for minerals by Fourier- Transform-IR spectroscopy, noting some of the challenges of this well-established technique and a wish list that would make this method faster and more accurate.

Jack Freed – High Frequency ESR: Biophysical Applications and Challenges

Jack Freed’s talk focused primarily on pulsed dipolar ESR, whereby pairs of nitroxide spin labels are added to biological molecules of interest, and the dipolar coupling between them is determined, thereby providing distance information (and also sometimes information on the relative orientations of the spin labels). The two pulse sequences described were DEER (Double Electron Electron Resonance) and DQC (Double Quantum Coherence). This technique is of

26 great importance in the study of intrinsically unstructured biomolecules that are not amenable to Xray structural determinations, and in cases where the distances are too large for NMR. Freed discussed molecular signal transduction, specifically the CheA/CheW/Receptor complex that detects food/poison and signals to cause a response by the flagellum. Most pulsed ESR is performed at 17 GHz and below. However, Freed highlighted several important advantages of going to higher frequencies, including concentration sensitivity, orientation selectivity, and sensitivity to dynamics. In the case of the latter, slow motions are best seen at low ESR frequencies and fast motions at high frequencies. The example of T4 Lysozyme was used for which simulations of 15 GHz continuous-wave ESR spectra are motionally narrowed, whereas spectra at 2 THz are seen to be in the rigid limit. Freed then cited certain example problems where higher frequencies can be of benefit, e.g., studies of HIV protease, where the work must be performed at low concentrations in order to avoid protein aggregation, and studies of inorganic ions such as Gd, where signals tend to be extremely broad at low frequencies. Freed concluded by discussing strategies and the outlook for extending ESR to higher frequencies. For pulsed ESR, the main limitation is power, which dictates the duration of nutation pulses and, hence, the time resolution of the technique: 3 kW is currently available at 95 GHz using commercial Extended Interaction Klystrons (EIKs); however, this drops to 50 W at 183 GHz. If future THz sources are to impact pulsed ESR, then it should be possible to generate arbitrary sequences of multiple phase coherent pulses with kW peak powers. In some cases, multiple frequencies are required, although rudimentary pulse sequences can be made up of just two or three pulses with variable delay. References [70,134-141].

Mark Sherwin – FEL based pulsed EPR at 240 GHz and beyond

Mark Sherwin continued discussion of future prospects for pulsed ESR at high frequencies, with most of the presentation focusing on efforts to develop a 240 GHz spectrometer based on the Santa Barbara electrostatic Free-Electron Laser (FEL) source. Further rationale was given for developing pulsed ESR capabilities at higher fields and frequencies, including the possibility for achieving very high degrees of electron spin polarization and consequent long transverse relaxation (T2, or coherence) times. This has been demonstrated via low-power pulsed measurements performed on nitrogen vacancy centers in diamond at 240 GHz at the National High Magnetic Field Laboratory. Sherwin also emphasized the need for large microwave powers for generation of high-bandwidth nutation pulses. The Santa Barbara FEL generates low repetition rate (~1 Hz) pulses of a few microseconds in duration in the 100 GHz to 100 THz range, with peak powers in excess of 1 kW possible. Sequences of THz pulses suitable for ESR nutation experiments are generated by slicing the longer FEL pulses into shorter ones that can be as short as 1 nanosecond. The pulse slicing is achieved using photo-activated Si reflection/transmission switches. A 532 nm Nd:YAG laser is used to activate the switches via the generation of an electron-hole plasma in the Si. Pulse-to- pulse phase coherence is established via injection locking of the FEL cavity using a low-power solid-state oscillator. Two-pulse experiments are currently feasible and several recent results 3+ were presented, including: Hahn-echo T2 measurements on a Gd salt; and quasi-continuous measurements of several candidate spin-labels used for biophysical measurements (e.g. TEMPO). Sherwin argued that the electrostatic FEL is better suited to this type of experiment in comparison to RF linac-driven FELs (Nijmegen, Dresden, TJNAF) that generate very short THz pulses (1-25 ps) at a high repetition rate (~10 MHz). Pulsed EPR has not yet been demonstrated with the latter type of FEL due to the need to coherently stack many pulses so as to accumulate

27 sufficient energy in a single pulse for appreciable spin nutation. It is also challenging to work below 1 THz with RF linac-driven FELs. Sherwin concluded that an electrostatic FEL would be the ideal THz source for performing pulsed ESR experiments in the magnetic fields available the US National High Magnetic Field Laboratory (0 – 45 T and 0.1 to 1.3 THz). References [67- 69,142-144].

Johan van Tol - Electrical and Optical detection of Coherent Spin Excitations at High Field

Johan van Tol talked about coherent electron spin excitations at high fields. Electron spins determine the magnetic properties of materials, and they also are basic building blocs of many proposals for quantum computation. It is possible to manipulate the spin states with strong electromagnetic pulses at the resonance frequency. This is a basic technique in NMR, and the basis for pulsed EPR. However, development to higher fields, frequencies and a higher time resolution is limited by the absence of powerful sources at THz frequencies. The few pulsed high field EPR instruments operate at relatively low power, and limited time resolution, but are delivering promising results. Van Tol showed that the spin dynamics of many systems change very significantly at high fields. Van Tol focused on recent experiments at the his lab where they studied Phosphorus in Silicon with electrically detected Pulsed EPR, measuring the change in electrical resistance when electron and nuclear spins are manipulated with millimeter waves and RF pulses at high frequencies and fields. Here the high fields increased the decoherence time of the electrically detected electron spins by two order of magnitude with respect to low frequency results. Also the high spin polarization at high fields enabled the storage and readout of quantum information in the nuclear spin system with several minutes relaxation times. Van Tol argued that it would be transformative if powerful sources existed that would enable the manipulation of a significant number of spins in very short time (~1 ns) at high frequencies. Just a train of single, strong coherent pulses could enable direct measurement of spin dynamics on unprecedented short timscales making use of electrical and/or optical detection techniques. He showed an example of very large resistance changes in silicon at high field by inverting the spins of the phosphorous dopants with pulses from the UCSB Free Electron Laser. References [145-151].

Janice Musfeldt – Accessing new states of matter with light: beyond traditional broadband spectroscopy

Musfeldt’s talk returned to the topic of electric-dipole allowed spectroscopy, with a focus on spin-lattice coupling. After a motivational/thought-provoking introduction, the concept of using intense photon fields to induce phase transitions in solids and, thus, new phases of matter with new properties, was discussed [42]. Experiments were described aimed at determining which (if any) phonon modes are involved in mediating superexchange interactions in molecular magnetic materials. The coordination polymer Mn dicyanomid, Mn(dca)2, was highlighted as an example. Data were presented demonstrating changes to phonon modes upon tuning Mn(dca)2 through a magnetic quantum phase transition using the strong magnetic fields available at the US National High Magnetic Field Laboratory as the external tuning parameter. The observed changes to the phonon spectra were attributed to changes in magneto-elastic couplings. Musfeldt then posed the question as to whether one can reverse such a process by pumping the phonon modes in order to excite the local structure and drive changes in the magnetism. An example of a target mode to pump might be the out-of-plane pyrazine bending mode, which was recently shown to influence

28 local structure and control the antiferromagnetic exchange interaction in copper halide coordination polymers [49]. Musfeldt concluded her presentation with a discussion of electromagnon excitations in multiferroics, i.e., electrically active spin-waves, and the possible consequences of non-linear (pulsed) excitation of such modes. Electromagnons have been observed in the terahertz regime in rare earth the manganites RMnO3 (R = Gd, Tb, Dy, Eu, Y, Lu), with intensity peaked at approximately 750 THz (25 cm-1).

Rick Averitt – A perspective on the potential of dynamic high-field excitation experiments on correlated electron materials

Averitt continued the theme of ultrafast dynamics and control in condensed matter systems, with particular emphasis on the correlated electron problem where there exist many competing degrees of freedom, e.g., lattice, spin, charge, and orbital. Up to now, most time-resolved pump- probe work in the THz has involved optical (IR/visible) pumping, due to the lack of availability of sufficiently bright sources of pulsed THz radiation. In such instances, subpicosecond laser pulses can selectively excite modes of strongly correlated electron systems and controllably push materials from one ordered phase to another. The probes have traditionally spanned the entire frequency range from the THz to hard x-rays, with THz Time-Domain Spectroscopy the method of choice for probing low energy electrodynamics. However, experiments nowadays are moving beyond the use of visible excitation pulses, with the advent of intense sources (up to 1 MV/cm) in the THz and mid-IR spectral ranges. Averitt surveyed the current state-of-the-art of such technology (details can be found in Ref. [11,12]). Averitt went on to discuss several examples of mode-selective excitations in correlated electron systems, leading to Photo-Induced Phase Transitions (PIPT). One of the main examples involved work performed at the Max-Planck Institute for Structural Dynamics (Hamburg), Centre for FEL Science (@ DESY). In this work, the superconducting transport between layers of La1.84Sr0.16CuO4 were gated with high-field terahertz pulses, leading to oscillations between superconductive and resistive states and a modulation of the dimensionality of the superconductivity in the material. A second example involved the Mott-Hubbard insulator VO2, where various mode-selective pump-probe measurements have demonstrated insulator-to-metal transitions, including work involving a THz-driven transition. Averitt concluded by highlighting efforts in his own lab to generate intense THz pulses by employing metamaterials with micron sized (i.e. sub-λ) features that enhance incident electric and magnetic fields. References [11- 12,152-157].

Junichiro Kono – Time-Domain Terahertz Magneto-Spectroscopy of Low-Dimensional Electron Plasmas

Kono discussed a range of new scientific opportunities for studying THz/far-IR phenomena in low-dimensional electron systems that have become possible due to recent advances in high- intensity, long-wavelength coherent sources, such as free-electron lasers (FELs), optical parametric amplifiers (OPAs), and Terahertz (THz) antennas. These advances permit both time domain and non-linear studies. In particular, far-infrared (FIR) pulses can directly excite low- energy dynamics in bulk and quantum-confined semiconductors, e.g., cyclotron resonance (CR), internal transitions of shallow donors and excitons, phonons, and intersubband transitions. An important theme of the presentation was the role of coherence as an important ingredient in

29 modern condensed matter physics. A coherent superposition of two levels is the basis in quantum computation, and the creation and manipulation of such superposition states is currently an enormously active and diverse field of research, encompassing quantum optics, condensed matter physics, and nanoscience. Kono then described a unique THz magneto-spectroscopy system, based on the technique of time-domain THz spectroscopy, ideally suited for coherent spectroscopy of solids at high magnetic fields and low temperatures. Using this setup, very long- lived (up to 50 ps) quantum coherence has been observed in a high-mobility two-dimensional electron gas in GaAs, whereby sequences of THz pulses are used to induce cyclotron resonance oscillations that are describable via single-particle Bloch equations, akin to pulsed NMR or EPR (described elsewhere in this report). In a second example, novel interference-induced THz transparency phenomena in InSb were described. Finally, circularly polarized mid-IR cyclotron resonance measurements in graphene using ultra high field pulsed magnets were presented. References [158-162].

Antoinette Taylor – Ultrafast Dynamics in Complex Materials: Probing to Control

Taylor discussed research in the area of complex functional (meta-) materials, with an emphasis on optical interactions and the development of optical-based techniques to study these phenomena. Metals provide the high conductivity needed to realize a strong resonant response in metamaterials; however, they contribute very little to tunability. By contrast, the complex conductivity in high-temperature superconducting films is highly sensitive to external perturbations, which provides new opportunities for achieving tunable metamaterials resulting directly from the resonant elements. Additionally, superconducting metamaterials are expected to provide a strong nonlinear response, particularly when Josephson junctions are integrated into the resonant elements. Photoexcitation using near infrared photons breaks the superconducting Cooper pairs, thus creating quasiparticles. This dramatically modifies the imaginary part of the complex conductivity in the THz regime and, consequently, the metamaterial resonance frequency on ultrafast timescales. Because of the non-linearity of the photo-response, the resonance frequency tuning depends on the infrared fluence. Meanwhile, the real part of the conductivity does not change significantly, so losses do not increase appreciably. This type of work represents an important step towards nonlinear THz metamaterials to yield functionalities that are difficult or impossible to achieve through conventional approaches in this technically challenging frequency range, as the superconducting films can naturally exhibit strong nonlinear response and additional quantum functionality – via Josephson junctions – can be integrated into the structures. References [163-165].

Larry Carr – Time-Resolved Studies of Materials using Accelerator-based THz Sources at the NSLS

After an introduction to the principles behind the generation of coherent THz radiation at the National Synchrotron Light Source (NSLS, at Brookhaven National Lab) facility, Carr described a range of applications in condensed matter and materials physics, with a focus on time-resolved capabilities. The first example involved the dynamics of phase excitations (vortices) – in particular, how do vortices initially form at the instant when the critical current is exceeded, and how does the superconducting state recover. Other examples included the use of strong transient E- and H-fields to induce very rapid of domain wall motion in ferroelectrics and ferromagnets, respectively, in order to test fundamental limits on polarization switching dynamics. In a final

30 example, Carr discussed the complex coupling between lattice vibrational modes and spin waves in multiferroic oxides, using THz pulses to pump electromagnons (coupled lattice/spin-wave excitations), whilst probing the lattice and magnetization with X-ray techniques. References [45- 48,121,122,166].

Mark Sherwin – Ultra-strong coupling of short and quasi-cw THz pulses with semiconductor nanostructures

Sherwin gave a second shorter presentation on the near-infrared interband absorption of semiconductor quantum wells driven by intense terahertz radiation in the regime of ultrastrong coupling, where the Rabi frequency is a significant fraction of the frequency of the strongly driven transition. With the driving frequency tuned just below the lowest frequency transition between valence subbands, a particularly interesting phenomenon is observed. As the THz power increases, a new peak emerges above the frequency of the undriven exciton peak that grows and eventually becomes the larger of the two. This reversal of relative peak intensity is inconsistent with the Autler-Townes effect in a three-state system while within the rotating wave approximation (RWA). In the samples investigated, the Bloch-Siegert shift (associated with abandoning the RWA), exciton binding energy, the Rabi energy, and non-resonant AC Stark effects are all of comparable magnitude. Solution of a semiconductor Bloch model with one conduction and multiple valence sub-bands indicates that the AC Stark effect is predominantly responsible for the observed phenomenon. References [167,168].

Charlie Schmuttenmaer – THz-TDS studies of low frequency modes in organic molecular crystals and time-resolved THz studies of electron injection on dye-sensitized metal oxide nanomaterials

Vibrational spectra of molecular crystals contain intermolecular modes that are not present in other phases. These vibrations typically occur at frequencies in the THz range and are very sensitive to the nature of the crystal lattice; these spectral features can be used to differentiate between materials that spectroscopy in the intramolecular frequency range cannot. For example, a particular molecule can often crystallize in different structures, with distinct properties such as solubility. This can make polymorph identification essential when designing optimal methods of pharmaceutical delivery. The intramolecular modes of two polymorphs can be extremely similar. However, the low-frequency intermolecular modes are directly affected by differences in crystal geometry and often allow one to immediately identify a particular polymorph. While the simplest picture of dynamics in molecular crystals assumes that the intermolecular modes are separated from the intramolecular modes, the low-frequency vibrations usually have more complicated, mixed character. It is in the THz frequency range where motions trend from intermolecular in character to intramolecular. The particular qualities of these vibrations result from the complex interplay of electrostatic, dipole-dipole, hydrogen bonding, and van der Waals interactions. THz spectroscopy is thus a direct method for exploring this complicated energy landscape. In the second part of his presentation, Schmuttenmaer focused on dye-sensitized metal- oxide nanomaterials, noting that THz spectroscopy is an ideal tool for studying the carrier dynamics because THz light is strongly absorbed by mobile electrons, while bound electrons such as those in the valence band are transparent to THz. TiO2 nanotubes have unique electron transport properties that make them a promising material for next generation solar cells and photoelectochemical cells. Both applications require that electrons be transported through a

31 network of TiO2 nanoparticles. Several studies have suggested that nanotubes might be superior to standard nanoparticle networks because electrons can travel down a continuous nanotube and not have to hop between as many particles. Evaluation of electron mobility through nanotubes in solar cells has, surprisingly, found nanotubes to be no better than nanoparticle films. Time- resolved THz measurements of the photoconductivity of nanotube films confirm the comparably low electron mobility of both nanoparticle and nanotube films but reveal different mechanisms for the same observed effect. In nanoparticles films, conductivity is inhibited by significant backscattering and/or disorder-induced localization. In nanotube films, the photoconductivity is limited by the formation of exciton-like trap states. References [169-174].

Carol Hirschmugl – Rapid and Contact-less Broadband IR Wide-field Imaging with Multiple Synchrotron Beams

Conventional Fourier-transform infrared (FTIR) micro-spectroscopic systems are limited by an inevitable trade-off between spatial resolution, acquisition time, signal-to-noise ratio (SNR) and sample coverage. Hirschmugl presented a recently developed FTIR imaging approach that substantially extends current capabilities by combining multiple (twelve) synchrotron beams with wide-field detection using multichannel focal plane array (FPA) detectors. This increases spatial coverage and imaging speed greatly, but the SNR using a thermal source limits pixel sizes to ~5 µm × 5 µm at the sample plane. Achieving a pixel size ~100 times smaller to correctly sample the diffraction-limited illumination is very ineffective using a thermal source, resulting in ~104-fold longer scanning time. Wide-field imaging with a small, low-emittance synchrotron beam seems counter-intuitive. However, these limitations can be overcome by extracting a large fan of synchrotron radiation from a dedicated bending magnet, which is then split into 12 beams to illuminate a large field of view in the sample plane. This advance allows truly diffraction- limited high-resolution imaging over the entire mid-infrared spectrum with high chemical sensitivity and fast acquisition speed, while maintaining high-quality SNR, thereby considerably extending the potential of infrared microscopy. The improvement in acquisition time opens the way to real-time noninvasive and label-free live-cell imaging. An example was presented in which wide-field multibeam synchrotron imaging revealed lymphocytes (diameter, ~2–7 µm) and other tissue features that were clearly visible in hematoxylin and eosin–stained images (the clinical gold standard for diagnosis). The same visualizations were impossible using currently existing table-top infrared systems. This work provides a rationale for future development of laser-based imaging systems and other multibeam synchrotron-based imaging beamlines. References [175-177].

Gwyn Williams – Low frequency electronic and vibrational dynamical coupling

Williams began by describing the THz/IR FEL facility at Thomas Jefferson National Accelerator Facility (TJNAF). The FEL is DoD funded, and can run for 8 hours per day. Operational funding for users must be included in their research grants. Three research areas were described. 1. Low frequency vibrational dynamics at metal surfaces reveal unusual electronic couplings that can be attributed to a breakdown of the Born‐Oppenheimer approximation. Absolute sources of IR and far‐IR, such as those found at accelerator facilities, are essential for probing these dynamics that can provide insights into the chemistry of interfaces, catalysis, and the functional behavior of nano‐particles. 2. High pressure and high magnetic fields are important thermodynamical variables. Studies of frequency‐dependent conductivity at high pressures are possible but require

32 high brightness sources of THz/IR radiation. Experiments were proposed on materials at high pressure in which the vibrational modes are modified and eventually frozen out to reveal distinctive novel physical properties such as metallic hydrogen. 3. Controlled far‐IR intramolecular coupling with high power THz light can be measured via a double resonance technique, whereby one pumps with broadband THz, then looks for modulations in the intramolecular bands. The technique is quite sensitive and can be extended to cover intermolecular modes as well. It has been postulated that THz dynamical coupling effects play an important role in some biological processes. Reference [178].

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45 Appendix A – Workshop Program

Workshop Location Westin Conference Room: Hemmingway 1 & 2 (Second Floor)

Wednesday April 13th 7-9pm All workshop attendees are encouraged to join workshop organizer Stephen Hill and others for an informal pre-workshop get together.

Thursday April 14th 07:30-08:30 Breakfast 08:30-08:50 Introduction: Krastan Blagoev, NSF Stephen Hill, NHMFL-FSU John Singleton, LANL Greg Boebinger, Director, NHMFL-FSU

08:50-12:30 Session 1: Chair, Stephen Hill, NHMFL-FSU

8:50 Martin Gruebele, University of Illinois "Protein-solvent dynamics: current and future THz experiments"

9:15 David Leitner, University of Nevada “Saccharides and proteins in solution: Computational studies of the molecular dynamics underlying THz spectra"

9:40 Glenn Edwards, Duke University "Potential Biological Applications of THz/IR"

10:05 Further discussion: Discussion Leader, Andrea Markelz, SUNY Buffalo

10:20-10:50 Break

10:50 Andrea Markelz, SUNY Buffalo "Over view of measurements on liquids and biomolecules using THz TDS"

11:15 David Plusquellic, NIST "State-resolved THz Spectroscopy: Phase Coherent Broadband Methods for High Sensitivity at Sub-us Scan Speeds"

11:40 Peter Weightman, University of Liverpool, Physics "Are there terahertz solutions to problems in understanding the physics of life"

12:05 Further discussion: Leader, Andrea Markelz, SUNY Buffalo

46 12:30-14:00 Working lunch

14:00-16:00 Session 2: Chair, Johan van Tol, NHMFL EPR

14:00 Peter Armitage, Johns Hopkins University "Probing correlated electrons in the THz at high magnetic fields and intensities"

14:25 Natalia Drichko, Johns Hopkins University "Interplay of Charge Order and Superconductivity in BEDT-TTF-based Materials with ¼-filled Conductance Band"

14:50 David Tanner, University of Florida "Photons, magnetic fields, Cooper pairs, and quasparticles: The use of synchrotron sources to study pairbreaking in superconductors"

15:15 Dmitry Smirnov, NHMFL-FSU “Prospects of broadband IR spectroscopy with split-helix magnet"

15:35 Further discussion: Leader, Peter Armitage, Johns Hopkins University

16:00-17:00 Break/refreshments

17:00-18:30 Session 3: Chair, David Tanner, University of Florida

17:00 Nick Polfer, University of Florida "Far infrared spectroscopy of mass-selected complexes pertaining to catalysis, astrochemistry and biochemistry"

17:25 Wim van der Zande, Radboud University Nijmegen, Holland "Three Short Aspects: Non-linear THz excitation, THz as a low frequency source and Serendipity"

17:50 Robert Peale, University of Central Florida "Anecdotes from THz science and about THz sources"

18:05 Further discussion: Leader, Nick Polfer, University of Florida

18:30-19:30 Further discussion and report planning

20:00 Group dinner

47 Friday April 15th 07:00-08:00 Breakfast

8:00-9:50 Session 4: Chair, Dmitry Smirnov, NHMFL-FSU

8:00 Jack Freed, Cornell University "High Frequency ESR: Biophysical Applications and Challenges"

8:25 Mark Sherwin, UC Santa Barbara “Free-Electron Laser-Based Pulsed Electron Paramagnetic Resonance at 240 GHZ and Beyond"

8:45 Johan van Tol, NHML-FSU "Electrical and Optical detection of Coherent Spin Excitations at High Field"

9:05 Janice Musfeldt, University of Tennessee "Light-induced, lattice-driven magnetic phase transitions in molecular solids"

9:30 Further discussion: Leader, Mark Sherwin, UC Santa Barbara

9:50-10:20 Break

10:20-12:30 Session 5: Chair, Janice Musfeldt, University of Tennessee

10:20 Rick Averitt, Boston University "A perspective on the potential of dynamic high-field excitation experiments on correlated electron materials"

10:45 Junichiro Kono, Rice University "Time-Domain Terahertz Magneto-Spectroscopy of Low-Dimensional Electron Plasmas"

11:10 Antoinette Taylor, Los Alamos National Laboratory "Ultrafast Dynamics in Complex Materials: Probing to Control"

11:35 Larry Carr, Brookhaven National Lab "Time-Resolved Studies of Materials using Accelerator-based THz Sources at the NSLS"

48 12:00 Mark Sherwin, UC Santa Barbara (short talk) "Ultra-strong coupling of short and quasi-cw THz pulses with semiconductor nanostructures"

12:10 Short discussion: Leader, Rick Averitt, BU

12:30-13:30 Working lunch

13:30-15:00 Session 6: Chair, Larry Carr, Brookhaven National Lab

13:30 Charlie Schmuttenmaer, Yale University "THz-TDS studies of low frequency modes in organic molecular crystals and time-resolved THz studies of electron injection on dye-sensitized metal oxide nanomaterials"

13:55 Carol Hirschmugl, University of Wisconsin, Milwaukee "Rapid and Contact-less Broadband IR Wide-field Imaging with Multiple Synchrotron Beams"

14:20 Gwyn Williams, Jlab "Low frequency electronic and vibrational dynamical coupling"

14:45 Further discussion: Leader, Charlie Schmuttenmaer, Yale

15:00 Wrap-up and start of report writing

49 Appendix B. Workshop Participants

Peter Armitage, Johns Hopkins University, Physics (http://www.pha.jhu.edu/~npa/npa.htm). The main interest of the group is exotic electronic states of matter at low temperatures, including exotic superconductors, novel magnetic states, electronic glasses, and materials in proximity to quantum critical points. Other areas of interest are nanostructures, biological physics, and aspects of physical chemistry and quantum optics. The group is also involved in the development of low-energy optical spectroscopic techniques in the so-called 'Terahertz gap'.

Rick Averitt, Boston University, Physics (http://physics.bu.edu/averittlab/): time-resolved optical spectroscopy spanning from the far-infrared through the visible to characterize the fundamental and technologically relevant properties of a host of interesting materials, with emphasis on unique aspects of short-pulse time-domain methods to investigate condensed matter. Examples include: photo-induced phase transitions in correlated electron materials; high peak field ps THz pulses (~0.5MV/cm or greater) for nonlinear far-infrared electrodynamic studies of materials.

Krastan Blagoev, NSF Program Director, Physics of Living Systems.

Gregory Boebinger, Director of the National High Magnetic Field Laboratory and Professor of Physics, Florida State University (http://www.magnet.fsu.edu/). Boebinger’s research focuses on high-temperature superconductivity and he maintains laboratories and close collaborations with colleagues at Los Alamos National Laboratory.

Larry Carr, Brookhaven National Lab (https://pubweb.bnl.gov/~carr/): infrared solid state physics with synchrotron radiation, in particular, sub-nanosecond time-resolved spectroscopy and studies of unusual (i.e. "bad") metals, superconductors, and semiconductors; infrared microspectroscopy and imaging using high-brightness synchrotron radiation; coherent THz radiation from accelerator-based sources; development and use of strong THz pulses for studying transient properties of materials.

Kelsey Cook, NSF Program Director, Division of Chemistry.

Natalia Drichko, Johns Hopkins University, Physics (http://www.pha.jhu.edu/~drichko/): Drichko is a condensed mater experimentalist, who is interested in charge and spin ordered ground states in strongly correlated electronic systems. Her current aim involves the employment of optical (Raman and Infrared) spectroscopies, to find a general view on these ground states in materials of different chemical origin, both inorganic and organic.

Glenn Edwards, Duke University, Physics (http://fds.duke.edu/db/aas/Physics/gedwards): Edwards is Director of the Duke Free-Electron Laser (FEL) Laboratory. His interests center on biological physics and FEL applications. Experimental research activities include vibrational dynamics of biological macromolecules with applications to protein disassembly and fracture, photothermal chemistry and photochemistry of biological macromolecules with applications to molecular and cell biology, and the development of novel spectroscopic techniques using FEL light sources. Theoretical research activities include modeling the solvent-DNA interface to better understand vibrational energy transfer.

Jack Freed, Cornell University, Chemistry (http://www.acert.cornell.edu/): application of magnetic resonance to problems in chemical physics and biophysics. The group studies

50 molecular dynamics, reactivity, and structure in condensed media, on clean surfaces, and in model membranes. The ability to study the participation of the solvent cage in liquid state dynamics is a unique feature of the group’s modern high-frequency ESR techniques.

Martin Gruebele, University of Illinois, Chemistry (http://www.scs.illinois.edu/mgweb/). The Gruebele group is engaged in experiments and computational modeling to study a broad range of fundamental problems in chemical and biological physics. A common theme in these experiments is the implementation of state-of-the-art laser techniques to interrogate and manipulate complex molecular systems, coupled with quantum or classical simulations. The results of these efforts are contributing to a deeper understanding of the way that proteins fold into functional 3-dimensional molecules, the details of how chemical bonds are broken by vibrational motion and how this can be controlled, and the switching of energy flow in large molecular structures on surfaces.

Stephen Hill (PI and Workshop Chair), National High Magnetic Field Laboratory and Florida State University, Physics (http://fs.magnet.fsu.edu/~shill/): Hill serves as Director of the Electron Magnetic Resonance (EMR) users program at the NHMFL. He has extensive experience performing microwave and far-infrared magneto-optical spectroscopy of materials in high magnetic fields, spanning the range from 0.3 to 200 cm 1 (9 GHz to 6 THz), using a wide array of compact, low-power sources and measurement techniques. Hill’s recent research has focused on fundamental studies of quantum phenomena in molecular magnets and correlated electron systems (quantum magnets and superconductors), as well as structure property relationships in a wide variety of molecule-based magnets, or molecular nanomagnets.

Carol Hirschmugl, University of Wisconsin - Milwaukee, Physics (http://www4.uwm.edu/letsci/physics/staff/carol_hirschmugl.cfm): Surface studies of adsorbates on epitaxial and bulk oxide systems, focusing on environmentally and technologically relevant problems. Investigative approaches include far and mid infrared absorption studies and picoampere low energy electron diffraction to study low energy dynamics and structure at aqueous-oxide interfaces. In addition, Hirschmugl is developing a rapid chemical imaging technique using infrared imaging microscope coupled to a synchrotron source, which will be used to examine real-time biochemical changes in vivo.

Junichiro Kono, Rice University, Electrical/Computer Engineering (http://www.ece.rice.edu/~kono/): interests include the physics and applications of semiconductor nanostructures and carbon-based nanomaterials. The group uses state-of-the-art spectroscopic techniques to study charge, spin, and vibrational dynamics in a variety of nanostructures, and has also developed a unique THz magneto-spectroscopy system based on the technique of time-domain THz spectroscopy, ideally suited for coherent spectroscopy of solids at high magnetic fields and low temperatures.

David Leitner, University of Nevada, Reno, Chemistry (http://www.chem.unr.edu/faculty/dml/). The group studies how energy flow within a molecule mediates the rate at which it reacts both in gas and condensed phases, including rather large molecules such as proteins or crystalline nanostructures. Rate theories developed for chemical reactions can also be usefully applied to describe the mobility of proteins in cells.

Walter Lowe, Howard University, Physics and Astronomy: Research interests include condensed matter physics and synchrotron radiation.

51 Andrea Markelz, SUNY Buffalo, Physics (http://www.physics.buffalo.edu/markelz/). The Markelz group studies physical dynamics at terahertz frequencies. This part of the electromagnetic spectrum corresponds to collective and diffusive motions in biomolecules and collective and single particle excitations in nanoelectronics. Our primary tools are terahertz spectroscopic studies using ultrafast lasers and molecular dynamic simulations.

Janice Musfeldt, University of Tennessee, Chemistry (http://web.utk.edu/~musfeldt/). The group focuses on the spectroscopy of novel electronic and magnetic materials, including: the interplay between charge, structure, and magnetism in complex materials; chemical and photochemical tuning effects in extended, molecular, and nanoscale systems; energetic materials; optical properties of polar oxides, electrode materials, battery compounds; finite length scale effects on charge and bonding in nanomaterials.

Robert Peale, University of Central Florida, Physics (http://physics.ucf.edu/~rep/): IR/Terahertz spectroscopy and technology development. Current projects include far-IR spectroscopy of mineral particles in support of Herschel Space Observatory data analysis, development of a long- wave IR intracavity laser absorption spectrometer for ultratrace vapor sensing, development of an infrared surface plasmon resonance biosensor, development of a tunable terahertz detector based on plasma resonances in a 2D electron gas, and development of thin-film THz optics.

Matthew Platz, NSF Program Director, Division of Chemistry.

David Plusquellic, NIST (http://www.nist.gov/pml/div685/grp08/plusquellic.cfm): studies of biomolecules and bichromophores in the microwave, terahertz (THz), and ultraviolet spectral regions; collective motions of biomolecular systems that are responsible for the large scale conformational changes associated with folding and activation of protein, polynucleotide and polysaccharide backbones. The group has pioneered the use of high-resolution THz laser sources to investigate the fully state-resolved vibrational spectra of polypeptides and to characterize the spectral response to hydration of hydrophobic and hydrophilic sites in a wide variety of crystalline structures.

Nick Polfer, University of Florida, Chemistry (http://www.chem.ufl.edu/~polfer/polfer.html): development of infrared (IR) photo-dissociation spectroscopy of mass-selected ions using the free electron laser FELIX; applications of this technique to a range of biomolecular systems, including carbohydrates, amino acids, peptides and their dissociation products, as well as whole proteins; vibrational spectroscopy in combination with theoretical approaches (e.g. DFT) to provide detailed information on gas-phase ions, e.g., vibrational frequencies which are subtly dependent on hydrogen bonding, the secondary structure, and the chemical moieties that are present/formed.

Charlie Schmuttenmaer, Yale University, Chemistry (http://www.chem.yale.edu/~cas/). Experimental Physical Chemistry and Chemical Physics: THz spectroscopy; Solar energy; Sub- ps time-resolved photoconductivity; Nanoscale properties and phenomena; Electron transfer, proton transfer; Solvation, and energy relaxation in liquids; Laser spectroscopy; Nonlinear dynamics..

Mark Sherwin, UC Santa Barbara, Physics (http://sherwingroup.itst.ucsb.edu/). The group is interested in the interaction of light and condensed matter in the terahertz (THz) frequency range. Specific projects include: nanostructures for THz frequencies; electron spin resonance; sensitive THz room temperature detectors; and THz photonic crystals and cavities.

52 Kamal Shukla, NSF Program Director, Division of Molecular and Cellular Biosciences.

John Singleton, National High Magnetic Field Laboratory and Los Alamos National Laboratory (http://www.magnet.fsu.edu/).

Dmitry Smirnov, National High Magnetic Field Laboratory (http://www.magnet.fsu.edu/).

David Tanner, University of Florida, Physics (http://www.phys.ufl.edu/~tanner/): studies of optical effects in solids occurring in the wavelength range from the far-infrared through the near ultraviolet. Among the topics being studied are high-temperature superconductors, conducting polymers, and low-dimensional organic systems. Dr. Tanner’s group are also very actively involved in the development of optical hardware used in gravitational wave and dark matter searches.

Antoinette Taylor, LANL: applications of THz to IR probes to condensed matter physics, including ultrafast dynamics in complex materials; the use of ultrafast THz to IR probes to not only understand the competing interactions in complex materials (correlated electron materials, nanomaterials, metamaterials, etc.) that result in emergent phenomena, but also to go beyond probing to control through directed synthesis, or external fields.

Guebre Tessema, NSF Program Director, Division of Materials Research.

Wim van der Zande, University of Nijmegen, The Netherlands, Molecular Biophysics (http://www.ru.nl/molphys/group/employees/wim_van_der_zande/). Dr. van der Zande is the head of the Molecular and Laser Physics group in Nijmegen. His interests include: dissociation dynamics of small molecules; molecular detection and molecular dynamics; THz spectroscopy and coherent control; atmospheric physics; electron recombination processes; and molecular biophysics.

Johan van Tol, National High Magnetic Field Laboratory (http://www.magnet.fsu.edu/): van Tol's research interests involve magnetic resonance of paramagnetic, anti-ferromagnetic, and ferromagnetic spin systems, as well as high-field/frequency EPR technique development.

Peter Weightman, University of Liverpool, Physics (http://pcwww.liv.ac.uk/~sc35/home.htm): development and application of Reflection Anisotropy Spectroscopy (RAS) for the study of biological systems and of a terahertz beamline and tissue culture facility on the ALICE accelerator at Daresbury for the study of biological systems.

Gwyn Williams, JLab (http://www.jlab.org/~gwyn/): understanding the fundamental physical behavior of materials and surfaces via infrared studies of the vibrational dynamics of adsorbates. Current research programs involve ultrafast pump-probe dynamics of novel materials and of bonding vibrational modes in both time and frequency domains.

53 Appendix C EPR Spectroscopy with THz Radiation in Very High Fields: Road map and Applications July 4, 2011, Radboud University Nijmegen, HFML 0220

Scope and goals of this meeting The ever increasing magnetic fields and new radiation sources in the THz part of the spectrum, which make pulsed EPR possible this frequency range, hold promise for a new step in EPR spectroscopy. Both the very high magnetic fields as well as the THz radiation sources will require a technical roadmap in order to arrive at the level of present EPR capabilities. This meeting is aimed at bringing together EPR scientists in order to provide answers on the following questions and challenges: - What are the promising applications of Pulsed EPR that explicitly benefit of very high fields and hence require THz radiation? - What are niche applications that make optimal use of high magnetic fields and/or the presence of intense THz radiation? - What is the technical road map for developing Free Electron Based THz sources and Very high Fields in high quality VHF pulsed EPR?

A secondary aim of this meeting is to discuss routes to combine initiatives as started in Germany, the United Kingdom, and the Netherlands in order to use the strength of an international approach to obtain funding either using bi-national programs or EU programs.

Program The program will be concentrated in one day and will include four invited talks of 30 minutes (including discussion). Discussion sessions will be organized in a “Faraday Discussion style”. We invite those interested to propose small contributions of five minutes (about five slides) as starting points of the discussion. Proposals can be made to Wim van der Zande ([email protected]) or Ed Reijerse ([email protected]). Mrs Miriam Heijmerink ([email protected]) will assist in the organisation. The meeting will be concluded with a diner.

The preliminary program reads: 9:00 Welcome 9:15 Graham M. Smith (St Andrews) “High Power, high sensitivity pulse EPR at 94GHz - extending pulse EPR to even higher frequencies”. 9:45 Joris van Slageren (Stuttgart) “Magnetic resonance in molecular magnetism”

54 10:15 Break

10:45- Discussion session 1: Promising Applications VHF Pulsed EPR 11:30 (including approx 4 x 5 minute presentations) 11.30- Discussion session 2: Applications with THz Sources and/or Very high Fields 12.30 (including approx 4 x 5 minute presentations)

12:30- 14.00 Lunch Buffet + Tour of Facilities in Nijmegen (in house)

14:00 Alexander Schnegg (Berlin), “Ultra-sensitive detection of device limiting paramagnetic defects in thin film Si solar cells”. 14:30 Thomas Prisner (Frankfurt) “DNP related EPR”. 15.00 Marina Bennati (Göttingen) “Concept and content of the German Priority Program”. 15.15 Gavin Morley (London) “EPR ambitions of the EPSRC program”. 15.30- Wim van der Zande (Nijmegen) “Specifications of the Nijmegen THz sources”. 15.45

15:45 Break

16:15- Discussion session 3: Technical Road Map Aspects of Identified Experiments 17.15 (Including 4x 5 minutes presentations)

17.15 Break-out Meetings on DFG/FOM/EPSRC/EU Programs (in small groups) 18:00 End

List of 5 minute contributions (under construction): • Graham Smith: Very high power, very high instantaneous bandwidth gyro-amplifiers at frequencies (using new technology) up to 400GHz • Szymon Smolarek: Pulse shaping Continuous FEL pulses for Pulsed ESR applications: a technical challenge • Gavin Morley: The possibility of electrically-detected EPR with FLARE • Sergei Zvyagin: High field ESR developments at the HFDZ-Dresden • Edward Reijerse: Proposed design of FLARE powered pulsed EPR setup • Igor Tkac: Hardware developments for high frequency EPR • Hans van Tol (NHFML): High-frequency Pulsed EPR of systems with very short T2 times

19:00 Dinner (Brakkenstein or Kasteel Heyendaal)

55 Appendix D

Report on Workshop to Initiate a Collaboration for Terahertz Microscopy July 11, 2011 Thomas Jefferson National Accelerator Facility, Newport News VA

A small group of THz researchers (see list below) met at JLab on July 11, 2011, to discuss research and research needs, organized around the following key points:

1. What are the theoretical or measured properties of the THz source(s) at your site? What yet needs to be further characterized? 2. At your source, what are the present THz experimental facilities that others could use? 3. What facilities could you bring to a THz source elsewhere to do experiments, if those experiments were desirable to you? 4. What science would you, your colleagues or users like to do using THz, but that is out of reach of your/their “home” source? What source characteristics are desired? 5. What experimental facilities do not now exist at any THz facility, that would allow important science if provided?

Workshop Participants

UK – funded by British Embassy

• Harvey N. Rutt – Southampton Develops and applies instrumentation for THZ research. Special interest in a microscope capable of simultaneous imaging and spectroscopy • Philip Taday – Teraview Develops and manufactures THZ instruments, mostly aimed at specific applications; e.g., non-invasive defect detection in microelectronics • Peter Weightman – Liverpool R&D on and with accelerator-based THZ sources, leader of THZ beamline at ALICE

US – funded by JLab or BNL

• G. Larry Carr – Brookhaven Synchrotron light source development and application, especially THZ sources at BNL for users • Michael Kelley – William & Mary Would like to use THZ to study surface water on minerals • J. Michael Klopf – JLab Instrumentation scientist. Steward of JLab THZ facilities • Giti Khodaparast – Virginia Tech Semiconductor materials science; THZ studies of narrow band gap semiconductors • Andrea Markelz – Buffalo Applies THZ to molecular biology; notably water in and as a probe of biological systems • Gwyn Williams – JLab Leads THZ effort at JLab

The outcome of the meeting was that a near-term path-forward emerged, pointing towards the first experiments. The major items are:

56 A. Needs/opportunities that can be addressed by sharing resources already in hand Teraview has an instrument located at a laboratory in the UK with an arrangement for access by Teraview collaborators. Work there could produce some scientific results, indicate the potential of commercial instruments for doing science, identify the potential of/need for instruments that could be develop from them, indicate the value of commercial style instruments for research use.

B. Needs/opportunities that span the community, especially ones that are foundational to progress Terahertz sources, e.g., the FEL at JLab and ALICE at Daresbury, are operational but have not been characterized in the depth or detail needed for broad, high-quality science. Working together could define what characterizations are needed and how they are best obtained. Doing so as a shared project among the source labs would foster valuable relationships.

C. What resources are needed widely now and how might they be acquired ? Studies using IR spectroscopy rely on well developed FTIR instruments for both spectroscopy and microscopy. Equivalent instruments do not now exist for THz, but are needed both for the science and to characterize the beams. The zeroth-level vision is a large aperture step-scan asymmetric interferometer with a wire-grid beamsplitter. The original notion of an instrument for spectroscopic microscopy is still appealing, but more foundation needs to be laid first.

57 Appendix E – White Papers

White papers were provided by many of the participants in advance of the workshop. These were posted on a secure workshop web site (http://biologyphysics.blogspot.com/) along with relevant references. The following participants provided white papers:

Peter Armitage Dimitry Basov (was unable to attend the workshop) Larry Carr Natalia Drichko Jack Freed Martin Gruebele Andrea Markelz Jan Musfeldt Robert Peale David Plusquellic Nick Polfer Mark Sherwin Wim van der Zande Hans van Tol Peter Weightman Gwyn Williams

Unedited versions of these white papers now follow in alphabetical order.

58 Probing correlated electrons in the THz at high magnetic ﬁelds and intensities

N.P. Armitage1 1The Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, MD 21218 USA. (Dated: April 11, 2011)

Material systems with strong electron-electron interactions, such as high-Tc superconductors, ‘quantum’ magnets, and materials in proximity to T = 0 quantum phase transitions, exhibit a multitude of particularly novel properties as a result of these interactions and coherent quantum effects. The complexity and the resultant effects of 1023 particles acting in quantum mechanical unison can give rise to a host of beautiful and striking phenomena like superconductivity and magnetism. Like waves on the sea, their behavior is intrinsically collective and is not easily reduced to the properties of individual particles i.e., they are emergent. A principal scientific interest in the field of strongly correlated electrons is understanding these effects in the electronic and magnetic properties of materials; how is it that large ensembles of strongly interacting, but fundamentally simple particles like electrons in solids can act collectively to exhibit complex emergent quantum phenomena (1)? Remarkably, strongly interacting electron systems, due to the collective effects and coherent motion of many particles, have characteristic energies much lower than the eV scale of the isolated atoms and chemical bonds that compose them. One can estimate the relevant frequencies and energies of these systems from their important temperature scales using the relation E = hν = kBT , where kB is Boltzmann’s constant. One such temperature scale may be the transition temperature Tc to an ordered state like superconductivity. For many interesting materials with strong interactions Tc ≈ 10K, which corresponds to 0.2 THz and an energy of 0.9 meV. Another important temperature scale is the sample temperature Ts itself. By using frequencies ν such that kBTs < hν, with hν still well below the energy of any relevant ordering temperature kBTc one can access the T=0 or so-called quantum limit and characterize intrinsic ground state properties (4). Since the lowest sample temperatures in ‘optical’ spectroscopy are limited to approximately 1 K, these considerations mean that experimental probes employing THz radiation should be optimal for the investigation of strongly interacting systems. Unfortunately, this spectral range has lay in the so-called “Ter- ahertz Gap” - above the capabilities of existing radio-frequency and microwave electronics, but below that of typical optical instrumentation. In recent years however, a number of exceptional technical advances (2; 3) have spanned this gap (Fig. ??) in some configurations. There are still notable gaps in the measurement possibilities however in this spectral range. Generally sources in this spectral range are weak and measurements are contaminated by the black-body bath of the surrounding environment, and so it can be difficult to incorporate samples into measurement configurations with, for instance, extremely low temperatures and high magnetic fields. The development of BigLight will allow measurements of both equilibrium and non-equilibrium (pumped) strongly correlated systems in the THz regime under high magnetic fields in a manner not previously possible. Below I detail a number of interesting strongly interacting systems that could be fundamentally impacted by BigLight. • Electronic Glasses - Glasses are ubiquitous states of matter with positionally or rotationally randomized frozen degrees of freedom. Their inherently frustrated interactions give rise to the existence of many energetically low- lying metastable states, which have a wide distribution of potential barriers separating them in configuration space. At low temperatures, such systems are kinematically constrained from reaching their true ground-state on typical experimental time scales. They are characterized by long relaxation times, memory, and aging effects (5–8). In analogy with structural glasses, electronic glasses can be defined as systems with a random distribution of localized charges (9–11). Here long relaxation times and glassy phenomena derive from a combination of disorder and long-range unscreened Coulomb interaction. The motion of any one charge manifestly necessitates a many-particle rearrangement of the other electron occupations to reach lower energy. Such many-particle processes are inherently slow and inefficient at finding the true ground-state configuration. Electronic glasses or ‘Coulomb glasses’ may be realized in granular metals and amorphous and lightly doped semiconductors, which are all expected to exhibit certain similar qualitative behavior. Recently there has been great progress in the understanding of how glassy non-equlibirum effects manifest themselves in these systems. In amorphous compounds like In2O3−x the natural history-free relaxation law of the DC conductivity after excitation is logarithmic over more than five decades in time (17), a behavior that may be rooted in a broad distribution of relaxation times. Interesting temperature dependencies have been found at low temperature, which have been ascribed to quantum tunneling instead of thermally activated hopping (18). The majority of such experiments have been on systems such as microcrystalline In2O3−x or granular metals (8; 17; 19) in which the charge 2 density is high. It would be interesting to look for such phenomena in systems with much lower densities like doped semiconductors. Although the equilibrium transport properties expected for electronic glasses have been reported in systems like Si:B or Si:P (14; 15; 20; 21), evidence for true glassy behavior in the conductivity, i.e. long-time non- exponential relaxation indicative of metastability and frustration was not been observed until recently. This is likely due to their much faster time scales owing to the much smaller charge densities, which is crucial for their quantum dynamics (18). We recently reported the use of ultrafast optical pump-terahertz probe (OPTP) spectroscopy to resolve glassy relaxation in the doped semiconductor Si:P (22). In this experiment a P doped Si sample was optically excited with a laser pulse and then probed at a later time with a terahertz (THz) pulse to measure the induced conductivity changes with picosecond resolution. While OPTP has been successfully applied to correlated electron systems such as high-Tc’s, manganites and semiconductors (23; 24) in unraveling the various dynamics by their different relaxation time scales, this was the first such study on glasses. The idea is that the charge was excited optically and then spends long times undergoing many-body rearrangements trying to find the true ground state. As this long time-scale frustrated relaxation is ongoing the THz conductivity decreases. In this work, we found strongly temperature and fluence dependent glassy non-exponential relaxation occurring on a sub-nanosecond time scale (100s of ps), which is in contrast to the much longer time scales observed in high electron density glassy systems. One of the problems with this study however, was that we were using optical pulses to pump the sample. Here the supposition is that although the charge is excited from the P impurity band by a 0.8 eV laser pulse it decays quickly back to the impurity band but to an excited configuration. This fast decay should take place on the ps scale and not influence the physics much, but we would have much preferred that we just excite the manifold of impurity band states directly. In general, with tabletop sources, it is difficult to make low energy pulses that one can pump only the impurity states. We’d like to do experiments where we pump in the MIR and probe with broadband THz. Such experiments should be possible with BigLight. • High-Tc Superconductors - The nature of the underdoped pseudogap regime of the high-temperature superconductors has been a matter of long-term debate. On quite general grounds, one expects that due to their low superfluid densities and short correlation lengths, superconducting fluctuations will be very significant for transport and thermodynamic properties in this part of the phase diagram. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap, and their significance for understanding high-temperature superconductivity. In our recent work published Nature Physics we have used or unique capabilities in THz time-domain spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity above Tc in La2−xSrxCuO4 (LSCO) thin films over a doping range that spans almost the entire superconducting dome (x =0.06 to 0.25). Through a collaboration with Ivan Bozovic of Brookhaven National Laboratory, we are investigating the fluctuation superconductivity in thin films of LSCO grown by molecular beam epitaxy (MBE). This synthesis technique provides exquisite control of the thickness and chemical composition of the films; the intrinsic chemical tunability of LSCO allows us to investigate essentially the entire phase diagram. Dynamical measurements in the THz range are a sensitive probe of the onset of superconductivity and measure its temporal correlations on the time scales of interest. In the presence of superconducting vortices such high-frequency measurements are not affected by effects like vortex pinning, creep, and edge barriers that often complicate interpre- tation of low frequency and DC results. Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, up to - at most - 16 K above Tc. (Fig. 1). In general, continuous phase transitions are typified by fluctuations with correlation length and time scales that diverge near Tc. For all samples, we find that the characteristic fluctuation rate increases quickly above Tc, but is then limited when it reaches a scale proportional to temperature. Our measurements show that superconducting correlations do not make an appreciable contribution to the transport anomalies of the pseudogap in LSCO at temperatures well above Tc. This is interesting because in La2−xSrxCuO4 a region of enhanced fluctuation diamagnetism extends almost 100 K above Tc (? ) while the THz fluctuation conductivity has an extent limited to 10 - 20 K above Tc (25). This is surprising as one might expect a close correspondence between these quantities (26). Similarly, it has been argued from Nernst and diamagnetism measurements that Hc2 may be as high as 150 T (? ), while the resistive transition is essentially complete in√ optimally and underdoped LSCO by 30 T (27; 28). Similar behavior has been found recently in YBCO in which a H to the heat capacity (indicative of nodal d-wave superconductivity) has been found up to fields well in excess of the resistive transition (29). The high magnetic fields of BigLight will allow for the first time spectroscopic measurements in the THz frequency ranges of interest up to fields well past the resistive tranitions of this material. The supposition from measurements other than charge transport is that vortices exist in this range, but that they are so fast that the superconducting response is shorted out from normal electrons. Although we don’t expect to see any sharp features at these high fields, it may be that high magnetic fields will show that the electronic background has a particular field sensitivity. This may indicate a superconducting origin. 3

120 Diamagnetism Onset (Li PRB 2010) THz conductivity onset T (Ω ∝ T) 100 L TC LSCO films

TC LSCO crystals (Wang PRB 2006) 80

Temperature [K] 40

0.00 0.05 0.10 0.15 0.20 0.25 0.30 Doping [x]

FIG. 1 Phase Diagram of the onset of superconducting correlations in LSCO. Temperature vs. doping phase diagram of La2−xSrxCuO4 comparing Tc of thin ﬁlms and bulk crystals with the THz conductivity onset To and the diamagnetism onset temperature from Ref. (? ). Here, To is expressed as a shaded region to convey the uncertainty in its determination. The temperature TQ at which the characteristic ﬂuctuation rate Ω becomes proportional to temperature is plotted in green.

FIG. 2 Spin ice in a rare-earth pyrochlore. (a) Magnetic moments in the ground state. (b) A misaligned magnetic moment adds two elementary excitations with opposite magnetic charges. (c) Magnetic charges are not conﬁned: they can be pulled far apart without incurring a large energy cost.

Even if we see no superconducting response, it will still be interesting to do THz spectroscopy in this range as this is the field range where quantum oscillations have been imagined. The inference has been that the high field state of the cuprates may be a Fermi liquid. It will be very interesting to probe quasi-particle coherence and transport in this spectral range. If these systems are truly Fermi liquids at high field then it will show up as a huge difference in the quasi-particle scattering rates as extracted from THz measurements at high field as compared to the normal state at low fields. The construction of BigLight will make these experiments possible. • Monopole transport and dynamics in spin-ice - The pyrochlore structure generally describes materials of the type A2B2O6 and A2B2O7 where A and B are generally rare-earth or transition-metal elements. Materials with these lattices have been the subject of interest for many years as they frequently host novel magnetic states because of strong geometrical frustration. In Ho2Ti2O7 and Dy2Ti2O7, rare-earth ions have large magnetic moments of order 10µB living on a lattice of corner-sharing tetrahedra (30). The crystal field induces a strong anisotropy aligning the magnetic moment with a local h111i axis, Fig. 2. The doubly-degenerate ground state of Ho3+ and Dy3+ lies a few hundred kelvins below the first excited state (30), so that at temperatures below 10 K, the excited states are frozen out creating effectively Ising spins with strong geometrical frustration. In low-energy states, two spins point into and two point out of each tetrahedron. This is analogous to the Bernal-Fowler rules (31) formulated for positions of protons in water ice (two protons next to an O2− and two farther away). These rules are satisfied by an exponentially large number of microstates, which leads results to a large residual entropy that persists to low temperature. It has recently been proposed that the low-energy excitations of the spin-ice state are magnetic monopoles (32). 4

Reversing a single spin in a ground state of spin ice violates the ice rules on two adjacent tetrahedra that now carry magnetic charges of opposite signs. The monopoles can be further separated by additional spin flips with no additional violations of the ice rules (See Fig. 2). At low temperatures, spin ice behaves as a gas of magnetic monopoles. This picture is consistent with experiments on neutron scattering, thermodynamics, dynamical magnetic susceptibility, and µSR measurements (33; 34). Although the existing experiment and theory are compelling, a quantitative understanding of the dynamics of magnetic monopoles is lacking. I believe that one may be able to study monopole transport in an ac magnetic field using the intense FIR fields available at BigLight. Typically one expects that magnetic dipole transitions are much weaker than electrical ones. Since these are insulating materials they should be not electrically active below their electronic band gap. So therefore, we expect that driven monopole motion will be evidenced by a magnetic dipole active absorption at energies below the spin-flip gap. Although magnetic dipole excitations should be active, they should still be weak and we shall need the strong magnetic fields of the the FIR FEL to see them with sufficient signal to noise. We expect that the spectral weight of any absorption will follow the expectation for the known magnetic field and temperature dependence of the monopole density using the magnetic dipole sum rule. One may also expect that if a monopole description of the low temperature state of the elementary degrees of freedom is valid then one may be able to describe their motion and absorption through some sort of a Boltzmann transport model. In this case, at low frequencies the monopole transport will be overdamped, but at high enough frequencies the transport may be dominated by inertia of the monopoles. Effects of inertia have been clearly observed in the motion of domain walls in ferromagnetic nanowires (35). A Boltzmann model description of the transport of these objects would be a beautiful confirmation of the relevance of the monopole description of the excitations of these systems. One may expect to extract from the AC measurements the monopole mass, charge, scattering times and density.

References

[1] R. B. Laughlin and David Pines, Proc. Nat. Acc. Sci. 97, 28-31 (2000). [2] http://en.wikipedia.org/wiki/Terahertz_time_domain_spectroscopy [3] http://www.er.doe.gov/bes/reports/abstracts.html#THz [4] S. L. Sondhi, S. M. Girvin, J. P. Carini, and D. Shahar, Rev. Mod. Phys. 69, 315 (1997). [5] L. Struik, Physical aging in amorphous polymers and other materials (Elsevier Scientific Pub. Co, 1978). [6] J. Bouchaud, Spin glasses and random fields p. 161 (1998). [7] E. Vincent et al., Complex Behaviour of Glassy Systems pp. 184219 (1997). [8] M. Ben-Chorin, Z. Ovadyahu, and M. Pollak, Phys. Rev. B 48, 15025 (1993). [9] J. H. Davies, P. A. Lee, and T. M. Rice, Phys. Rev. Lett. 49, 758 (1982). [10] N. Mott and E. Davis, Electronic processes in non- crystalline materials (Clarendon Press Oxford, 1979). [11] M. Pollak and M. Ortuno, Sol. Energy Mater. 8, 81 (1982). [12] A. Efros and B. Shklovskii, J. Phys. C 8, L49 (1975). [13] B. I. Shklovskii and A. L. Efros, Sov. Phys. JETP 54, 218 (1981). [14] M. Lee and M. L. Stutzmann, Phys.Rev.Lett. 87, 056402 (2001). [15] E. Helgren, N. P. Armitage, and G. Gru?ner, Phys. Rev. Lett. 89, 246601 (2002). [16] E. Helgren, N. P. Armitage, and G. Gru?ner, Phys. Rev. B 69, 014201 (2004). [17] A. Vaknin, Z. Ovadyahu, and M. Pollak, Phys. Rev. B 61, 6692 (2000). [18] Z. Ovadyahu, Phys. Rev. Lett. 99, 226603 (2007). [19] T. Grenet et al., Euro. Phys. Jour. B 56, 183 (2007). [20] M. Lee et al., Phys. Rev. B 60, 1582 (1999). [21] P. Dai, Y. Zhang, and M. Sarachik, Phys. Rev. Lett. 66, 1914 (1991). [22] V. K. Thorsmolle and N. P. Armitage, Phys. Rev. Lett. 105, 086601 (2010). [23] R. Averitt and A. Taylor, Journal of Physics: Condensed Matter 14, R1357 (2002). [24] P. Jepsen et al., App. Phys. Lett. 79, 1291 (2001). [25] L. S. Bilbro et al., Nature Physics (2011) doi:10.1038/nphys1912. [26] B. I. Halperin and D. R. Nelson, J. Low Temp. Phys., 36, 599 (1979). [27] N. Miura et al., Phys. B, 319, 310 (2002). [28] Y. Ando et al., Phys. Rev. Lett., 75, 4662 (1995). [29] Scott C. Riggs et al., Nature Physics 7, 332335 (2011). [30] Gardner, Jason S. and Gingras, Michel J. P. and Greedan, John E., Rev. Mod. Phys. 82, 53 (2010). [31] J. D. Bernal and R. H. Fowler, J. Chem. Phys. 1, 515 (1933). [32] Castelnovo, C. and Moessner, R. and Sondhi, S. L., Nature 451, 42-45 (2008). [33] Hiroaki Kadowaki, Naohiro Doi, Yuji Aoki, Yoshikazu Tabata, Taku J Sato, Jeffrey W. Lynn, Kazuyuki Matsuhira, Zenji Hiroi, J. Phys. Soc. Jpn. 78, 103706 (2009). [34] Jaubert, L. D. C. and Holdsworth, P. C. W., Nat. Phys. 5, 258 (2009). [35] Eiji Saitoh, Hideki Miyajima, Takehiro Yamaoka and Gen Tatara, Nature 432, 203 (2004). Infrared nano-scopy of complex functional systems

D.N. Basov University of California, San Diego http://infrared.ucsd.edu [email protected]

Introduction. Applications of infrared (IR) spectro-microscopy in science and technology extend to such diverse areas as physics, chemistry, life sciences and biology, materials science and engineering, forensics and national security. The underlying reason behind such an unprecedented scope is that many fundamental properties of matter have characteristic energy scales falling in the infrared range. Conventional and synchrotron- based IR microscopy enables characterization of these properties in inhomogeneous substances with the diffraction-limited spatial resolution (10-50 µm). The impact of IR microscopy both in life and materials/physical sciences strongly motivates the development of experimental approaches suitable for an infrared probe of matter at the nanoscale. Recently, significant progress in infrared nano-scopy has been achieved by several research groups through an innovative combination of atomic force microscopy and IR lasers. Novel scanning near field IR instrumentation facilitates both spectroscopy and imaging with the spatial Scanning Focusing mirror resolution down to 10 nano- IR optics meters or better. beam splitter IR Basics of a near field IR laser nano-scope. Figure 1 shows a AFM schematic of a near field nano- cantilever HeIR-cooled illuminationfar field scope originally developed by Ωcryostat KBr Keilmann and collaborators1. window nΩ Here, the tip of an atomic force microscope (AFM) is illuminated with radiation from a tunable IR gas laser. The detector cantilever tip, an IR laser, and Figure 1. Schematics (not to scale) of the Scattering detector are arranged in the Scanning Near Field Infrared Nanoscope. The basic Michelson interferometer building block of the apparatus is an AFM microscope. The scheme. The principal task of tip of the AFM is illuminated with an infrared laser. The the interferometer is to enable scattered signal is registered using an interferometric measurements of both the scheme enabling direct measurements of both amplitude and phase of the scattered light. Interferometric detection is amplitude and phase of back- imperative to produce images of local values of optical scattered radiation. Since both constants free of topographic artifacts. the amplitude and phase are directly accessible, this instrument allows one to infer local values of the optical constants of studied specimens at the frequency of the laser source2. Importantly, the spatial resolution is determined solely by the radius of the tip appex and not by the wavelength of light. This latter circumstance allows one to achieve imaging with the resolution of the order of 10 nm irrespective of the frequency of the IR source. These functionalities of near field IR nano-scopy have already enabled nano-scale exploration of previously unattainable characteristics of a variety of materials including semiconductors, polymers1 as well as single viruses3. A combination of broad-band ellipsometry and near-field nano-imaging has facilitated significant advances in the understanding of the electronic correlations in transition metal oxides4,5,6.

Need for broad band tunable lasers. The operation principle displayed in Fig. 1 is compatible with spectroscopy and imaging across a broad region of electromagnetic spectrum: from GHz7,8 to visible light. Because the near field tip-sample interaction is weak, the nano-scopy experiment critically relies on the availability of fairly powerwul infrared lasers delivering 1-10 mW. This condition is relatively easy to realize only in the 10 mkm range and in the 5 mkm owing to well developed technology of CO2 and CO lasers respectively. Outside of these two ranges, tunable radiation with sufficient power can be readily provided by Free Electron Lasers. Extension of nano-imaging/spectroscopy to far-IR and THz frequencies is particularly challenging and beneficial at the same time.

Impact. Near-field nanoscopy is likely to enable breakthrough results in studies of imhomogeneous and phase separated systems. Such inhomogeneties are known to occur in many systems of high current interest in the context of both energy and information technologies including but not limited to: high-Tc superconductors, plastic solar cells, electrochromics, materials employed for hydrogen storage, etc. In systems where multiple phases coexist on the nanometer scale, the dynamical properties of these individual electronic phases remain unexplored because methods appropriate to study charge dynamics (transport, infrared/optical, and many other spectroscopies) lack the required spatial resolution. Scanning near-field infrared nano-scopy can circumvent this long- standing limitation. For that reason one can anticipate a major impact of the near-field nano-scopy experiments in a variety of subfields of condensed matter physics and materials science.

1Fritz Keilmann and Rainer Hillenbrand, “Near-field microscopy by elastic light scattering from a tip,” Phil. Trans. R. Soc. Lond A 362, 787–805 (2004). 2 R. Hillenbrand, F. Keilmann, “Complex optical constants on a subwavelength scale.” Phys. Rev. Lett. 85, 3029 (2000). 3 M. Brehm, T. Taubner, R. Hillenbrand, F. Keilmann, “Infrared Spectroscopic Mapping of Single Nanoparticles and Viruses at Nanoscale Resolution”, Nanoletters 6, 1307 (2006). 4 M. M. Qazilbash, M. Brehm, Byung-Gyu Chae, P.-C. Ho, G. O. Andreev, Bong-Jun Kim, Sun Jin Yun, A. V. Balatsky, M. B. Maple, F. Keilmann, Hyun-Tak Kim, and D. N. Basov “Mott Transition in VO2 Revealed by Infrared Spectroscopy and Nano-Imaging” Science 318, 1750 (2007). 5 T. Driscoll, S. Palit, M. M. Qazilbash, M. Brehm, F. Keilmann, Byung-Gyu Chae, Sun-Jin Yun, Hyun-Tak Kim, S. Y. Cho, N. Marie Jokerst, D. R. Smith, and D. N. Basov “Dynamic tuning of an infrared hybrid-metamaterial resonance using vanadium dioxide” Appl. Phys. Lett. 93, 024101 (2008). 6 M. M. Qazilbash, Z. Q. Li, V. Podzorov, M. Brehm, F. Keilmann, B. G. Chae, H. T. Kim, and D. N. Basov “Electrostatic modification of infrared response in gated structures based on VO2 ” Appl. Phys. Lett. 92, 241906 (2008). 7 K. Lai, M. B. Ji, N. Leindecker, M. A. Kelly, and Z. X. Shen, “Atomic-force-microscope- compatible near-field scanning microwave microscope with separated excitation and sensing probes” Rev. Sci. Instrum. 78, 063702 (2007). 8 H. Ma and J. Levy “GHz Apertureless Near-Field Scanning Optical Microscopy of Ferroelectric Nanodomain Dynamics” Nano Letters 6, 341 (2006). Applications of the NHMFL "Big Light" FEL for Two-Color, Pump-Probe Studies of Condensed Matter Dynamics

Larry Carr, Brookhaven Nat'l Lab, [email protected]

The proposed Big Light FEL facility offers a powerful combination of tunability, power, pulse energy and repetition rate for studying dynamics. The multiple source capability is particularly well suited to two-color pump-probe studies. Of particular interest is the THz FEL branch and the possibility for producing strong-field single-cycle pulses. Listed below are a few science problems in condensed matter & materials physics that can be addressed by two-color pump- probe studies.

1. Correlated Electron Systems & Emergent Behavior. Novel electronic properties, such as

high-TC superconductivity, often emerge at the boundary between two competing electronic phases. Measurements that probe the interactions between these phases are therefore of interest [1]. A candidate measurement approach is to transiently force the system across a phase boundary while studying how the characteristics of a given phase appear and evolve with time.

Experimental methods: Two-color pump-probe using light pulses to drive the system toward or across a phase boundary while probing with FEL pulses tuned to an energy for sensing the competing phase(s) or to limit the excitation range in a continuum system. The pump could be conventional photo-excitation to create mobile carriers from an insulating phase (“photo-doping”), or by tuning to a relevant excitation such as the electromagnon in multiferroics [2]. An applied DC magnetic field could be used to bias the sample toward a phase boundary. A 3rd type of excitation makes use of coherent THz pulses where the transient E-field directly drives a response such as current in a superconductor or the polarization state in a ferroelectric. Recently, the B-field of a strong pulse was used to drive soin wave excitations in antiferromagnetic NiO [3].

2. Electronic Energy Relaxation in Nanomaterials and Structures : The study of electronic relaxation processes in photo-excited materials is relevant both for a fundamental understanding of electronic materials and also for applications such as harvesting solar energy. Systems of interest include semiconductor nanostructures based on III-V or II-VI materials as well as novel materials such as graphene. A 3rd class of materials are the non- crystalline organic photovoltaic materials. In contrast to crystalline semiconductors, these materials suffer from poor mobilities and the need to dissociate rather strongly bound excitons [4].

Experimental methods: Two-color pump-probe using to probe the production and relaxation of free carriers, excitons and other electronic states. a) Free and weakly bound carriers in crystalline semiconductors can be sensed by THz spectroscopy, which can include standard FTIR methods that exploit the bandwidth available in these ultra-fast pulses.. A magnetic field gives rise to Landau levels and the ability to track the mass and energy of carriers [5]. b) Electron transfer in organic photovoltaics (OPVs) can be sensed through some mid-IR vibrational modes (6). Of special interest would be spatial probing to observe the process in heterogeneous materials (a microscope equipped with an array detector or a near-field probe method). The high intensity of the FEL is likely to be a necessary ingredient for the latter.

1. See, for example, review articles by R. A. Kaindl, R. D. Averitt, in “THz Spectroscopy: Principles and Applications ", S. Dexheimer, ed., (2007). and R. D. Averitt, A. J. Taylor, J. Phys: Cond. Matt 14, R1357-R1390 (2002).

2. See, for example, A. Pimenov et al, Nat. Phys. 2, 97 (2006) ; A. Sushkov, J. Phys: Cond. Mat 20, 434210 (2008); R. Valde´s Aguilar, Phys. Rev Lett. 102, 047203 (2009)

3 T. Kampfrath, Nature Photonics 5, 31 (2011).

4. See, for example, S.R. Forrest, MRS Bull. 30, 28 (2005), C.-X. Sheng et al, Phys. Rev. B75, 085206 (2007),

5. J. Kono et al, Appl. Phys. Lett. 75, 1119 (1999); S.N. Gilbert and G.L. Carr (in prep).

6. R.D. Pensack and J. Asbury J. Am. Chem. Soc. 131: 15986-15987 (2009). Interplay of Charge Order and Superconductivity in quasi-two- dimensional organic conductors with 1/4-ﬁlled Conductance Band

Research related to unconventional superconductivity is one of the most actively developing topics in solid state physics. While the applied studies are focused on superconductors with high Tc, the fundamental research involves studies of various classes of materials, also those with much lower temperature of a superconducting transition, as well as related insulating materials. A reason to study materials with these various properties is clarified by a phase diagram in Fig.1: a general feature of systems with strong electron-electron correlations is that on changing of a tuning parameter, a ground state changes from an ordered (insulating) to a metallic state, and the superconducting state found in between may originate from the fluctuations of the ordered state.(e.g. [1, 2]) A tuning parameter is different for different materials, for example it is doping in cuprates or pressure in pnectides and organic conductors.

Figure 1: General phase diagram of correlated electron systems where an electronically ordered ground state crosses over to a metallic state as a tuning parameter is changed; at low temperatures superconductivity develops right between these states. The electronic density corresponds to the oxygen content for doped cuprates, resulting in the generic phase diagram of high-temperature superconductors [3]. In heavy fermions, organic conductors or pnectides hydrostatic pressure is the parameter to drive the system from an antiferromagnetic state to an unconventional metal [1]

Studies of superconductors with relatively low Tc, as well as metals with strong electronic correlations, which mean narrow bands and high effective masses of conductance electrons, as well as spectral features due to fluctuations of an ordered state involve optical measurements at low frequencies and low temperatures. In particular, for the studies of a whole phase diagram, a possibility to tune a parameter that can control a ground state of a material, for example, pressure is very attractive. Optical observation of a superconducting gap and a reaction of supreconducting system on the application of magnetic field are also very important. While studies of this kind can be relatively easily performed for d.c. or thermal properties [4], they are technically challenging and some times not possible at all with a conventional lab equipment that involves optical technique. An example of studies of strongly correlated electronic systems by the low-frequency optical measurements is our work on tuning of a ground state in BEDT-TTF-based quasi-two-dimensional organic conductors by so-called ”chemical pressure”. Quasi-two- dimensional organic conductors are formed by layers of BEDT-TTF molecules, sand- witched between the layers of anions that serve as charge reservoirs. These material are widely used as model systems to study the effects of electronic correlations and unconventional superconductivity. We used optical reflectance methods in a wide frequency range, going as low as 10 cm−1, to investigate the response of conductance electrons in these materials as well as to register the superconducting gap in the β′′-

1 (BEDT-TTF)2SF5CH2CF2SO3 below Tc of 5.2 K [5]. We observe signatures of charge- order fluctuations in the in-plane conductivity spectra of the organic superconductor β′′- (BEDT-TTF)2SF5CH2CF2SO3, and compare the results to a relative metallic compound ′′ β -(BEDT-TTF)2SO3CHFSF5 which shows no signatures of the ordered state. We show that on the decrease of effective electronic interaction in an isostructural metal both charge-order fluctuations and superconductivity are suppressed. While the present study, which was using ”chemical pressure” as a tuning parameter, was possible using a laboratory equipment, an important extension of it which would use hydrostatic pressure to suppress both superconductivity and charge order, to directly confirm the unconventional origin of superconductivity. A conventional lab equipment does not have enough source intensity for performing low-frequency measurements under pressure. An application of magnetic field for a study of superconducting state and the metallic state which occurs on the suppression of superconductivity is beyond a standard lab possibilities, while it would be extremely interesting to study a response of the conducting electrons in the magnetic-field induced metallic state. These studies a facility with low- frequency radiation source of high intensity, coupled with an optical measurement system and magnetic field would be necessary.

References

[1] N. D. Mathur, F. M. Grosche, S. R. Julian, I. R. Walker, D. M. Freye, R. K. W. Haselwimmer and G. G. Lonzarich Nature 394, 39 (1998).

[2] M. Dumm, D. Faltermeier, N. Drichko und M. Dressel Phys. Rev. B 79, 195106 (2009).

[3] B. Batlogg and C.M. Varma, Physics World, February 2000.

[4] F. Levy et al., Science 309, 1343 (2005).

[5] S. Kaiser, M. Dressel, Y. Sun, A. Greco, J. Schlueter, G.L. Gard und N. Drichko Phys. Rev. Lett. 105, 206402 (2010).

2 Terahertz Infrafred, Arlington VA, Apr 2011

Applications of Terahertz-to-Infrared Probes in Molecular & Materials Sciences April 14 & 15, 2011 The Westin Gateway in Arlington, VA

High Frequency ESR: Biophysical Applications & Challenges Jack H. Freed National Biomedical Center for Advanced ESR Technology Department of Chemistry and Chemical Biology Cornell University, Ithaca, NYNY,, USA

OUTLINEOUTLINE 1.1. Pulse Dipolar ESR: DEER & DQC Are Higher Frequencies Helpful? 2.2. 2D2D--FTFT--DQCDQC 3.3. MultiMulti--FrequencyFrequency ESR & Protein DynamicsDynamics 4.4. TwoTwo--DimensionalDimensional ESR: 2D2D--ELDORELDOR & Membrane Dynamics 5.5. Taking Pulse 2D2D--ESRESR to Higher Frequencies

1 Biological THz spectroscopy Problems that can be tackled with tunable, high intensity sources

Terahertz radiation, spanning frequencies from about 0.3 THz

Prospects for imaging and THz spectroscopy Macromolecules like proteins and DNA have distinctive spectral signatures at 0.5-1.5 THz related to large amplitude vibrational and functional modes, and so do their solvation water molecules. The problem is that these features are broad, so spectra can be less than information rich. Imaging can help with this: if the molecule is excited to saturation by a powerful THz source, so the molecule undergoes a large amplitude motion, this motion can be imaged at atomic resolution, yielding dynamical strcutural information To build a THz imaging spectrometer, we will adapt a technique we used successfully to study single molecule infrared absorption with Ångstrom resolution independent of wavelength or focusing. The schematic in Figure 1 illustrates the essential concept, which combines THz absorption and tunneling microscopy: a frequency-modulated THz beam will be coupled from the THz source into a transparent substrate via a silicon wedge or gold thin film. The wedge transmits the THz radiation to its front surface, where the radiation is totally internally reflected. The front surface of the wedge is derivatized with a biomolecule-friendly organic layer, either directly (by derivatized alkyl chains), or by depositing a self-assembled monolayer on a 5-10 nm thick gold layer evaporated onto the chip (a technique we recently published in J. Phys. Chem.) The evanescent wave penetrates about 0.5 mm into the solvent above the chip, fully illuminating any biomolecule on the surface. By modulating the THz beam between a resonant and a non- resonant wavelength, the molecular Figure 1. A schematic diagram of the tip-molecule-surface motion is modulated, resulting in a junction on logarithmic horizontal and vertical scales. The enhanced optical field is shaded in red. A phosphoglycerate change of the local electronic kinase molecule is adsorbed onto the substrate, illuminated by density of states on the surface at modulated THz radiation through the back of a p rism to the location of the molecule. The induce large amplitude motion, which is then imaged by the STM tip with 0.1 nm lateral resolution. Phase-sensitive lock- change is detected by scanning over in detection at the modulation frequency yields the absorption the molecule with an STM tip that signal in addition to the usual STM topography image. can detect displacements < 0.05 nm. Thus, a topographic image (average current), and a single molecule absorption image in the THz range (modulated current) can be obtained simultaneously. Besides providing the modulated tunneling current, the STM tip also enhances the THz electric field ~1000x, saturating the molecular transition, even though the beam itself is <50 mW/mm2. Wax coating, a standard procedure in electrochemical STM experiments, is used to reduce Faraday currents into the solvent. The enzyme phosphoglycerate kinase (PGK), which produces ATP in cells, is a good example of a target protein. PGK has a large amplitude hinge motion, and we showed recently that solvent crowding can modulate the stability of the ‘crystal’ and ‘compact’ structures of PGK resulting from hinge motion. Thus PGK can be ‘primed’ to undergo large amplitude motions induced by the THz field. We also have worked extensively with PGK and site-specific mutants that modify the hinge using fluorescence and infrared spectroscopy. The high power of Big Light will make saturation of large amplitude motions possible, something that cannot be accomplished with current table top sources. To saturate the molecular transition requires electric field strength in excess of 107V/cm at THz frequencies.

Prospects for two dimensional THz-THz and THz-IR spectroscopy Another way of creating richer information from broad THz transitions is to use 2D techniques. Water dynamics has been looked at by 2D IR-IR experiments by Zanni and others, but the most direct information on water dynamics lies in the 10-100 cm-1 range. There Gruebele and Havenith showed that solvation water absorbs more strongly than bulk water around proteins. Big Light offers the exciting prospect of a source where THz-(THz->IR) multidimensional experiments can be carried out. It is unique in offering both the necessary power for nonlinear spectroscopy, and the tunability to cover the far infrared to infrared range. As a bonus, THz 2D experiments could turn out to be easier than the current IR experiments, assuming a proper source is available: the optical requirements for coherent experiments are not as exacting at longer wavelengths, and the transitoin dipoles for large amplitude motions of polar biomolecules (e.g. helix dipole moments) are quite large. As an example of a project to link biomolecule and solvent dynamics, consider the correlation between amide modes at the surface of a protein and water dynamics. By labeling only charged and polar amino acid carbonyl groups with 13C, the surface of a protein can be overwhelmingly labeled compared to the hydrophobic core. A 2D IR-THz experiment at (1600-1800 cm-1) – (0.5-2.5 THz) coverage can look at the correlations between water dipoles and surface amide I’ band dipoles (in deuterated solvent). Selective labeling of different sidechains can localize the information, and with several such “orthogonal” patches on the surface labeled, the contribution of specific surface epitopes on the protein to modulating water THz absorption can be probed. There is evidence that proteins modulate solvation shells out far form the surface. It is currently not known how much the modulation of the solvation shell depends on localized variations in surface polarity and charge. What is well known is that long-range electrostatics play a role in enzyme activity (for example, reeling in substrates towards active sites), and a THz-IR double resonance experiment would provide a unique opportunity to determine how protein surface composition controls water-mediated long range electrostatic interactions. White Paper: Protein Structural Mode Separation with Modulated Orientation Sensitive THz Spectroscopy

Rohit Singha, Deepu Koshy Georgea, Kristin Suttonb, Edward Snellb and Andrea Markelza

aDepartment of Physics, University at Buffalo

bDepartment of Structural Biology, Hauptman-Woodward Medical Research Institute

Abstract

The objective of this research is to develop a table top technique to characterize large scale correlated motions in proteins. Correlated motions means individual atomic motions of the macromolecule are correlated on the length scale of the entire protein.

The energies of protein correlated motions lie in the far infrared or THz frequency range (l = 1 cm – 50 mm, n = 0.03 – 6 THz). The existence of correlated motions has been confirmed by neutron spin echo and inelastic x-ray scattering measurements. These techniques require large sample volumes and specialized facilities, limiting their application to systematic studies of changes in correlated motions with functional state and allosteric interactions. THz spectroscopy is table top and requires 2 orders of magnitude smaller sample volumes. While standard terahertz spectroscopy measurements have shown sensitivity to protein-ligand binding, oxidation state, conformation, temperature and hydration, the response is broad, in part from the large vibrational density of states and in part from the dielectric response contribution from surface water and side-chains.

It is difficult using standard techniques to determine if the THz sensitivity arises from changes in the correlated motions or from changes in interfacial water and/or surface side chain motions. These local librational motions give give rise to a dielectric relaxational loss contribution to the signal.

However there is a key difference between the dielectric response from the surface water and side chains and the correlated motions. It is the dependence of the transition dipole relative to the protein structure. Because of the nearly homogeneous distribution of surface side chains and water, there should be no orientational dependence for relaxational contributions from aligned samples, however this is not the case for the correlated motions. Determination of specific structural modes from THz spectroscopy is complicated by the large density of states of correlated motions. For a homogenously oriented sample all optically active motions will be excited leading to a nearly featureless spectrum.

However, for aligned samples, mode excitation and therefore absorption will dependent on the relative alignment of the transition dipole (as determined by the dipole derivative in MD simulations)and the light polarization:

We can achieve this alignment with protein crystals. For sufficiently large crystals we can do standard THz time domain spectroscopy. However for smaller crystals, we must use THz microscopy. We propose to build THz systems that allow for polarization sensitive measurements of aligned protein samples, and rapid determination of the polarization difference spectroscopy. The figure shows the change in the THz absorption as a function of rotation of a crystal of hen egg white lysozyme relative to the THz polarization direction.

Light-induced, lattice-driven magnetic phase transitions in molecular solids

Jan Musfeldt, U. Tennessee

One initiative of emerging interest to our team is the use of intense infrared pumping to drive lattice-induced phase transitions. Certainly the idea of photoinduced phase transitions and photoinduced magnetism is not new [1]. What is different here is that we propose to use our understanding of spin-lattice coupling in several model molecular materials to pump a specific phonon that is known to be connected with the magnetic state. The targeted phonon, for instance in copper halide coordination polymers, might be the out-of-plane pyrazine bending mode, which was recently shown to influence local structure and control the antiferromagnetic exchange interaction [2]. The ultimate goal of this project is to tune or even block the magnetic quantum critical transition via light-induced local structure changes. Proof-of-concept work will focus on continuous wave experiments. Time-resolved measurements will be pursued to reveal the dynamics of any lattice-induced magnetic transitions. In addition to advancing our understanding of coupling between structure and magnetism, this effort will forward the use of light as a tuning parameter to control the state of matter.

[1] Y. Tokura, Photoinduced phase transition: A took for generating a hidden state of matter, J. Phys. Soc. Jpn. 75, 011001-011003 (2006).

[2] J.L. Musfeldt, L.I. Vergara, T.V. Brinzari, C. Lee, L.C. Tung, J. Kang, Y.J. Wang, J.A. Schlueter, J.L. Manson, and M.-H. Whangbo, Magnetoelastic coupling in quasi-two-dimentional Heisenberg antiferromagnet, Phys. Rev. Lett. 103, 157401 (2009). Anecdotes from THz science and about THz sources Robert E. Peale, University of Central Florida Grating-gated transistors have potential as tunable THz detectors based on plasmon resonances in the 2DEG. Such resonances have been observed in transmission by FTIR for a variety of materials systems. To observe and optimize a resonant electrical photo-response requires a tunable THz source with high spectral density. I describe our experiences in this application with p-Ge-, quantum cascade-, and free-electron-lasers, and with backward wave oscillators. The good and bad points of each source are discussed. Astrophysics continues to be the application where THz (here known as far-infrared or sub-mm wave) reigns supreme. With the Herschel Space Observatory now operating and with the Stratospheric Observatory for Infrared-red Astronomy beginning to obtain results, there is a need for far-IR laboratory spectroscopy of gases and minerals. I describe our work in collecting reference spectra for minerals by FTIR, some of the challenges of this well-established technique, and a wish list that would make this technique faster and more accurate.

State-resolved THz Spectroscopy: Phase Coherent Broadband Methods for High Sensitivity at Sub-us Scan Speeds

Eyal Gerecht, Kevin Douglass, and David F. Plusquellic We report the development of a new phase-coherent THz heterodyne detection method for performing high speed broadband measurements at frequencies between 200 GHz and 2 THz [1]. The sensitivity and speed of the method have been demonstrated for detection of trace gases in direct absorption and by the free induction decay response. A schematic overview of the instrument is shown in Fig. 1. The technique is based on the chirped-pulse polarization method reported in the MW [2]. Linear chirped pulses spanning 230 MHz and 25 ns in length are generated in an Arbitrary Waveform Generator (AWG) at > 500 kHz repetition rates and mixed with a MW signal from a phase-locked synthesizer. The upper sideband of the mixed signal is filtered out using an Yttrium Iron Garnet (YIG) filter and amplified to 18 dBm at the input port of a x48 harmonic multiplier chain. The output signal of the multiplier chain (300 uW max) is quasi-optically coupled into a 25 m long White cell. The chirped pulse signal spanning 11 GHz (230 MHz * 48) is focused onto a phase-locked sub-harmonic heterodyne detector (mixAMC - x24) using an off-axis parabola mirror at the output port of the White cell. The AMC (and mixAMC) has been adapted to enable rapid response to < 1 usec variations of the driving MW signals. The IF signal is detected directly on the oscilloscope following amplification by a 32 dB amplifier. Three wavelength bands of multiplier sources are available and include (260 to 290) GHz, (530 to 620) GHz and (780 to 870) GHz but the technique would perform equally well for any frequency range where AMCs or phase- coherent THz sources are available.[1] Each chirped pulse is precisely phase locked to a Rb reference standard and therefore, may be signal averaged directly in the time domain for any length of time in order to improve S/N. The chirped pulse near 548 GHz is shown in the upper panel of Fig. 2 and the corresponding squared magnitude of its Fourier transform are in the lower panel. The Fourier transform (squared magnitude) of the free induction decay following the chirped excitation pulse is shown in Fig. 3. The background free nature of the measurement leads to high detection sensitivities shown as inverted traces for each of the gas components in Fig. 3. Other data reported in Ref. 1 have been taken in 500 ns (25 ns chirp + 475 ns FID) where absorption lines are phase coherently excited in less than 5 ps (sub-ps rates will soon be possible). The precise control of the timing and bandwidth for excitation and detection enables methods that probe the dynamics associated with hydration and conformation changes in biomolecular systems as well as a variety of other transient processes in the gas and condensed phases.

References 1. E. Gerecht, K. Douglass and D. F. Plusquellic, “Chirped-pulse Terahertz Spectroscopy for Broadband Trace Gas Sensing”, OPTICS EXPRESS, in press, (2011). 2. G. G. Brown, B. C. Dian, K. O. Douglass, S. M. Geyer, S. T. Shipman and B. H. Pate, “A Broadband Fourier Transform Microwave Spectrometer Based on Chirped Pulse Excitation,” Review of Scientific Instruments, 79, 053103 (2008).

Fig. 1. The left panel illustrates the power spectrum of a 100 ns duration chirp pulse with 450 MHz bandwidth at a center frequency of 11.48 GHz. The upper sideband of the mixer output from the AWG and synthesizer was filtered with tunable YIG filter. The synthesizer was set to 9.08 GHz and the center frequency of the AWG was 2.4 GHz. The center frequency offset of the AWG was optimized for spectral purity. The final chirp pulse has a high level of spectral purity, which is required for efficient multiplication of the chirp pulse in order to avoid parasitic mixing effects. The largest spur is down over 40 dB. Note that the spikes at 5 GHz, 10 GHz, and 15 GHz are from the oscilloscope. The right panel is a schematic of the experimental setup.

Fig. 2. Time domain signal of a 10 GHz chirped pulse with 5 component gas mix (upper panel) and squared magnitude of the Fourier transform of the time domain signals (top trace in lower panel - blue) and that of the empty cell (bottom trace in lower panel - red).

Fig. 3. Measured FID spectrum of the five component gas mixture over a 10 GHz bandwidth. Predictions from HITRAN and JPL databases are shown inverted and below the measured spectrum. The simulated line intensities have been corrected according to Eq. (1) in Ref. 1. The insert shows a 45 MHz section containing the 110←101 transition of the 17 H2 O isotopologue at 552.020 GHz and the corresponding signal obtained when the cell was evacuated using a diffusion pump (EC). The parent isotopologue of this transition at 556.837 GHz is saturated (2773 times stronger) and not shown.

BigLight NSF Workshop April 14-15 2011

Far infrared spectroscopy of mass-selected complexes pertaining to catalysis, astrochemistry and biochemistry

The combination of mass spectrometry with tunable infrared lasers offers the advantage that structures of discrete, mass-selected complexes can be investigated. This allows size-specific trends to be established. Such experiments also serve as a direct benchmark for quantum-chemical calculations. Most of the published work from the free electron laser (FEL) facilities FELIX and CLIO have employed the technique of infrared multiple photon dissociation (IRMPD) spectroscopy to record infrared spectra of ions in the mid-IR range (500-2000 cm-1) [1]. Such experiments are much more challenging at longer wavelengths (<500 cm-1), due to the lower photon flux for those FEL designs, and the lower energy per photon. The usefulness of extending the wavelength region to the far IR has already been demonstrated for metal clusters. In the Figure to the left, the low-

frequency metal cage vibrations of the Au7 cluster are diagnostic in order to identify the symmetry of the proposed complexes [2]. Experiments on size-selected clusters can give insights into the growth mechanism of these clusters, and can rationalize how structure is related to their catalytic properties. In these carefully controlled experimental conditions, the reactivity of the clusters can even be induced by infrared radiation [3]. A similar argument about the usefulness of the far-IR range can be made for astrophysically relevant molecules. While the signature of unidentified infrared bands (UIB) in the mid-IR range has strongly suggested the presence of polyaromatic hydrocarbons (PAHs) in interstellar space, laboratory and simulated mid-IR spectra of PAHs are too similar to give insights into their exact chemical composition. On the other hand, far-IR spectra of PAHs have been proposed to be much more diagnostic with respect to structure [4], due to “drumbeat” modes (see Figure to the right) [5]. Coincidentally, a number of on-going astronomical missions are aimed at gathering far-IR spectroscopic data from interstellar clouds, such as the Herschel Space Observatory (launched May 2009), and the Stratospheric Observatory for Infrared Astronomy (SOFIA). Many studies at FEL facilities have involved molecules of biological interest [1]. As a proof-of-principle experiment, the mid-IR spectrum of an entire protein in the gas phase has been recorded [6]. The positions of the amide I (C=O stretch) and amide II (N-H bend) modes were found to be consistent with a protein largely composed of α-helical motifs. In principle, gas-phase IR spectroscopy is hence well suited

Nicolas Polfer – University of Florida

BigLight NSF Workshop April 14-15 2011

to the fundamental study of protein folding diseases (linked to Alzheimer’s or Parkinson’s Disease), which are known to progress via β-sheet formation. The combination of mass selectivity with laser spectroscopy allows IR spectra of size-selected aggregate states of the protein to be obtained. This is expected to yield information on the early stages of the aggregation process, which remain poorly understood. The large molecular sizes of these aggregate complexes require the high laser fluences of FELs, even in the mid-IR range. While it is not clear yet what additional structural information will come from far-IR radiation, it is likely the dynamics of large amplitude motions will become visible.

References [1] Polfer, et al., Vibrational spectra of bare and solvated ionic complexes of biological relevance, Mass Spectrom. Rev., 28, 468-494 (2009).

[2] Gruene, et al., Structures of neutral Au7, Au19, and Au20 clusters in the gas phase, Science, 321, 674- 676 (2008). [3] Hamilton, et al., Infrared induced reactivity on the surface of isolated size-selected clusters:

Dissociation of N2O on rhodium clusters, J. Am. Chem. Soc., 132, 1448-1449 (2010). [4] Zhang, et al., Far-infrared emission spectra of selected gas-phase PAHs: Spectroscopic fingerprints, Science, 274, 582-583 (1996). [5] Ricca, et al. The far-infrared spectroscopy of very large neutral polycyclic aromatic hydrocarbons, Astrophys. J., 709, 42-52 (2010). [6] Oomens, et al., Charge-state resolved mid-infrared spectroscopy of a gas-phase protein, Phys Chem Chem Phys, 7, 1345-1348 (2005).

Nicolas Polfer – University of Florida

“Free-Electron Laser-Based Pulsed Electron Paramagnetic Resonance at 240 GHz and Beyond.”

Mark Sherwin

Institute for Terahertz Science and Technology (ITST) and Physics Department, UCSB.

An electron’s spin is an exquisite probe of its local environment in condensed matter, including solids, liquids, biological molecules and devices. Using electron paramagnetic resonance (EPR), one can interrogate electron spins to extract information about local structure and dynamics. However, the state of the art in EPR allows the exploration of only a tiny fraction of the information that can be extracted from electron spin dynamics. In a collaboration between UCSB and the NHMFL, FEL-based pulsed EPR is now rapidly expanding the frontiers of EPR. This technique is poised to become a critical tool for unlocking some of nature’s most closely guarded secrets.

Fig. 1: Free-electron Larmor frequency (EPR frequency) vs. magnetic field up to 45 T, the highest DC magnetic field available on earth (NHMFL). Without a FEL, high-power (>100 W) pulsed EPR is available only up to magnetic fields of 3.5 T (95 GHz), shown in blue rectangle. FEL-based pulsed EPR has now been demonstrated at 240 GHz. This technology is scalable to 45 T and beyond.

Ultimate goal for pulsed EPR: The ability to probe the structure and dynamics of hard, soft and living matter on length scales between 3 and 10 nm up to room temperature on time scales of 1 ps and longer. (Good topic for discussion)

To reach goal: Like NMR, EPR becomes much more powerful at high fields, high frequencies, pulsed rather than cw, with high power, programmable pulses.

State of the art without a FEL: High-power (>100W) pulsed EPR can be performed up to 95 GHz (B=3.5 T). Maximum distance that can be measured is~7 nm, but must be measured at low temperatures (<50K).

UCSB-FEL-enabled breakthroughs: In a collaboration between UCSB and the NHMFL, high-power pulsed EPR has been demonstrated at 240 GHz (8.5 T). Pi/2 pulses as short as 6 ns have been measured for spin 1/2, 50 times shorter than can be achieved with the best existing alternative, a solid-state source at 240 GHz. Phase memory times (T2) shorter than 50 ns have been measured recently using a sequence of two pulses, ten times shorter than have been measured for spin-1/2 systems with a solid-state source at 240 GHz. Hahn echoes have been measured at temperatures approaching 200K.

Electrostatic-accelerator-based FELs for pulsed EPR up to the highest magnetic fields on earth: Unlike almost every other FEL in the world, the UCSB FEL is driven by an electrostatic rather than a rf linear accelerator FEL. The UCSB FEL can generate few- microsecond long pulses with ~kW peak powers and linewidths of less than 1 MHz, and is tunable from 0.12-4.7 THz. Using shutters activated by ultrafast laser pulses, the long FEL pulses can be “sliced” into a series of short, mutually-phase-coherent pulses for pulsed EPR. The duration and separation between the sliced pulses can be controlled with sub-ns precision. For comparison, the FELBE superconducting rf-linac-based FEL produces 1-25 ps pulses in the 1.2-85 THz frequency range.

Pulsed EPR spectrometers powered by electrostatic accelerator-driven FELs are straightforwardly scalable to frequencies limited only by foreseeable magnets. The free- electron Larmor precession frequency at the current world-record 45 T DC magnetic field in Tallahassee is 1.26 THz.

Location Source, magnet EPR Power Rep. rate B1 π/2 Bandwidth frequency pulse

NHMFL, Synthesizer + 240 GHz 0.03W Electronic 0.03 300 ns 3 MHz UCSB, … multipliers, 8.5 T s limited mT

UCSB MM-FEL , 8.5 T 0.24 THz 300 W 1 Hz 1.4 mT 6 ns 200 MHz (now)

UCSB EPR-FEL, 8.5 T 0.24 THz >10 10-1000 >8 mT <1 ns >1 GHz (planned) kW Hz

UCSB EPR-FEL+17 T 0.48 THz >10 10-1000 >16 mT <0.5 ns >2 GHz (potential) magnet kW Hz

NHMFL Electrostatic FEL + 1.26 THz >10 10- >42mT <0.2 ns >5 GHz (potential) 45 T Hybrid kW 1000Hz

Table 1: Present and future capabilities of pulsed EPR spectrometers based on electrostatic-accelerator driven FELs. B1, Pi/2 pulse duration for spin 1/2, and bandwidth all assume sample is not in a resonator. In order to achieve the 1000 Hz repetition rate, a high-current electrostatic accelerator is required, such as a “fixed-field alternating gradient” accelerator or a “dynamitron.”

Electrostatic accelerator-driven FELs are relatively inexpensive to operate and maintain— the UCSB FELs are operated and maintained by a staff of two. The electrostatic accelerators themselves are based on very well-established technology. A 4 MeV (rather than 6 MeV) accelerator would likely be sufficient for a FEL facility that was solely dedicated to pulsed EPR.

The existing UCSB FELs were not designed with pulsed EPR in mind. A FEL designed specifically for pulsed EPR would provide significantly improved performance, as shown in Table 1. We would like to broaden the impact of the FEL-based EPR technology that continues to be developed at UCSB by building a new undulator that is specifically designed for EPR and upgrading UCSB’s electrostatic accelerator; establishing a mechanism that funds research by users outside UCSB; and helping to build new electrostatic accelerator- based FELs at other facilities.

Collaborators: Prof. Songi Han, Dept. of Chemistry and Biochemistry, UCSB Prof. Susumu Takahashi, Dept. of Chemistry, USC Dr. Louis-Claude Brunel, Institute for Terahertz Science and Technology, UCSB Dr. Hans van Tol, EMR group, National High Magnetic Field Laboratory Devin Edwards, Physics Department and ITST, UCSB.

Support: NSF IMR-MIP program, NSF MRI program, W. M Keck Foundation.

Preparations for a WHITE paper for BIG LIGHT. A Far infrared /THz light source based on a Free Electron Laser.

Introduction.

In the Netherlands, we are building at this moment a THz Free Electron Laser, called FLARE: spectral region 100 micron (3 THz) – 1500 micron (0.2 THz). Two types of specifications will be offered: a 10 Hz mode in which pulses have a length of 10 microsecond (macropulses) consisting of up to 3000 pulses (micropulses) per microsecond with a length of 10-100 picosecond. We believe that the coherent properties of our laser can be made such that the spectrum of the optical output has the form of a frequency comb with tooth separation of 3 GHz and a width of 1 MHz. Using a Fabry-Perot filter, the outpu will be transformed in a continuous 10 microsecond ling few 100 Watt power Thz beam with a 1 MHz bandwidth. Our FEL will be ‘small’ in the sense that we have not looked to the frontiers of technology. As examples worldwide show, the peak power and CW power can be very much higher (Jlab, Budker Institute), the average could have been higher (such as in Dresden, or by increasing our repetition rate). The technology is not limiting; the combination of finite resources and realistic expectations determine the limit.

In this short contribution, I want to describe three aspects connected to the success of the proposal of which FLARE is a part. These aspects are: the role of the facility in the success of the proposal, the importance of non-linear THz physics and finally, the role of serendipity in the possible future success of FLARE and the remainder of the facility.

Facility Instrumentation with a national and international character.

Around 2005, the Netherlands National Science Foundation realized that Netherlands scientists had ample choice to go abroad to use high quality instrumentation, but that the Netherlands had very little to offer to high quality researchers from abroad in spite of the fact that the Netherlands as an economy is larger than countries such as Sweden and Denmark together. A call for proposals followed and Nijmegen proposed:

- Creating a 45 Tesla Hybrid Magnet - Creating a THz source for spectroscopy with and without high magnetic fields - Creating a facility structure including the NMR instrumentation and the scanning probe instrumentation

Our proposal was probably selected by the following reasons:

- The central position in the proposal of our existing facility: the High Field Magnet Laboratory (HFML) - The promise of a quantum step in research, namely achieving “NMR resolution and flexibility” in EPR around 40 Tesla. - The filling of a frequency gap (THz radiation) with new instrumentation I do not believe that the explicit research plan that I proposed (to use THz radiation for bringing forward the field of (bio-)molecular physics) has been crucial in the success of the proposal. It may well be that the integration of our four directions (Magnets, NMR instrumentation, Scanning Probe Technology, and Laser spectroscopy) in an explicit spectroscopic facility has been appreciated.

The promise of a quantum step in research because of a new facility is in a way orthogonal on exploiting past scientific success stories with similar instrumentation to bring the strengths of a new facility.

Non-linear physics with THz radiation: a quantum experimental leap?

Free Electron Lasers are powerful instruments. The positive keywords are: peak power, bandwidth limited picoseconds pulses and ease of tunability. The negative keywords are: not table top, serious laboratory preparations (radiation protection and complex light distribution systems), single user.

The power should allow one to make the step to non-linear THz physics from purely linear spectroscopy. Following the example of the facility IR facility in Rijnhuizen with the FEL called FELIX (3 – 200 micron), a large step has been made by coupling (F)IR absorption to laser ionization (in ion-dip spectroscopy) or to fragmentation (in IRMPD-IR multi-photon dissociation) or to subtle changes as removal of a messenger rare gas atom. In all cases, the whole process is non-linear.

A second example of non-linearity is using “saturation while keeping quantum coherence”. This is an alternative wording for using Rai-cycles, π-pulses, π/2-pulses and variants thereof, in order to distinguish homogeneous effects from heterogeneous effects in absorption spectra. Clearly, the ambition with HFML to achieve such a possibility routinely, is both highly ambitious and highly rewarding when it will work.

Achieving non-linearity can be achieved in two ways. One can increase the power of the light but one also can enhance the susceptibility of the system under study. I have been involved in THz excitation studies of Rydberg atoms. Here many non-linear effects were directly obvious using already Table Top source of broad band THz pulses using femtosecond pulses in combination with biased GaAs wafer material. In the physics of systems with loosely bound electrons, the direct classical effect of an external field in the form of so-called electron quiver motion scales with ω-2, and hence is six orders higher at 1 THz than at visible wavelengths. The fact that at our low frequencies pulses become longer and hence the peak power smaller, takes away some of this enhancement.

I am convinced that multi-photon THz physics and chemistry will add to our understanding of materials and molecules.

The role of serendipity: too important to ne dependent on good plans based on past experience only.

Purcell made excuses when explaining his first NMR attempts as he did not recognize any useful applications. Although maybe not historic correct, it is clear that NMR/MRI techniques have become extremely important societal techniques. Serendipity is not automatically present. It requires talent to recognize the unusual amidst of the normal and the attitude to go for what is intriguing more than what is useful. The program within the IR facility in Rijnhuizen is dominated by experiments employing a serendipitous discovery. With only high power IR, C60 could be excited so efficiently, that electrons started to evaporate. The wavelengths where this phenomenon was detected resembles the linear absorption spectrum of C60. Nowadays, biomolecular ions are stored inside an FTICR mass spectrometer, illuminated by IR photons; the wavelengths at which molecular dissociation occurs is now routinely interpreted as a linear spectrum (in spite of the fact that many identical photons must be absorbed) and used for comparison with ab initio DFT calculations.

In short, I expect a novel instrument as FLARE (and hence as BIG LIGHT) to become known internationally not because of the success in reproducing the plans in white papers, but because of new fields of science that become accessible, initially unexpected and later on completely logic “after the fact”.

Wim J. van der Zande, Radboud University Nijmegen

In coherent (pulsed) EPR experiments, the general detection scheme is based on the creation of a component of the total net magnetization vector in the plane perpendicular to the external magnetic field. The (Larmor) procession of this magnetization around the magnetic field direction produces radiation that can be detected in a phase sensitive detection scheme. The number of spins necessary to make up a macroscopic magnetization component large enough to be detected is of the order of 1010 at conventional frequencies to about 107 at higher frequencies, and requires a detection scheme optimized for a specific frequency. Also it requires that the relaxation slow enough to allow for this net magnetization in this perpendicular field to exist for a sufficiently long time.

In order to increase the sensitivity of magnetic resonance experiments, one can use alternative methods, like squid detection of the longitudinal magnetization, force or torque detection, optical detection, or electrical detection. Many of these methods have shown single spin sensitivity, and all of them depend of the detection of changes of the magnetization component along the external field (longitudinal magnetization). The large spin polarizations obtained at high magnetic field can be an important advantage for many of these methods, however, applications of these methods at high fields have been limited, due to the absence of monochromatic (sub)THz sources with sufficient power to drive the energy level populations out of equilibrium. As an illustration of what can be achieved, we will focus on optical and electrical detection methods that have recently been employed in relation to quantum information, as they can be used as a sensitive spin read-out mechanism, or even as a spin manipulation mechanism. Also, these systems have relatively long relaxation times, making them accessible with relatively low-power sources. In experiments on donors in silicon, the magnetic field has proven to be an important factor in the spin-dependent resistance, and large nuclear polarizations and long (electrically-detected) coherence times can be achieved. Recent experiments with the high power Free Electron Laser source at UCSB show a very large spin-dependent resistance in these silicon systems.

As a general outlook, these methods allow a detailed study of spin-dependent resistivity and/or spin- dependent luminescence/absorption in many condensed matter systems, as well as a field dependence of the spin-lattice couplings. One can think of studying the interplay of magnetic order and resistivity in manganites, organic conductors, superconductors; the study of recombination processes in organic solar cells, or organic LEDs; the study of conducting thin films and graphene, etc. The measurement of the response of a system when tickling it with (in)coherent spin excitations hold promise!

The extracellular matrix

High performance accelerator based light sources are needed to study a very important problem in medical and biological science: the structure and function of the extracellular matrix (ECM). This is probably the most extreme combination of importance and ignorance in the whole of medicine. The ECM consists of molecular assemblies of proteins and polysaccharides (glycosaminoglycans) located on the outside of cells. There are an enormous number of different species, some in very low concentrations, that self assemble and interact on length scales of 3 to 103 nm and over timescales of 10-15 to 103 seconds. They are key regulators of cell function, and hence organ and organism function.

The central problem is how does the structure of glycosaminoglycans drive their functional interactions with other molecules of the ECM and the cell surface to regulate cell activity? This question is relevant to medical research on cancer, neurodegeneration (Alzheimer’s and PrP in CJD), inflammation, congenital disorders and pathogens such as HIV, herpes, malaria, and chlamydia.

High intensity light sources are needed because although the subject is tractable by a number of standard techniques such circular dichroism, infrared spectroscopy, 2D IR and a variety of pump probe techniques, particularly involving terahertz radiation, the experiments need to be done on fast timescales and at very low concentrations: µg/ml. Table top instruments can be used to study concentrations of 100 mg/ml, but this chemistry not biology.

Peter Weightman April 2011 Low frequency electronic and vibrational dynamical coupling Gwyn P. Williams, Jefferson Lab, representing the JLAB FEL Team High power/brightness IR and THz light are invaluable tools for studies of dynamical coupling in gases, solids and surfaces embracing fields of physics, chemistry and biology, relevant to energy transfer and environmental issues, and extending to properties of nano and other novel materials. Specific area 1: Surfaces Low frequency vibrational dynamics at metal surfaces reveals unusual electronic coupling which can be attributed to a breakdown of the Born‐Oppenheimer approximation. Absolute sources of IR and far‐IR, such as are produced by accelerators, are essential. Understanding the mechanisms provides insight into the chemistry of interfaces, catalysis, and the functional behavior of nano‐particles (Hirschmugl, U. Wisconsin, Milwaukee). Specific area 2: Solids/liquids Intramolecular dynamical coupling, plays a critical role in energy transfer, and in materials preparation. For example the rapid and unexpected thermalization of resonantly‐excited large molecules, allows novel films to be prepared. (Haglund, Vanderbilt). Specific area 3: THz/IR dynamical coupling (double resonance). Controlled far‐IR intramolecular coupling with JLab high power THz light can be measured via a double resonance technique (Rutt, Southampton U.), and can be extended to cover intermolecular modes. In biology various postulates concern THz induced collective and other behavior (Weightman, Liverpool U.). Specific area 4: Extreme conditions. High pressure and high magnetic fields are important thermodynamical variables. Studies of frequency‐dependent conductivity are possible but require high brightness beams. Studies are planned on materials at high pressure in which the vibrational modes are modified and eventually frozen out to reveal distinctive novel physical properties such as metallic hydrogen (Goncharov, Carnegie Geophysical Institution).