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Near-Field Infrared Vibrational Dynamics and Tip-Enhanced Decoherence Xiaoji G

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Near-Field Infrared Vibrational Dynamics and Tip-Enhanced Decoherence Xiaoji G Letter pubs.acs.org/NanoLett Near-Field Infrared Vibrational Dynamics and Tip-Enhanced Decoherence Xiaoji G. Xu and Markus B. Raschke* Department of Physics, Department of Chemistry, and JILA, University of Colorado, Boulder, Colorado 80309, United States *S Supporting Information ABSTRACT: Ultrafast infrared spectroscopy can reveal the dynamics of vibrational excitations in matter. In its conventional far-field implementation, however, it provides only limited insight into nanoscale sample volumes due to insufficient spatial resolution and sensitivity. Here, we combine scattering- scanning near-field optical microscopy (s-SNOM) with femtosecond infrared vibrational spectroscopy to characterize the coherent vibrational dynamics of a nanoscopic ensemble of C−F vibrational oscillators of polytetrafluoroethylene (PTFE). The near-field mode transfer between the induced vibrational molecular coherence and the metallic scanning probe tip gives rise to a tip-mediated radiative IR emission of the vibrational free-induction decay (FID). By increasing the tip− sample coupling, we can enhance the vibrational dephasing of the induced coherent vibrational polarization and associated IR NF ≃ fi FF ≃ emission, with dephasing times up to T2 370 fs in competition against the intrinsic far- eld lifetime of T2 680 fs as dominated by nonradiative damping. Near-field antenna-coupling thus provides for a new way to modify vibrational decoherence. This approach of ultrafast s-SNOM enables the investigation of spatiotemporal dynamics and correlations with nanometer spatial and femtosecond temporal resolution. KEYWORDS: Near-field microscopy, ultrafast infrared spectroscopy, free-induction decay, nano-optics he aim of ultrafast spectroscopy is to characterize electron In this work, we combine scattering-scanning near-field T dynamics, nuclear motions, and their coupling induced by optical microscopy (s-SNOM) with femtosecond infrared transient optical excitations in molecules and condensed excitation for ultrafast coherent molecular vibrational nano- matter.1,2 The extension of ultrafast spectroscopy to two- and spectroscopy. Mid-IR femtosecond pulses have already been higher-dimensional wavemixing techniques enables the study of applied for their broad bandwidth in s-SNOM vibrational IR 14−18 intra- and intermolecular coupling in the presence of structural spectroscopy. Building on these results, we demonstrate inhomogeneities and disorder.3,4 While the established the extension of s-SNOM to time-domain IR vibrational nanospectroscopy. The metallic scanning probe tip localizes the technique of photon echo spectroscopy delivers information fi fi about the dynamics of the homogeneous subensemble, it infrared eld to the nanoscale for a spatially con ned molecular provides no insight into the underlying spatial distribution and excitation. The tip in turn acquires the transient coherent vibrational polarization through near-field coupling. This leads correlations of the inhomogeneities.5 The combination of Downloaded via UNIV OF COLORADO BOULDER on January 7, 2021 at 18:43:10 (UTC). to radiative infrared emission through the optical antenna ultrafast spectroscopy with microscopy, however, has remained See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. 6−9 properties of the tip. Using interferometric heterodyne challenging because of dispersion and aberration of the amplification, we detect the associated coherent emission of − microscope optics, low signal levels, and limited k-vector the vibrational free-induction decay (FID)19 21 of a nanoscale control. Progress has been made to extend ultrafast microscopy ensemble of molecular oscillators in a spatially confined volume fi beyond the optical far- eld spatial resolution limit using of 10−20 nm radius. Increasing the near-field dipole−dipole ff photoelectron emission, electron di raction, and X-ray spec- coupling between resonant molecular excitation and the tip 10−12 troscopy. However, the general applicability of these nano-antenna, we can increase the mode transfer between emergent techniques in particular to molecular and soft matter molecular and tip transient polarizations and therefore enhance systems is still lacking, because they require special sample the vibrational decoherence rate of the molecular excitation. preparation and are instrumentally demanding. The approach provides for a new way to modify vibrational Mid-infrared radiation couples directly to nuclear motions via coherence by antenna coupling and extends ultrafast IR IR-active vibrational modes in molecules or optical phonons in spectroscopy to the nanoscale for spatiotemporal vibrational solids. In ultrafast infrared spectroscopy, structural dynamics imaging on femtosecond time and nanometer length scales. can be probedinformation otherwise not always accessible by 13 continuous wave spectroscopies. However, because of its long Received: December 31, 2012 wavelength, the extension of IR spectroscopy into the micro- Revised: January 31, 2013 and nanoscale is particularly challenging. Published: February 6, 2013 © 2013 American Chemical Society 1588 dx.doi.org/10.1021/nl304804p | Nano Lett. 2013, 13, 1588−1595 Nano Letters Letter Figure 1. Femtosecond infrared vibrational scattering-scanning near-field optical microscopy (s-SNOM). Experimental setup (a). Interferogram of the 220 fs mid-IR laser pulse (b), with Fourier transform of the interferogram (c). Wavenumber dependence of refractive index n and absorption ffi − ν −1 ν −1 24 coe cient k of PTFE (d) are characterized by C F symmetric and antisymmetric modes at 1 = 1160 cm and 2 = 1220 cm , respectively. Experiment. A schematics of our experiment is shown in dipole moment of the C−F bond and high oscillator density. Figure 1a. Mid-infrared femtosecond radiation, tunable from 5 Figure 1d shows the complex refractive index ñ = n + ik of to 10 μm, with an average power of ∼1 mW, pulse duration of PTFE obtained from the literature.24 The two dominant ∼ ∼ − −1 ν −1 220 fs, and bandwidth of 50 100 cm (fwhm) is obtained vibrational modes are 1 = 1160 cm (symmetric stretch) and ff 22 ν −1 25 − by di erence frequency generation in GaSe, of the signal and 2 = 1220 cm (antisymmetric stretch). For enhanced tip idler output of an optical parametric oscillator (OPO, sample coupling efficiency, we evaporate a gold film with a Chameleon, APE, Germany) pumped by a high power thickness gradient up to a few tens of nanometers onto the femtosecond Ti:S laser (MIRA, Coherent, 3.5 W). The IR PTFE samples, forming isolated gold clusters at low coverage beam is collimated to a diameter of ∼10 mm, passes through a that percolate with increasing nominal film thickness. beam splitter (BS), and is focused by a parabolic mirror (focal Results. The measurement and analysis of the FID length f = 25 mm, effective NA = 0.25, spot size diameter ∼20 interferograms of time-domain s-SNOM directly allow for IR μm) onto the scanning probe tip of an atomic force microscope vibrational nanospectroscopy.18 As an example, Figure 2a (blue (AFM, Innova, Bruker Inc.) modified for s-SNOM. The AFM is trace) shows an s-SNOM interferogram detected at the second- operated in dynamic noncontact mode, and platinum coated harmonic (2Ω) of the cantilever oscillation with the tip AFM probes (Arrow NCPT, Nanosensors) are used for the engaged in noncontact mode feedback with the PTFE sample. experiments. The tip cantilever oscillates at a frequency of Ω ≈ As a reference, we use the spectrally broad-band and 254 kHz with an estimated tapping amplitude of 25 nm. nonresonant near-field response from a gold surface. The The IR light is p-polarized with respect to the tip shaft. The envelope of its interferogram acquired under otherwise tip-scattered light is collected in epi-detection, interferometri- identical experimental conditions is shown in part a (dashed cally heterodyned23 with the reference field with variable time red). A clear resonant near-field FID behavior is discerned. The ν delay through a dispersion compensation plate (CP), and FID trace is dominated by 1, with a weaker signal contribution ν detected by an MCT detector (J15D12 M204 Judson). A lock- from 2, as seen from the Fourier transform in Figure 2b. The in amplifier (HF2Li, Zurich Instruments, Switzerland) is used vibrational levels are indicated by blue dashed lines (for for signal demodulation with the reference frequency set to the corresponding results with the central wavelength tuned toward Ω Ω ν tip oscillation frequency . Multiple harmonics (n , n =1,2,3, 2, see the Supporting Information, Figure S1). The beating ...) of the optical signal are simultaneously recorded while observed in the FID interferogram is due to the interference of scanning the reference mirror position for heterodyne record- these two modes with each other and with the nonresonant ing of the interferograms. signal contributions. Figure 1b shows the interferometric linear autocorrelation of As opposed to conventional symmetric Fourier transform a typical mid-IR pulse, corresponding to a duration of ∼220 fs. infrared (FTIR) spectroscopy interferograms, the Fourier Figure 1c shows the Fourier transform of that pulse with a transform of the asymmetric FID interferograms directly central wavenumber of 1170 cm−1 and a fwhm bandwidth of 70 provides the spectral amplitude and phase of the different cm−1. In our linear optical experiment, the interpretation of the vibrational modes.18 The resulting normalized spectral ultrafast dynamics is based on the assumption that
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