SFG Spectrometers Sum Frequency Generation Vibrational SPECTROSCOPY SYSTEMS SFG spectrometers

Sum Frequency Generation Vibrational Spectroscopy

Sum Frequency Generation vibrational modes of molecules ADVANTAGES Vibrational Spectroscopy (SFG-VS) at the interface give a resonant ▶ Intrinsically surface specific is powerful and versatile method for contribution to SF signal. The resonant Selective to adsorbed species in-situ investigation of surfaces and enhancement provides spectral ▶ Sensitive to submonolayer of interfaces. In SFG-VS experiment a information on surface characteristic ▶ molecules pulsed tunable infrared IR (ωIR) laser vibrational transitions. beam is mixed with a visible VIS ▶ Applicable to all interfaces accessible to light (ωVIS) beam to produce an output Z IR at the sum frequency (ωSFG = ωIR + w wSFG ▶ Nondestructive ωVIS). SFG is second order nonlinear wVIS ▶ Capable of high spectral and process, which is allowed only in spatial resolution media without inversion symmetry. At surfaces or interfaces inversion X symmetry is necessarily broken, that makes SFG highly surface specific. As the IR wavelength is scanned, active APPLICATIONS

▶ Investigation of surfaces and interfaces of solids, liquids, polymers, biological membranes SPECTRA EXAMPLES and other systems

▶ Studies of surface structure, 0.4 1.0 chemical composition and molecular orientation 0.3 0.8 ▶ Remote sensing in hostile environment

0.2 0.6 ▶ Investigation of surface reactions under real atmosphere, catalysis,

SFG signal, a.u.

SFG signal, a.u. 0.1 0.4 surface dynamics

▶ Studies of epitaxial growth, 0 0.2 electrochemistry, material and environmental problems 1700 1710 1720 1730 1740 1750 1760 2800 3000 3200 3400 3600 3800 4000 Wavelength, cm-1 Wavelength, cm-1

SFG spectra of monoolein surface, 1 cm-1 Water-air interface spectra, scan step, 200 acquisitions per step. 200 acquisitions per step. Courtesy of University of Michigan

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Design of the SFG Spectrometer

beam dedicated for VIS channel SYSTEM COMPONENTS passes through OPG compartment ▶ Picosecond mode-locked only to minimize the risk of accident Nd:YAG laser with dangerous high intensity laser Multichannel beam delivery unit radiation. It makes Ekspla spectrometer ▶ substantially safer comparing to ▶ Picosecond optical parametric home-made SFG-VS setups. Also generator optical parameters, like beam diameter, ▶ Spectroscopy module pulse energy, delays between channels Monochromator are perfectly matched. Motorised ▶ PMT based signal detectors switch of IR Polarisation, motorisation ▶

Sum frequency generation (SFG) of delay line are included in standard ▶ Data acquisition system spectrometer is based on picosecond configuration. ® ▶ Dedicated LabView software pump laser and optical parametric We designed our spectrometer package for system control generator (OPG) with difference thinking about user friendly operation. frequency generation (DFG) extension. Many components of the system Solid state mode-locked Nd:YAG laser are automated and controlled from featuring high pulse duration and PC. The opto-mechanical holders energy stability is used in the system. that need to be tuned often during Fundamental laser radiation splits routine operation are located around into several channels in multichannel sample area and can be easily recording, dynamics monitoring, X-Y beams delivery unit. Two of these accessed without walking around the sample mapping, azimuthal scan and beams are used for pumping OPG optical table. According to user needs system parameters monitoring. and DFG. Small part of laser output different level of automation can be Ekspla offers three common SFG beam, usually with doubled frequency proposed, starting from most simple spectrometer models for classical (532 nm), is directed to VIS channel mechanical setup to most advanced picosecond scanning SFG vibrational of SFG spectrometer. IR channel of fully motorized version. spectroscopy and several specialized spectrometer is pumped by DFG Detection system consists of filter models for most demanding users. output beam. with high stray light rejection and Basic models are: SFG Classic, SFG The sizes of individual compartments, gated PMT based SF signal detector. Advanced and Double resonance positions of apertures and beams The feature of such design is ability SFG, Phase sensitive. They differ by heights are fitted. As a result SFG to perform measurements in room IR beam tuning ranges and available spectrometer takes less space in lighting. Second parallel detection VIS beam wavelengths (please see laboratory. Standard versions usually channel is available as an option. All specifications page). Other models: fit on 1000×2400 mm optical table. system components are controlled Classic + Phase-sensitive SFG and SFG All beams among laser, harmonics from single dedicated software. microscope provides unique features, module, parametric generator, SFG Program contains many useful which are described on next pages of box are enclosed in tubes. For example instruments for automatic SFG spectra this brochure.

1064 nm PL2231-50 SFGH500-2H PM

532 nm PG501 DFG1 T

532 nm IR 2.3 – 10 µm SFG-V532

1064 nm VIS 532 nm

Schematic layout of SFG Classic spectrometer.

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SFG Spectrometer Modifications and Options

DOUBLE RESONANCE MODEL MODIFICATIONS Double resonance SFG Both IR and VIS wavelengths are mode coupling to electron states at a ▶ spectrometer – allows tunable in Double resonance SFG surface. investigation of vibrational spectrometer model. This two- Double resonance enables the use mode coupling to electron dimensional spectroscopy is more of another wavelength for VIS beam states at a surface selective than single resonant SFG if the sample has strong absorption Phase sensitive SFG and applicable even to media with at 532 nm and 1064 nm. Two ▶ spectrometer – allows strong fluorescence. Double resonant outputs PL2230 laser is used for this measurement of the complex SFG allows investigation of vibrational spectrometer. spectra of surface nonlinear response coefficients

OPTIONS

▶ Single or double wavelength VIS beam: 532 nm and/or 1064 nm

▶ One or two detection channels: main signal and reference

▶ Second harmonic generation surface spectroscopy option

▶ High resolution option – down to 2 cm-1

▶ Motorized VIS and IR beams alignment system

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PHASE-SENSITIVE SFG SPECTROMETER

Phase sensitive measurements optical wave requires an interference all three beams are retranslated to with spectral resolution scheme. Mixing the wave of interest the second sample, where another up to 6 cm⁻¹ (2 cm⁻¹) with a reference wave of known SFG beam is generated. Due to In conventional SFG-VS intensity of SF phase generates an interference electromagnetic waves coherence signal is measured. It is proportional to pattern, from which the phase of both SFG beam are interfering. Setup the square of second order nonlinear the wave can be deduced. contains the phase modular located (2) 2 In practice Phase-sensitive SFG on the SFG beam path between susceptibility ISF ~ | χ | . However, χ(2) is complex, and for complete experimental setup includes two samples. We are able to change the information, we need to know both samples generating SF signal phase of SFG beam by rotating it. This the amplitude and the phase. This will simultaneously. One sample (usually way we are recording two-dimensional allow us to determine the absolute called local oscillator) has well known interfererogram with wavelength and direction in which the bonds are and flat spectral response. Second one phase shift on x and y axis. Using pointing and characterize their tilt is investigated sample. The excitation fitting algorithms we are able to angle with respect to the surface. beams are directed to first sample, calculate the amplitude and phase of Measurement of the phase of an where SFG beam is generated. Later SF signal.

Spectroscopy module of Phase-sensitive SFG spectrometer: internal view (left) and 3D drawing (right).

Phase-sensitive SFG spectrometer software window showing interferograms of AZO (azophenylcarbazole) dyes on the Au surface and fitted SF spectra with amplitude and phase distinguished.

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PHASE SENSITIVE SFG + CLASSIC SFG SPECTROMETER IN ONE UNIT

Interference measurements of SFG signals from reference sample and the OPTIONS investigated sample for Phase-sensitive configuration. Spectrometer has "classic" and "Phase-sensitive" properties:

▶ Easy switching between setups

▶ Adjustable spot size for classic configuration

(“Classic”) configuration

Classic + Phase sensitive versions in one unit

Switchable setup. Phase sensitiv / “Classic” (“Advanced”) ; Top/ Bottom configuration. Switch: VIS beam manual. IR mirrors motorised, BaF₂ lens manual. Path length to the sample is same in all configuration Tunable beam size for IR beam. Motorised polarisation control. Beams are Focused with Lens. (BaF₂ lens for IR beam). VIS beam 532 nm. IR 2,3- up to 10 (16) µm. “Classic” configuration. IR 2.3-10 µm (up to 16 µm).

Phase sensitive, Top (Reflection- Reflection) configuration Phase sensitive, Transmission-reflection configuration

Fixed beams sizes on the sample. VIS and IR beams. Fixed beams sizes on the sample. VIS and IR beams. Beams are Focused with Parabolic mirrors. Beams are Focused with Parabolic mirrors. Interference configuration for Phase measurement. Interference configuration for Phase measurement. IR 2.3-10 µm. IR 2.3-3,5 µm .

NARROWBAND SFG SYSTEM <2 cm-1 0.4 Spectral resolution in of narrowband SFG is determined by light source – OPA.

Monochromator is used only as filter. 0.3 Light source for IR: PG511. Line width of mid-IR < 2 cm-1.

Synchronously pumped optical parametric generator with OPO with long focal 0.2 length resonator.

SFG signal, a.u. 0.1

0

SFG spectra of monoolein surface, 1700 1710 1720 1730 1740 1750 1760 1 cm-1 scan step, 200 acquisitions per step. Wavelength, cm-1

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Main System Components

PICOSECOND MODE-LOCKED Nd:YAG LASER

The heart of the spectrometer is minimal maintenance requirements. solid-state picosecond laser. Its This model provides up to 40 mJ per reliability is critical to perfect pulse output energy, which in most spectrometer operation and relevance cases is enough for pumping OPG and of measured data. Two standard VIS channel of SFG spectrometer. models of high energy are Model PL2230 is available for dedicated for SFG spectrometers. double resonance SFG. This model Model PL2230 is fully diode pumped, usually is used for pumping of which means that master oscillator two independent OPG’s. Such and all following amplification stages configuration is used in double are diode pumped. It features great resonance SFG version. long term parameters stability and

MULTICHANNEL BEAMS DELIVERY UNIT

Fundamental laser radiation needs wavelength (1064 nm) or tunable in to be split into several channels and visible range, when second OPG is converted to different wavelenghts. used. Tunable IR radiation is generated Multichannel beams delivery unit in picosecond optical parametric SFGHX00 series provides all these generator (OPG). Large portion of laser features. Additionally it contains output is converted into second or automatized VIS channel input energy third harmonics and used for monitoring and control. OPG pumping. Residual beam is The VIS channel wavelength (if spatially filtered, delayed and directed double wavelenght option is included) into SFG spectrometer as VIS channel. is changed manually. Setup also Usually it is converted into second includes all needed separators and harmonic (532 nm), but in some cases filters to block residual radiation and can be used also at fundamental prevent it from reaching a sample.

TO SFG SPECTROMETER AS VIS CHANNEL

TO PG401 TO PG501 TO DFG

355 nm

1064 nm

532 nm 1064 nm 532 nm or/and

FROM LASER 1064 nm Multichannel beam delivery unit SFGH1000

An example of Multichannel beams delivery unit used for Double resonance SFG spectrometer.

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PICOSECOND OPTICAL PARAMETRIC GENERATOR

PG501 series picosecond optical sample. Visible laser pointer is installed parametric generator (OPG) feature inside each unit and aligned in-line high pulse energy and narrow with IR beam. It helps to manage linewidth. It is used for generation of invisible mid IR radiation and direct it tunable wavelength in broad spectral through multiple optical elements into range. In SFG spectrometer it provides a sample. middle infrared radiation for IR Some SFG-VS studies require better channel. than 6 cm-1 spectral resolution. In such cases Ekspla offers unique design DFG stage extends tuning range PG511 series OPG. In this system seed to mid IR, which corresponds to is generated in synchronously pumped molecular vibrational fingerprints. optical parametric oscillator (SPOPO), Depending of OPG model, DFG output which is temporally synchronized with can cover spectral range 2.3 – 10 µm or laser regenerative amplifier. In this 2.3 – 16 µm. All residual wavelengths configuration radiation spectral width are carefully filtered preventing is narrowed down to 2 cm-1 in mid IR residual radiation from reaching a range.

3-1 -1 Wavenumber, 10 × cm Wavenumber, cm 5.00 3.33 2.50 2.00 1.66 1.43 1.25 1.11 1.00 1250.0 1000.0 833.3 714.2 625.0 350 120 –DFG2 300 –DFG1P or DFG2 option 100

, µJ 250

, µJ gy 80 200 gy

150 60

Pulse ener

100 Pulse ener 40

50 20 Air absorption –DFG1 option 0 0 2000 3000 4000 5000 6000 7000 8000 9000 10000 8000 10000 12000 14000 16000 Wavelength, nm Wavelength, nm

Typical PG501-DFGx output energy curve in 2 300 – 10 000 nm and in 8 000 – 16 000 nm.

APPLICATION EXAMPLE: TWO PG511 FOR SFG PUMP PROBE

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However, in some experiments one The special design of SFG spectrometer Sample surface and beams overlap layer of the sample can be transparent provides possibility to change can be monitored using camera only for VIS beam, but not for IR angles of interaction. This feature installed above sample area. This beam and vice versa. In such case together with different polarization utility is integrated into every SFG experimental setup requires different combinations helps better understand spectrometer. On a special request geometries. This problem can be molecular dipoles orientation. In our sample visualization system can be solved, if we can access interface from spectrometer we use large aperture combined with motorized beams different sides, for example directing parabolic mirror. The sample is adjustment. This allows to align SFG VIS beam from the top and IR beam places in focal point of parabolic spectrometer from PC, even being from the bottom. Ekspla offers several mirror. Such solution makes optical physically far from it. It essentially standard geometries: top side, bottom system extremely simple in operation, solves safety issues and opens new side, top-bottom side and total because it guarantee the same beams possibilities for multiple long time internal reflection. All of them can be position on the sample surface and experiments without accessing implemented in single spectroscopy perfect overlap, when incidence angle spectroscopy box. unit and easy interchangeable. is changed.

SFG Spectrometer Accessories

OPTIONAL ACCESSORIES

▶ Six axis sample holder

▶ Sealed temperature controlled sample chamber

▶ Larger sample area – space for Langmuir trough

▶ Motorisation of polarisation control of VIS and IR beams, SFG spectrometer with Langmuir polarisation analyser for SFG trough used for studies of the unique signal properties of molecules in monolayers.

Hermetically sealed sample cell with heater, specially designed for SFG Compact and stable six axis spectrometer, allows experiments manipulator for precise sample under controlled environmental positioning. conditions.

More detailed information about accessories for SFG spectrometers will be available soon. Please check Ekspla website: http://www.ekspla.com/product/sfg-spectrometer

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Technical Specifications 1)

Version SFG Classic SFG Advanced SFG Double resonance SFG Phase sensitive

SYSTEM (GENERAL) Spectral range 1000 – 4300 cm-1 625 – 4300 cm-1 1000 – 4300 cm-1 1000 – 4300 cm-1 <6 cm-1 <6 cm-1 <6 cm-1 Spectral resolution <10 cm-1 (optional <2 cm-1) (optional <2 cm-1) (optional <2 cm-1) Spectra acquisition method Scanning Sample illumination geometry Top side, reflection (optional: bottom side, top-bottom side ) Incidence beams geometry Co-propagating, non-colinear Incidence angles Fixed, VIS ~60 °, IR ~55 ° (optional: tunable) Fixed, VIS ~60 °, IR ~55 ° 532 nm and 532 nm 532 nm VIS beam wavelength tunable 420 – 680 nm 532 nm (optional: 1064 nm) (optional: 1064 nm) (optional: 210 – 680 nm) Polarization (VIS, IR, SFG) Linear, selectable “s” or “p”, purity > 1:100 IR Beam spot on the sample Selectable, ~150 – 600 µm Fixed Sensitivity Air-water spectra Solid sample

PUMP LASERS 2) Model PL2231-50 PL2231-50 PL2231A PL2231-50 Pulse energy Optimised to pump PG Pulse duration 29 ± 4 ps Pulse repetition rate 50 Hz

OPTICAL PARAMETRIC GENERATORS IR source with standard PG501-DFG1P PG501-DFG2 PG501-DFG1P linewidth (<6 cm-1) IR source with narrow PG511-DFG PG511-DFG2 inquire PG511-DFG linewidth (<2 cm-1) UV-VIS source for PG401 – – Double resonance SFG (optional: PG401-SH) For standard specifications please check the brochure of particular model.

PHYSICAL DIMENSIONS (FOOTPRINT) Standard 2500 × 1200 mm 3000 × 1500 mm 2600 × 1200 mm Extended (with special options 2700 × 1200 mm 3000 × 1500 mm 2700 × 1200 mm or large ccessories)

1) Due to continuous improvement, all specifications are subject to change without advance notice. 2) Laser is optimised for pumping parametrical generator, maximum output energy may be different than specified for stand alone application.

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Revision – 20210630