SFG Spectrometer

The SFG Spectrometer is an ideal laser spectroscopy tool for in-situ investigation of surfaces and interfaces. The system operates from 4300 to 625 cm-1 and provides < 6 cm-1 spectral resolution. The heart of the system is a picosecond Nd:YAG laser generating 25 ps pulses that pump an OPG/DFG delivering up to 300 µJ per pulse in the IR range

Description 

Features and Advantages

  • Characterization of vibrational bonds of molecules at surfaces or interfaces
  • Intrinsically surface specific
  • Selective to adsorbed species
  • Sensitive to submonolayer of molecules
  • Applicable to all interfaces accessible to light
  • Nondestructive
  • Capable of high spectral and spatial resolution

Applications

  • Investigation of surfaces and interfaces of solids, liquids, polymers, biological membranes and other systems
  • Studies of surface structure, chemical composition and molecular orientation
  • Remote sensing in hostile environment
  • Investigation of surface reactions under real atmosphere, catalysis, surface dynamics
  • Studies of epitaxial growth, electrochemistry, material and environmental problems

Principle of SFG spectroscopy

Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) is powerful and versatile method for in-situ investigation of surfaces and interfaces. In SFG-VS experiment a pulsed tunable infrared IR (ωIR) laser beam is mixed with a visible VIS (ωVIS) beam to produce an output at the sum frequency (ωSFG = ωIR + ωVIS). SFG is second order nonlinear process, which is allowed only in media without inversion symmetry. At surfaces or interfaces inversion symmetry is necessarily broken, that makes SFG highly surface specific. As the IR wavelength is scanned, active vibrational modes of molecules at the interface give a resonant contribution to SF signal. The resonant enhancement provides spectral information on surface characteristic vibrational transitions.

Design of the SFG Spectrometer


Sum frequency generation (SFG) spectrometer is based on picosecond pump laser and optical parametric generator (OPG) with difference frequency generation (DFG) extension. Solid state mode-locked Nd:YAG laser featuring high pulse duration and energy stability is used in the system. Fundamental laser radiation splits into several channels in multichannel beams delivery unit. Two of these beams are used for pumping OPG and DFG. Small part of laser output beam, usually with doubled frequency (532 nm), is directed to VIS channel of SFG spectrometer. IR channel of spectrometer is pumped by DFG output beam.
The sizes of individual compartments, positions of apertures and beams heights are fitted. As a result SFG spectrometer takes less space in laboratory. Standard versions usually fit on 1000×2400 mm optical table. All beams among laser, harmonics module, parametric generator, SFG box are enclosed in tubes. For example beam dedicated for VIS channel passes through OPG compartment only to minimize the risk of accident with dangerous high intensity laser radiation. It makes SFG spectrometer substantially safer comparing to home-made SFG-VS setups. Also optical parameters, like beam diameter, pulse energy, delays between channels are perfectly matched. Motorised switch of IR Polarisation, motorisation of delay line are included in standard configuration.
The spectrometer is designed towards user friendly operation. Many components of the system are automated and controlled from PC. The opto-mechanical holders that need to be tuned often during routine operation are located around sample area and can be easily accessed without walking around the optical table. According to user needs different level of automation can be proposed, starting from most simple mechanical setup to most advanced fully motorized version.
Detection system consists of monochromator with high stray light rejection and gated PMT based SF signal detector. The feature of such design is ability to perform measurements in room lighting. Second parallel detection channel is available as an option. All system components are controlled from single dedicated software. Program contains many useful instruments for automatic SFG spectra recording, dynamics monitoring, X-Y sample mapping, azimuthal scan and system parameters monitoring.
TOPAG offers three common SFG spectrometer models from Ekspla for classical picosecond scanning SFG vibrational spectroscopy and several specialized models for most demanding users. Basic models are SFG Classic, SFG Advanced, Double Resonance SFG and Phase Sensitive SFG. They differ by IR beam tuning ranges and available VIS beam wavelengths (please see specifications page). The Classic and Phase-sensitive SFG models can be combined in one spectrometer. Further SFG microscope provides unique ability to investigate spatial variations across a sample surface at micrometer resolution as a function of time.

System Components

  • Picosecond mode-locked Nd:YAG laser
  • Multichannel beam delivery unit
  • Picosecond optical parametric generator
  • Spectroscopy module
  • Monochromator
  • PMT based signal detectors
  • Data acquisition system
  • Dedicated LabView® software package for system control

Classic and Advanced SFG spectrometer

Classic configuration of SFG spectrometer covers tunable IR laser range from 4300cm-1 to 1000cm-1 (10µm). Using additional DFG crystal in Advanced SFG spectrometer the detection range can be extended to longer wavelengths until 625cm-1 (16µm)

Phase-sensitive SFG spectrometer

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

Combination of Phase sensitive and Classic SFG Spectrometer

This combinations allows a switchable setup between phase sensitive and Classic/Advanced SFG spectrometer in top/ bottom configuration. Switch for VIS beam is manually. IR mirrors are motorised, BaF₂ lens manually. Path length to the sample is the same in all configuration. Motorised polarisation control can be used.

Double Resonance SFG Spectrometer

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

Narrow Band SFG Spectrometer

The spectral resolution in of narrowband SFG is determined by light source, which is a ps YAG laser pumped OPA with DFG extension. The monochromator is used only as filter. Using a narrowband OPA model PG511 line width of mid-IR can be reduced to < 2 cm-1. This tunable laser is a synchronously pumped optical parametric generator with OPO with long focal length resonator.

SFG Microscope

SFG spectroscopy combined with high spatial resolution in micrometer range provides a unique ability to investigate spatial and chemical variations across the surface as a function of time. An example of such application is chemical imaging of corrosion. SFG microscopy reveals presence of highly-coordinated complexes of molecules at particular stage of this process.
SFG microscope spectrometer uses far-field image formation technique. Illuminated area on the sample surface is substantially bigger than in regular SFG spectrometer. Using blazed grating and unique design optical system, image of surface plane is translated to matrix of ICCD camera. This way we can record distribution of SF signal at particular wavelength. For complete spectral and spatial information it is necessary to record multiple surface pictures at different wavelength. Integrated software package provides ability to visualize measured data making various cross sections: position-, wavelength- or time-dependent.

Specifications 
Model 1) SFG Classic SFG Advanced SFG Double resonance SFG Phase Sensitive
Spectral range 1000- 4300 cm-1 625- 4300 cm-1 1000- 4300 cm-1
Spectral resolution <6 cm-1(optional <2 cm-1) <6 cm-1(optional <2 cm-1) <10 cm-1 <6 cm-1(optional <2 cm-1)
Spectra acquisition method Scanning
Sample illumination geometry Top side, reflection (optional: bottom side, top-bottom side, total internal reflection)
Incidence beams geometry Co-propagating, non-colinear (optional: colinear) non-collinear
Incidence angles Fixed, VIS ~60 deg, IR ~55 deg (optional: tunable) not tunable
VIS beam wavelength 532 nm (optional: 1064 nm) Tunable 420 – 680 nm (optional: 210 – 680 nm) 532 nm
Polarization (VIS, IR, SFG) linear, selectable “s” or “p”, purity > 1:100
Beam spot on the sample selectable, ~150 – 600 µm fixed
Sensitivity air-water spectra solid sample
Pump lasers PL2230
Optical parametric generators
IR source with standard linewidth (<6 cm-1) PG501-DFG1P PG501-DFG2 PG501-DFG1P
IR source with narrow linewidth (<2 cm-1) PG511-DFG PG511-DFG2 - PG511-DFG
UV-VIS source for Double resonance SFG - - PG401 (optional: PG401-SH) -

1) Due to continuous product improvements, specifications are subject to changes without advance notice.

Options 

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

Optional Accessories

  • Six axis sample holder
  • Motorization of different axis
  • Sealed temperature controlled sample chamber
  • Langmuir trough
  • Motorization of polarization central of VIS beam and polarization analyzer

 

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Features and Advantages

  • Characterization of vibrational bonds of molecules at surfaces or interfaces
  • Intrinsically surface specific
  • Selective to adsorbed species
  • Sensitive to submonolayer of molecules
  • Applicable to all interfaces accessible to light
  • Nondestructive
  • Capable of high spectral and spatial resolution

Applications

  • Investigation of surfaces and interfaces of solids, liquids, polymers, biological membranes and other systems
  • Studies of surface structure, chemical composition and molecular orientation
  • Remote sensing in hostile environment
  • Investigation of surface reactions under real atmosphere, catalysis, surface dynamics
  • Studies of epitaxial growth, electrochemistry, material and environmental problems

Principle of SFG spectroscopy

Sum Frequency Generation Vibrational Spectroscopy (SFG-VS) is powerful and versatile method for in-situ investigation of surfaces and interfaces. In SFG-VS experiment a pulsed tunable infrared IR (ωIR) laser beam is mixed with a visible VIS (ωVIS) beam to produce an output at the sum frequency (ωSFG = ωIR + ωVIS). SFG is second order nonlinear process, which is allowed only in media without inversion symmetry. At surfaces or interfaces inversion symmetry is necessarily broken, that makes SFG highly surface specific. As the IR wavelength is scanned, active vibrational modes of molecules at the interface give a resonant contribution to SF signal. The resonant enhancement provides spectral information on surface characteristic vibrational transitions.

Design of the SFG Spectrometer


Sum frequency generation (SFG) spectrometer is based on picosecond pump laser and optical parametric generator (OPG) with difference frequency generation (DFG) extension. Solid state mode-locked Nd:YAG laser featuring high pulse duration and energy stability is used in the system. Fundamental laser radiation splits into several channels in multichannel beams delivery unit. Two of these beams are used for pumping OPG and DFG. Small part of laser output beam, usually with doubled frequency (532 nm), is directed to VIS channel of SFG spectrometer. IR channel of spectrometer is pumped by DFG output beam.
The sizes of individual compartments, positions of apertures and beams heights are fitted. As a result SFG spectrometer takes less space in laboratory. Standard versions usually fit on 1000×2400 mm optical table. All beams among laser, harmonics module, parametric generator, SFG box are enclosed in tubes. For example beam dedicated for VIS channel passes through OPG compartment only to minimize the risk of accident with dangerous high intensity laser radiation. It makes SFG spectrometer substantially safer comparing to home-made SFG-VS setups. Also optical parameters, like beam diameter, pulse energy, delays between channels are perfectly matched. Motorised switch of IR Polarisation, motorisation of delay line are included in standard configuration.
The spectrometer is designed towards user friendly operation. Many components of the system are automated and controlled from PC. The opto-mechanical holders that need to be tuned often during routine operation are located around sample area and can be easily accessed without walking around the optical table. According to user needs different level of automation can be proposed, starting from most simple mechanical setup to most advanced fully motorized version.
Detection system consists of monochromator with high stray light rejection and gated PMT based SF signal detector. The feature of such design is ability to perform measurements in room lighting. Second parallel detection channel is available as an option. All system components are controlled from single dedicated software. Program contains many useful instruments for automatic SFG spectra recording, dynamics monitoring, X-Y sample mapping, azimuthal scan and system parameters monitoring.
TOPAG offers three common SFG spectrometer models from Ekspla for classical picosecond scanning SFG vibrational spectroscopy and several specialized models for most demanding users. Basic models are SFG Classic, SFG Advanced, Double Resonance SFG and Phase Sensitive SFG. They differ by IR beam tuning ranges and available VIS beam wavelengths (please see specifications page). The Classic and Phase-sensitive SFG models can be combined in one spectrometer. Further SFG microscope provides unique ability to investigate spatial variations across a sample surface at micrometer resolution as a function of time.

System Components

  • Picosecond mode-locked Nd:YAG laser
  • Multichannel beam delivery unit
  • Picosecond optical parametric generator
  • Spectroscopy module
  • Monochromator
  • PMT based signal detectors
  • Data acquisition system
  • Dedicated LabView® software package for system control

Classic and Advanced SFG spectrometer

Classic configuration of SFG spectrometer covers tunable IR laser range from 4300cm-1 to 1000cm-1 (10µm). Using additional DFG crystal in Advanced SFG spectrometer the detection range can be extended to longer wavelengths until 625cm-1 (16µm)

Phase-sensitive SFG spectrometer

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

Combination of Phase sensitive and Classic SFG Spectrometer

This combinations allows a switchable setup between phase sensitive and Classic/Advanced SFG spectrometer in top/ bottom configuration. Switch for VIS beam is manually. IR mirrors are motorised, BaF₂ lens manually. Path length to the sample is the same in all configuration. Motorised polarisation control can be used.

Double Resonance SFG Spectrometer

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

Narrow Band SFG Spectrometer

The spectral resolution in of narrowband SFG is determined by light source, which is a ps YAG laser pumped OPA with DFG extension. The monochromator is used only as filter. Using a narrowband OPA model PG511 line width of mid-IR can be reduced to < 2 cm-1. This tunable laser is a synchronously pumped optical parametric generator with OPO with long focal length resonator.

SFG Microscope

SFG spectroscopy combined with high spatial resolution in micrometer range provides a unique ability to investigate spatial and chemical variations across the surface as a function of time. An example of such application is chemical imaging of corrosion. SFG microscopy reveals presence of highly-coordinated complexes of molecules at particular stage of this process.
SFG microscope spectrometer uses far-field image formation technique. Illuminated area on the sample surface is substantially bigger than in regular SFG spectrometer. Using blazed grating and unique design optical system, image of surface plane is translated to matrix of ICCD camera. This way we can record distribution of SF signal at particular wavelength. For complete spectral and spatial information it is necessary to record multiple surface pictures at different wavelength. Integrated software package provides ability to visualize measured data making various cross sections: position-, wavelength- or time-dependent.

Model 1) SFG Classic SFG Advanced SFG Double resonance SFG Phase Sensitive
Spectral range 1000- 4300 cm-1 625- 4300 cm-1 1000- 4300 cm-1
Spectral resolution <6 cm-1(optional <2 cm-1) <6 cm-1(optional <2 cm-1) <10 cm-1 <6 cm-1(optional <2 cm-1)
Spectra acquisition method Scanning
Sample illumination geometry Top side, reflection (optional: bottom side, top-bottom side, total internal reflection)
Incidence beams geometry Co-propagating, non-colinear (optional: colinear) non-collinear
Incidence angles Fixed, VIS ~60 deg, IR ~55 deg (optional: tunable) not tunable
VIS beam wavelength 532 nm (optional: 1064 nm) Tunable 420 – 680 nm (optional: 210 – 680 nm) 532 nm
Polarization (VIS, IR, SFG) linear, selectable “s” or “p”, purity > 1:100
Beam spot on the sample selectable, ~150 – 600 µm fixed
Sensitivity air-water spectra solid sample
Pump lasers PL2230
Optical parametric generators
IR source with standard linewidth (<6 cm-1) PG501-DFG1P PG501-DFG2 PG501-DFG1P
IR source with narrow linewidth (<2 cm-1) PG511-DFG PG511-DFG2 - PG511-DFG
UV-VIS source for Double resonance SFG - - PG401 (optional: PG401-SH) -

1) Due to continuous product improvements, specifications are subject to changes without advance notice.

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

Optional Accessories

  • Six axis sample holder
  • Motorization of different axis
  • Sealed temperature controlled sample chamber
  • Langmuir trough
  • Motorization of polarization central of VIS beam and polarization analyzer

 

Do you have questions about SFG?
Your details will be gathered and handled to respond to your request.
Detailed information on this topic can be retrieved from our privacy policy.

Do you have questions about our products?

Write to us | info@topag.de

Give us a call | +49 6151 425978

TOPAG Lasertechnik GmbH
Nieder-Ramstädter Str. 247
64285 Darmstadt, Germany
Phone: +49 6151 4259 78
Fax: +49 6151 4259 88
E-mail: info@topag.de