Frequency comb SFG: a new approach to multiplex detection

Abstract

Determination of molecular orientation at interfaces by vibrational sum frequency generation spectroscopy (VSFG) requires measurements using at least two different polarization combinations of the incoming visible, IR, and generated SFG beams. We present a new method for the simultaneous collection of different VSFG polarization outputs by use of a modified 4f pulseshaper to create a simple frequency comb. Via the frequency comb, two visible pulses are separated spectrally but aligned in space and time to interact at the sample with mixed polarization IR light. This produces two different VSFG outputs that are separated by their frequencies at the monochromator rather than their polarizations. Spectra were collected from organic thin films with different polarization combinations to show the reliability of the method. The results show that the optical arrangement is immune to fluctuations in laser power, beam pointing, and IR spectral shape.

Thin Films and Bulk Phases Conucleate at the Interfaces of Pentacene Thin Films

“…The VSFG laser system has been described elsewhere . A pair of narrow-band visible pulses was created using a Fabry–Perot etalon (TecOptics, 12 cm –1 FWHM), as published previously .…”

“…The VSFG laser system has been described elsewhere. 29 A pair of narrow-band visible pulses was created using a Fabry−Perot etalon (TecOptics, 12 cm −1 FWHM), as published previously. 30 The center frequencies of the visible pulses were tuned to 807.40 and 789.75 nm and separated from the laser output.…”

Facile Background Discrimination in Femtosecond Stimulated Raman Spectroscopy Using a Dual-Frequency Raman Pump Technique

“…To more easily produce simultaneous yet spectrally different Raman pump pulses for a SERDS-inspired approach to FSRS, we took inspiration from the work of Kearns et al, who built a simple frequency comb generation setup for vibrational sum frequency generation (VSFG) experiments. 34 To collect two different VSFG spectra simultaneously, Kearns et al used a pair of mirrors to divide the Ti:Sapphire fundamental output into two visible beams in a grating filter. 34 This simple addition of two mirrors in the grating filter produced two visible pulses that were spectrally separated but spatially and temporally overlapped resulting in two different VSFG signals for multiplex detection.…”

“…34 To collect two different VSFG spectra simultaneously, Kearns et al used a pair of mirrors to divide the Ti:Sapphire fundamental output into two visible beams in a grating filter. 34 This simple addition of two mirrors in the grating filter produced two visible pulses that were spectrally separated but spatially and temporally overlapped resulting in two different VSFG signals for multiplex detection. Given that a standard FSRS approach uses a grating filter to generate the picosecond Raman pump pulse, we utilize a similar design here.…”

“…Various specialized light sources have been previously employed for SERDS, such as the external cavity diode laser, , distributed Bragg reflector (DBR) diode laser, and a monolithic dual-wavelength diode laser at 785 nm. , By subtracting Raman spectra obtained with two different but close in frequency (∼5–10 cm –1 ) excitation wavelengths, dispersive-shaped Raman features are obtained and all other spectral contributions such as fluorescence , are removed from the wavelength-dependent Raman peaks . A background-free Raman spectrum can then be recovered using curve fitting and integration algorithms. ,,,,,,− The choice of reconstruction method is application dependent and can range from being relatively simple to complicated integration algorithms requiring user intervention to manually specify the limits. ,, Therefore, a simple and straightforward technique is desired to produce an excitation pulse at two wavelengths to distinguish the Raman features from the background.…”