Abstract

Sum frequency generation (SFG) spectroscopy is a powerful technique for characterization of vibrational bonds of molecules at surfaces and interfaces [1]. The principle of SFG is based on resonant nonlinear mixing of two optical pulses – typically one in MIR range and another visible (VIS). There are two main SFG spectroscopy techniques. Narrowband SFG (NB-SFG) is based on picosecond MIR and picosecond VIS source. Full spectral information is obtained by tuning MIR pulses through vibrational lines of the molecules. The typical resolution in NB-SFG is a few cm^-1 [2]. In broadband SFG (BB-SFG) femtosecond MIR pulses with spectrum spanning a few hundred cm^-1 are used. This allows for simultaneous recording of complete SFG spectra and reduces systematic errors due to sample degradation during the measurement. However in most experimental BB-SFG setups the same femtosecond laser source is used to produce narrowband VIS pulses. Therefore typical resolution obtained with BB-SFG is ~10-20 cm^{- 1}, which limits ability to analyze vibrational line-shapes [2]. Finally a hybrid system was demonstrated where two separate laser sources – femtosecond and picosecond – were synchronized to achieve both very high spectral resolution (<1cm^-1) and simultaneous spectral recording [3]. However the system required two complex independent synchronized laser systems which would be difficult to implement in most laboratories.

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