Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Abstract

We present a new technique of broad-band heterodyne-detected sum frequency generation (HD-SFG) spectroscopy and demonstrate its high sensitivity allowing surface-selective measurements of vibrational spectra at submonolayer surface coverage, as low as a few percent of a monolayer. This was achieved without the help of surface enhancement phenomena, on a transparent dielectric substrate (water), and without introducing fluorescent labels, in fact, without utilizing any electronic resonances. Only the intrinsic vibrational transitions were employed for the detection of the analyte molecules (1-octanol). Unlike conventional (homodyne-detected) SFG spectroscopy, where the signal intensity decreases quadratically with decreasing surface coverage, in HD-SFG, the scaling is linear, and the signal is amplified by interference with a reference beam, significantly improving sensitivity and detection limits. At the same time, HD-SFG provides the phase as well as the amplitude of the signal and thus allows accurate subtraction of the non-resonant background--a common problem for surfaces with low concentrations of analyte molecules (i.e., weak resonant signals).

Vibrational sum frequency generation spectroscopy using inverted visible pulses

“…In Fig. 5(c), we show the SFG spectrum for well known model system of 1-octanol at the air-water interface [16]. ), showing good agreement with prior studies.…”

Two-Dimensional Correlation Analysis of Sum-Frequency Vibrational Spectra of Langmuir Monolayers

“…Recently S. Roy and coworkers reported that a spectral shift can be seen as an artifact when 2D correlation analysis is applied to the coherent spectroscopy [25]. To avoid this artifact, in the future study one can apply 2D correlation analysis to 1D imaginary spectra, which can be either deduced by fitting the spectrum with a multi-peak Lorentzian or measured with heterodyne SFG experiments [29].…”

Probing Electronic Structures of Organic Semiconductors at Buried Interfaces by Electronic Sum Frequency Generation Spectroscopy

“…We emphasize this equation differs from the formula for vibrational sum frequency generation (VSFG) 4,[41][42][43][44][45][46][47][48][49][50] , in which one of the two incoming pulses are tuned in resonant with molecular transitions. First, all states are electronic states and the contribution from vibrational states are treated implicitly.…”