A new optical configuration is described for sum-frequency generation spectroscopy in which the two fundamental beams approach the interface from different phases. In this geometry one beam approaches the interface at its critical angle in the denser medium, while the other beam approaches the interface in external reflection from the rarer phase. The proposed configuration permits the signal enhancement obtained with total internal reflection sum-frequency generation to be applied to interfaces for which the denser medium is nontransparent in the infrared spectral region. Calculations predict the optimal incident angles for the externally reflecting infrared beam and the internally reflecting visible beam for a series of polarization combinations. Experimental measurements of the sum-frequency intensity as a function of the visible-beam incident angle are presented and compared with the calculated angular dependence. Resulting sum-frequency spectra obtained with a LiNbO<sub>3</sub> optical parametric oscillator nanosecond system are shown for a surfactant system at the D<sub>2</sub>O–air interface.
© 2000 Optical Society of America
(190.4350) Nonlinear optics : Nonlinear optics at surfaces
(240.6490) Optics at surfaces : Spectroscopy, surface
(300.6420) Spectroscopy : Spectroscopy, nonlinear
(300.6490) Spectroscopy : Spectroscopy, surface
Yarjing J. Yang, Rebecca L. Pizzolatto, and Marie C. Messmer, "Internal–external reflection sum-frequency generation spectroscopy at an interface," J. Opt. Soc. Am. B 17, 638-645 (2000)