Enhanced nonlinear spectroscopy for monolayers and thin films in near-Brewster’s angle reflection pump-probe geometry

Abstract

We experimentally demonstrate and theoretically explicate a method that greatly enhances the detection of third-order nonlinear signals from monolayers and thin films on dielectric substrates. Nonlinear infrared signals, including two dimensional infrared (2D IR) vibrational echo signals, were detected from a functionalized alkyl chain monolayer on a dielectric SiO2 surface in a near-Brewster’s angle reflection pump-probe geometry. We observed a tremendous enhancement of the signal-to-noise (S/N) ratio in this geometry compared with a conventional transmission pump-probe geometry signal. The S/N enhancement is achieved by the greatly increased modulation of the local oscillator (LO) field that is induced by the nonlinear signal field. By reducing the LO field without loss of the signal field, the modulation amplitude acquired in this geometry was enhanced by more than a factor of 50. The incident angle dependence of the enhancement was measured and the result agreed remarkably well with theoretical calculations. We combined this geometry with a germanium acousto-optic modulator pulse shaping system to apply 2D IR spectroscopy to the monolayer. The enhanced and phase-stable 2D IR spectra gave detailed dynamical information for the functionalized alkyl chain monolayer. The application of the method to films with finite thickness was described theoretically. The range of film thicknesses over which the method is applicable is delineated, and we demonstrate that accurate dynamical information from thin films can be obtained in spite of dispersive contributions that increase with the film thickness. While we focus on infrared experiments in this article, the method and the theory are applicable to visible and ultraviolet experiments as well.