Extending the Capabilities of Heterodyne-Detected Sum-Frequency Generation Spectroscopy: Probing Any Interface in Any Polarization Combination

Vibrational sum-frequency generation spectroscopy (SFG) is a powerful tool for studying noncentrosymmetric environments, particularly interfaces. Conventional homodyne-detected SFG inherently detects the intensity of the emitted light and thus forfeits the ability to directly measure the complex components, that is, phase, of the second-order nonlinear susceptibility, which contains the molecular response of interest. Heterodyne-detected SFG (HD-SFG) has recently been employed to recover this lost information, but has not been broadly adopted due to restrictions in the technical implementation. Presented in this Article is a HD-SFG geometry that fills a need for ease of use and increased versatility; our flexible and convenient design provides the capability to probe any interface in any polarization combination with exceptional phase stability. We demonstrate this ability by collecting the SFG signal from an octadecyltrichlorosilane monolayer on the front of a solid fused silica substrate and determine, for the first time with broadband HD-SFG, the complex spectrum of buried dry and solvated interfaces, collected in both ppp and ssp polarization combinations. This experimental design does not display any appreciable phase shift for over 10 h, which is a necessity for inclusion in more advanced methods such as time-resolved HD-SFG and 2D-HD-SFG.