Nonlinear interferometer: Design, implementation, and phase-sensitive sum frequency measurement.

Sum frequency generation (SFG) spectroscopy is a unique tool for probing the vibrational structure of numerous interfaces. Since SFG is a nonlinear spectroscopy, it has long been recognized that measuring only the intensity-the absolute square of the surface response-limits the potential of SFG for examining interfacial interactions and dynamics. The potential is unlocked by measuring the phase-sensitive or imaginary response. As with any phase, the phase-sensitive SFG response is measured relative to a reference; the spatial relationship between the phase reference and the sample modulates the observed interference intensity and impacts sensitivity and accuracy. We have designed and implemented a nonlinear interferometer to directly measure the phase-sensitive response. If the phase of the reference is known, then the interferometer produces an absolute phase of the surface. Compared to current configurations, phase accuracy and stability are greatly improved due to active stabilization of the sample-reference position. The design is versatile and thus can be used for any system that can be probed with SFG including buried interfaces and those with high vapor pressure. Feasibility and advantages of the interferometer are demonstrated using an octadecyltrichlorosilane film on fused silica.