Detecting weak signals from interfaces by high accuracy phase-resolved SFG spectroscopy

Much work over the last 25 years has demonstrated that the interface-specific, all-optical technique, vibrational sum frequency generation (v-SFG) spectroscopy, is often uniquely capable of characterizing the structure and dynamics of interfacial species. The desired information in such a measurement is the complex second order susceptibility which gives rise to the nonlinear response from interfacial molecules. The ability to detect molecular species yielding only small contributions to the susceptibility is meanwhile limited by the precision by which the spectral phase and amplitude can be determined. In this study we describe a new spectrometer design that offers unprecedented phase and amplitude accuracy for extended studies that involve multiple spectral acquisitions while modifying sample properties. The key to this significant improvement to the sensitivity of the technique is the combination of a full collinear beam geometry with broadband spectral sampling and the ability to simultaneously measure the complex sample and reference spectrum. We show that using this technique uncertainties in the reference phase and amplitude can be greatly reduced. Furthermore, we show that using balanced detection, the signal to noise ratio can be increased by one order of magnitude. The capabilities of the spectrometer are demonstrated by the isolation of a small isotropic surface signal from the bulk dominated nonlinear optical response of z-cut quartz. The achieved precision of our spectrometer enables measurements not currently feasible in v-SFG spectroscopy.