Detecting Weak Signals from Interfaces by High Accuracy Phase-Resolved SFG Spectroscopy

TL;DR

This paper introduces a high-precision, sensitive v-SFG spectrometer that significantly enhances phase and amplitude accuracy, enabling detection of weak interfacial signals previously difficult to measure.

Contribution

The study presents a novel spectrometer design with collinear geometry and balanced detection, greatly improving phase accuracy and sensitivity in v-SFG spectroscopy.

Findings

Unprecedented phase and amplitude accuracy achieved.

Sensitivity increased by an order of magnitude.

Successful detection of weak surface signals from quartz.

Abstract

Much work over the last 25 years has demonstrated that the interface-specific, alloptical 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 while significantly improving the sensitivity of the technique. Combining a full collinear beam geometry with a technique enabling the simultaneous measurement of the complex sample and reference spectrum, uncertainties in the reference phase and amplitude are shown to 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.