Phase Tracking for Accurate Interferometric Measurements of Heterodyne Detected e-SFG Spectra

Heterodyne detection (HD) is a ubiquitous tool in spectroscopy for the simultaneous detection of intensity and phase of light. As an interference measurement, heterodyne detection hinges on the phase stable relationship between the signal and a local oscillator (LO). Short UV-Vis wavelengths used in electronic spectroscopy are susceptible to phase instability due to drifts in optical path length, posing a challenge for heterodyne detection. We present an interferometric design that utilizes phase modulation of one arm of a Mach Zehnder interferometer to directly measure the phase between the two arms. “Tracking” the phase allows us to correct for errors in the path length difference caused by drifts in the optics, offering unprecedented stability. We have applied this method to measurement of electronic sum frequency generation (e-SFG) spectra in the time domain with lock-in amplification, accessing the weak complex second order susceptibility, χ⁽²⁾, which reports on interfacial energy states in centrosymmetric media. In conjunction with broadband ultrafast pulses, this method will enable in-situ probing of electronic states and charge transfer dynamics at interfaces between materials.