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Abstract
Sum Frequency generation has emerged as a powerful all optical technique for probing surface and interfacial phenomena. Recent progress in developing phase-sensitive (heterodyne) SFG techniques with IR spectroscopy has enabled unprecedented, direct access to vibrational energies and structural dynamics of interfaces. The ability to collect phase as well as magnitude in nonlinear spectra has several advantages including distinguishing between real and imaginary components of the emitted nonlinear field to separate resonant and non-resonant responses, access the sign of the electric field and infer the orientation of dipoles at the interface, and improve signal to noise ratio. While many interfacial systems relevant to open questions in science and industry, including battery and solar cell design, heterogeneous catalysis, and semiconductor device design, would benefit from electronic spectroscopy to elucidate the mechanisms of charge and energy transfer across their active interfaces, electronic SFG has not achieved the same prevalence that vibrational spectroscopy enjoys. These interfaces are often dynamic, with properties that evolve over time or under varying conditions. Capturing electronic dynamics necessitates time-resolved techniques capable of tracking the interface's evolution, commonly occurring on ultrafast timescales. Interpretation of time-resolved SFG spectra benefits greatly from heterodyne detection where a local oscillator field is used as a reference for the simultaneous measurement of the phase and amplitude of the SFG signal. However, phase-sensitive measurements rely on interference and demand high precision in controlling the relative phase between the SFG signal and the local oscillator. This is particularly challenging for electronic transitions with resonances on the visible spectrum of light, and their sum frequency signals near to UV, where phase is much more sensitive to instabilities in the optics. This dissertation presents an interferometric design for a phase-sensitive electronic sum frequency generation (e-SFG) spectrometer with lock-in detection. Our method of continuous phase modulation of one arm of the interferometer affords direct measurement of the phase between SFG and LO fields. Errors in the path length difference caused by drifts in the optics are corrected, offering unprecedented stability. This spectrometer has the added advantage of collinear fundamental beams. The capabilities of the spectrometer are demonstrated with proof-of-principle experiments with GaAs e-SFG spectra, where we see significantly improved signal to noise ratio, spectral accuracy, and lineshapes. Efforts to extend the spectrometer’s capabilities to broadband experiments, as well as challenges that remain to be addressed are also outlined.