Spectral narrowing of a phonon resonance in time-domain sum-frequency spectroscopy

TL;DR

This study demonstrates spectral narrowing of a phonon resonance in time-domain sum-frequency spectroscopy using tunable IR pulses, revealing insights into phonon dephasing and nonlinear optical responses in 4H-SiC.

Contribution

It introduces a method to achieve spectral narrowing of phonon resonances in SFG spectroscopy by controlling temporal delays, supported by experimental and model data.

Findings

Spectral narrowing observed near the phonon frequency.

Slower polarization dephasing at resonance.

Model calculations corroborate experimental results.

Abstract

Sum-frequency generation (SFG) spectroscopy provides a versatile method for the investigation of non-centrosymmetric media and interfaces. Here, using tunable picosecond infrared (IR) pulses from a free-electron laser, the nonlinear optical response of 4H-SiC, a common polytype of silicon carbide, has been probed in the frequency- and time-domain by infrared-visible vibrational SFG spectroscopy. In the SFG spectra we observe a sharp resonance near the longitudinal optical phonon frequency, arising from linear optical effects due the epsilon-near-zero regime of the IR permittivity. In the time domain, the build-up of the SFG intensity is linked to the free-induction decay of the induced coherent IR polarization. When approaching the frequency of the phonon resonance, a slower polarization dephasing is observed as compared to off-resonant IR excitation. Thus, by introducing a temporal delay between the IR and the visible up-conversion pulse we are able to demonstrate spectral narrowing of the phonon SFG resonance, as corroborated by model calculations.