Terahertz waveform considerations for nonlinearly driving lattice vibrations

Abstract

High-field terahertz (THz) spectroscopy is applied to nonlinearly excite the E phonon-polariton vibrational coordinate in LiNbO3. We compare three THz sources to show that by optimizing the THz waveform, we can drive the atomic motion to large amplitudes and observe nonlinear effects. To maximize the atomic displacement, we show that the spectral amplitude at the resonant frequency of the mode (3.8 THz) is more important than the THz peak electric-field strength. Z-scan and simple 2-dimensional (2D) THz pulse-shaping measurements confirm this. In addition to the effects of an anharmonic potential energy surface that can describe the nonlinear behavior of the excited mode, we also consider a 2-photon absorption mechanism that may be a competing nonlinear excitation pathway. We consider the effects of each model on the observed responses in single-pulse power-dependent measurements, z-scan measurements, and simple 2D measurements, providing important guidance for future measurements to experimentally investigate nonlinear vibrational excitation in solid materials.