Terahertz Waveform Considerations for Driving Lattice Vibrations into the Anharmonic Regime

The potential energy surface (PES) is a manifestation of the forces that connect elements together and govern nearly all material properties, from thermal expansion to phase transitions to energy transport, making the PES central to condensed-matter physics and ultimately device design. The shape of the PES in solids has only recently begun to be determined experimentally. We extract the PES along the E phonon-polariton vibrational coordinate in LiNbO3 using high-field terahertz (THz) spectroscopy, and optimize the THz waveform to drive atomic motion to large amplitudes by comparing three different THz generation crystals. To maximize atomic displacement, we show that the spectral amplitude at the resonant mode frequency (3.8THz) is more important to consider than the THz peak electric-field strength. We confirm this with Z-scan and 2D THz pulse shaping measurements and provide important guidance for future experimental investigations of the anharmonic PES in solid materials.