Title: Examining Nonlinear Terahertz Photonic and Phononic Excitation with Two-Dimensional Spectroscopy

Abstract: High-field terahertz (THz) spectroscopy is enabling the ultrafast study and control of matter in new and exciting ways. However, when intense electromagnetic pulses are used in any kind of pump-probe spectroscopy, several nonlinear excitation pathways can result, leading to scenarios that required the development of multi-dimensional spectroscopies to illuminate the observed dynamics. I’ll describe a clear example where 2-dimensional (2D) THz vibrational spectroscopy is needed to distinguish between nonlinear-excitation pathways in crystalline CdWO4. We nonlinearly excite a set of Raman-active vibrational modes in CdWO4 with broadband THz pulses, and we considered anharmonic phonon coupling and 2-photon absorption excitation routes; single-pulse excitation measurements were not enough to unambiguously distinguish the dominant excitation pathway. 2D THz-Raman spectroscopic analysis allowed us to distinguish between photonic and phononic excitation pathways and determine that 2-photon absorption is the dominant excitation pathway in CdWO4. As more systems are studied with high-field THz radiation to explore fundamental material properties and cutting-edge device development, multi-dimensional measurements will be needed to elucidate energy transfer pathways.