Report Summaries:

Terahertz fields are now available with sufficient strengths to drive nonlinear responses in a wide range of materials. THz field interactions with lattice, electronic, and spin degrees of freedom and monitoring of THz-induced responses with THz, optical, and x-ray probes will be illustrated, with emphasis on THz-driven structural and electronic quantum phase transitions. Single-shot measurement of time-dependent signals will be highlighted for enabling study of irreversible phase transitions that do not permit repeated excitation of the initial, pristine state and for enabling 2-dimensional THz spectroscopy with data collection times measured in minutes or hours instead of days.

Optical excitation of acoustic and shock waves in a geometry that permits continuous viewing of wave propagation and its effects on materials will also be discussed briefly. It is now possible to excite acoustic waves up to 1-2% strain without damage to the optically irradiated region, permitting repeated excitation and time-resolved measurement as well as observation of cumulative effects. Current efforts to combine acoustic, THz, and optical excitation for multimodal control will be discussed. Above-damage-threshold excitation of shocks reaching pressures of tens of GPa, with prospects for much higher pressures, will also be demonstrated.