New developments in nonlinear terahertz spectroscopy have revealed important new insights into quantum material behavior and have opened up new possibilities for practical applications including advanced material characterization and control. Terahertz field interactions with electronic, lattice, and spin degrees of freedom have all been exploited in recent measurements. Terahertz-induced responses including quantum material phase transitions, colossal Stark shifts, and magnetization control will be illustrated. Terahertz-frequency electron paramagnetic resonance (EPR) including two-dimensional terahertz magnetic resonance will be discussed.
Time-resolved transient grating measurements of acoustic and thermal responses, in which crossed optical pulses generate an interference pattern at the sample whose spatial period determines the acoustic wavelength or the thermal transport length scale, have been conducted for some time, with applications already developed including noncontact metrology of thin films in microelectronics. Recently the measurements have revealed nondiffusive thermal transport kinetics, which provides fundamental insight into the phonons that carry thermal energy and which is a key consideration in nanoscale device applications. Experiments with extreme UV pulses from a soft x-ray free-electron laser have enabled the thermal transport length scale to be reduced to tens of nanometers, revealing essentially purely ballistic kinetics. New modes of thermal transport may become possible in some materials.