Keynote
Linear and nonlinear terahertz spectroscopy of molecules and materials

The advent of tabletop sources of intense terahertz (THz) pulses has enabled rapid advances in the nascent subfield of nonlinear THz spectroscopy. Terahertz fields can be used to drive collective and molecular electronic, vibrational, and spin responses as well as gas-phase molecular rotations. In some cases, far-from-equilibrium responses including electronic and structural phase transitions and drastic changes in electronic energetics and spectra can be induced by THz fields.

Generation and enhancement of strong THz fields will be discussed briefly, and THz light-matter interactions will be reviewed. Examples of highly nonlinear responses to THz electric fields, including colossal Stark shifts and electroluminescence of quantum dots and transient or permanent phase transitions in several prototype quantum materials, will be shown. Measurement of the responses using THz, optical, and x-ray probes will be illustrated.

Terahertz magnetic fields have been used to drive collective spin waves (magnons) in crystalline solids and zero-field transitions of high-spin transition metal complexes. Linear THz electron paramagnetic resonance (EPR) spectroscopy of molecular compounds will be reviewed, and nonlinear two-dimensional (2D) THz EPR measurements of magnons will be discussed. Prospects for extension of 2D THz EPR spectroscopy to molecular spins, for picosecond time-resolved EPR spectroscopy of photoexcited high-spin molecular complexes, and for a novel type of magnon Bose-Einstein condensation will be discussed.