Terahertz spectroscopy of quantum materials

Quantum materials lying at the frontier of condensed matter physics call for access to increasingly low frequencies in the electromagnetic spectrum. In particular, the terahertz region is a rich arena for probing exotic quasiparticles associated with novel quantum phenomena. Ultrafast femtosecond lasers developed in the late 1990s brought an unexpected opportunity to implement terahertz time-domain spectroscopy (THz-TDS). Here, we introduce the standard method of THz-TDS and illustratethis technique by presenting case studies in which the distinct advantages of THz-TDS were utilized to obtain key information on representative quantum materials. Recent results on the gapless superconductivity and fractional excitations in quantum spin liquid candidate materials will be presented along with a perspective on future developments in techniques and applications.