Terahertz time-domain spectroscopy (THz-TDS) is a powerful tool for probing the optical response of materials in the THz frequency range. THz spectra are rich in low-energy collective excitations that serve as fingerprints of quantum phases, including magnons, phonons, and superconducting gaps (see Fig. 1 in Kumar et al. 2019).

Using electro-optic sampling, the instantaneous electric field of a THz pulse can be measured with ~50 fs resolution, allowing the time-domain waveform of the pulse lasting only a few ps (10^-12 s) to be directly recorded.