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.

Scattering-type scanning near-field optical microscopy (s-SNOM) combines atomic force microscopy (AFM) and optical spectroscopy to measure a material's optical response at the nanometer length scale, far below the diffraction limit of light. 

The metallic AFM tip acts like a nano-antenna that concentrates light at its apex, couples to evanescent near fields, and scatters them into detectable far-field radiation. This enables nanometer-resolved access to local optical properties and high-momentum excitations such as polaritons.