In our laboratory, we generate THz pulses based on nonlinear optical methods (optial rectification, difference frequency generation). The experimental approach used is pump-probe spectrosopy,  where the excitation of the material is realized via an initial intense pulse (pump pulse) and whose ensuing dynamics is measured by a secondary pulse (probe pulse) delayed in time, so as to take snapshots of the state of the system as a function of time.

Conventional THz spectroscopy has a built-in limitation: the diffraction limit. At THz frequencies, the wavelength of light ranges from 300 µm to 3 mm, meaning that any material smaller than this cannot be studied with standard free-space optics. This leaves an entire class of systems out of reach, in particular two-dimensional (2D) superconductors that can be mechanically exfoliated down to a single atomic layer, but whose flakes are only a few micrometers across.

On-chip THz spectroscopy offers a way to circumvent this constraint. In this technique, THz pulses are generated and detected by photoconductive switches, and guided through sub-wavelength coplanar waveguides directly onto the material of interest. The THz pulse interacts with the sample at its own scale rather than at the scale of the wavelength, giving access to the optical conductivity of micron-scale samples that are otherwise invisible to conventional THz probes.