UC Santa Barbara

Free-space time domain THz spectroscopy accesses the electrodynamic responses of quantum materials at frequencies ideally matched to the energy scales of interacting condensed matter systems. THz spectroscopy, however, is challenging when samples are physically smaller than the diffraction limit of ~0.5 mm, as is typical, for example, in van der Waals materials and heterostructures. We examine foundational electrodynamics and spectroscopic principles relevant at THz frequencies. We present THz engineering challenges and overcome them by designing an on-chip, time-domain THz spectrometer with an interchangeable sample architecture and a bandwidth of 750 GHz. We use this spectrometer and extract the optical conductivity of a 7.5-um wide NbN film across the superconducting transition. We then extensively benchmark the spectrometer's operation in the context of THz engineering principles, and conclude by looking forward to the measurements this technology enables, including superconductivity, magnetism, and charge order in sub-diffraction materials and van der Waals heterostructures.