Free-space time domain THz spectroscopy accesses electrodynamic responses in a frequency regime ideally matched to interacting condensed matter systems. However, THz spectroscopy 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. Here, we present an on-chip, time-domain THz spectrometer based on semiconducting photoconductive switches with a bandwidth of 200 GHz to 750 GHz. We measure the optical conductivity of a 7.5-μm wide NbN film across the superconducting transition, demonstrating spectroscopic signatures of the superconducting gap in a sample smaller than 2% of the Rayleigh diffraction limit. Our spectrometer features an interchangeable sample architecture, making it ideal for probing superconductivity, magnetism, and charge order in strongly correlated van der Waals materials.
@article{arxiv.2302.05434,
title = {On-chip time-domain terahertz spectroscopy of superconducting films below the diffraction limit},
author = {Alex Potts and Abhay Nayak and Michael Nagel and Kelson Kaj and Biljana Stamenic and Demis D. John and Richard D. Averitt and Andrea F. Young},
journal= {arXiv preprint arXiv:2302.05434},
year = {2023}
}