On-chip broadband THz time domain spectroscopy of van der Waals heterostructures*

Free space THz spectroscopy is generally limited to samples with sizes greater than the diffraction limited resolution of the experimental system. This restricts sample sizes to several hundreds of microns excluding investigations of smaller, exfoliable van der Waals materials such as graphene and transition metal dichalcogenides which have raised great interest. One method to beat the THz diffraction limit is to couple the radiation to an on-chip waveguide. Here, we present our progress towards development of a completely on-chip THz spectroscopy technique to investigate van der Waals heterostructures with sizes well below the diffraction limit. We employ photoconductive switches and coplanar waveguides to generate and control THz transients and detect the waveforms with picosecond time resolution. The versatility of our waveguide geometry allows for transmission, reflection, and emission spectroscopy all on a single chip. I will describe our efforts to investigate equilibrium and non-equilibrium phases of van der Waals heterostructures with these THz transients.