Graphene-enabled coherent terahertz wave detection and thickness determination

Coherent detection and interferometry in the terahertz (THz) regime are key capabilities that enable applications ranging from astronomy to non-destructive testing. Phase-sensitive THz detection is currently achieved using nonlinear crystals or external interferometers and photomixers. However, the former approach requires femtosecond pulsed radiation, and all approaches suffer from a large footprint and sensitive alignment. Here, we demonstrate a graphene-enabled, on-chip, integrated THz detector-interferometer with optical cavity and antenna, exhibiting high sensitivity to the phase of incident THz light. We exploit this by determining the thickness of thin films placed in front of the detector-interferometer, obtaining a strongly sub-wavelength thickness accuracy of $\sim$5 $μ$m, while we predict that an accuracy of 10 nm is within reach. This is relevant for a range of industrial application domains, including automotive, construction, and health. The detector-interferometer moreover exhibits a record-high external responsivity - without any normalization to a diffraction-limited spot size - of 73 mA/W and a noise-equivalent power of 79 pW$~\rm{Hz}^{-1/2}$. This performance is due to enhanced absorption at the cavity mode around 89 GHz, in agreement with multi-physics simulations. These results pave the way to exploiting coherent wave detection in the THz regime with utility in spectroscopy, next-generation wireless communication, and beyond.