Twisted graphene-based Josephson junction detectors

This Phase II STTR is a collaboration between Truventic and the University of Central Florida (UCF) to develop polarization-sensitive single-photon detectors for the mm-wave and THz bands. These detectors will be based upon antenna-coupled superconducting magic-angle twisted multilayer graphene Josephson-junctions. Phase I theoretically demonstrated the potential for single photon detection, developed tools for fabricating devices with precisely controlled twist angles, and acquired or committed major instrumentation for cryogenic spectral characterization.  Phase II Objectives include detector fabrication, characterization, and applications development with transitioning to DoD.  The fabricated device will comprise four sheets of graphene with magic twist angle of ~1.1 deg between each sandwiched between insulating flakes of hexagonal boron nitride, with back gate to control the Fermi level, and with narrow bisecting top gate to create the Josephson-junction weak link.  The device will be located at the feed of a polarization sensitive wavelength-selective bowtie antenna, which will function as the receiver of free-space radiation and the source of high-frequency AC currents driven across the Josephson-junction.  This junction is DC biased at the maximum zero-voltage current, and the AC current depresses this maximum causing a DC voltage to appear.  The high sensitivity is based on the very large dynamic resistance at the bias point.  As a non-thermal detection mechanism, it can be very fast, giving simultaneously high responsivity and speed.  We estimate a noise-equivalent photon flux of 1 every 5 ns.  Characterization will use a 20 mK cryogen-free dilution refrigerator operated as a user facility at U Mass Boston, a 1.7 K optical cryostat at UCF, and a narrow-band Backward Wave Oscillator tunable from 160 GHz to 1.4 THz at UCF.  Experiments will be designed and performed to further certain applications for the detector, to help identify potentially interested stakeholders within DoD, and to encourage transitioning.