SBIR-STTR Award

This Phase I STTR will model, design, fabricate and characterize twisted graphene heterostructure Josephson junction detectors of mm-wave and THz radiation. For a range of potential Josephson junction designs, we will calculate the IV curve, the zero-magnetic-field value for the maximum zero-voltage current, the dependence of this current on rf fields and temperature, and the dynamic resistance near this current value. These quantities will enable a prediction of responsivity for broad-band detection mechanism for various device structures. We will calculate frequencies for the AC-Josephson-effect emission, which will serve as local oscillators for narrow-band heterodyne detection, the spectral purity of emission, and its power. We will fabricate Moire-patterned few layer graphene at the magic angles for superconductivity, as well as Josephson junctions based on theoretically optimized designs. The electrical properties of the fabricated Josephson junctions will be characterized down to 40 mK temperatures, and photoresponse at 15 GHz will be studied using a microstrip resonator. These Phase I theoretical and experimental efforts will inform Phase II device prototype development and characterization up to 50 GHz frequencies using existing apparatus and to THz frequencies with cryostat modifications to enable optical access.

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.