Development of Gap-Plasmon-Enhanced NbN Superconducting Photodetectors and Its Applications

Superconducting photon detectors have a low dark count rate and short timing jitter; thus, they provide outstanding detection performance. However, most reports focus on optimization within IR telecom wavelengths. Rarely do researchers focus on the optimization within the visible spectrum range. On the one hand, superconducting nanostrip single-photon detectors have been widely studied in the past decades because their intrinsic detection efficiency is higher than ones with microstrip. On the other hand, microstrip photon detectors have lower kinetic inductance and well fiber coupling efficiency that worth developing.<br/>In this work, we deposited niobium nitride (NbN) thin film by ultrahigh vacuum radio-frequency (RF) magnetron sputtering on MgO (100) substrate at 800 °C with high crystalline quality. The superconductivity of the NbN film can be modified by different growth parameters, such as the argon/nitrogen flow rate, target, RF power, growth temperature, and growth substrate during sputtering. Thus, we optimized the metallic and superconductivity properties (with T_c ~15.5 K) of NbN film by adjusting the parameters above. We characterized the material properties of the NbN film by ellipsometry, X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), transmission electron microscopy (TEM) X-ray diffractometer and superconducting quantum interference device (SQUID).<br/>To increase photodetectivity, we deposited 5 nm-Al₂O₃ and Ag nanocube on NbN film, where the Ag nanocube was designed as nanoantenna, which induced gap plasmon resonance in the visible range, further to engineer the optical response for the superconducting devices. The superconducting states broke down by localizing strong electromagnetic fields such that the photodetectivity of the device was promoted. To design plasmonic nanostructures, we calculated the electromagnetic field distribution of the Ag nanocube/Al₂O₃/NbN structure by finite-difference time-domain (FDTD). We discovered a localized plasmonic resonance, which resonated at 532 nm wavelength, at the edge of Ag nanocube with 40 nm long and 30 nm thick. Hence, we can enhance the photon response of the detector to the visible range which can attribute to the gap plasmon resonance. Noted that the lowest power of light can be detected is 4.4 nW. In the end, we discuss the potential for NbN superconducting single-photon detector applications, such as large active area and polarization independence, <i>etc</i>.