Gap-plasmon-resonance Enhanced Photodetection in NbN Superconducting Microstrip Photodetectors

We deposited niobium nitride (NbN) thin film by radio-frequency (RF) magnetron sputtering on MgO substrate at 800 ºC (with Tc ~15 K). To increase the photodetectivity, we design an Ag nanocube nanoresonator with a strong gap-plasmon resonance in the visible range to further enhance the photoresponsivity by engineering the optical response for the NbN superconducting devices. This is due to the superconducting states are broken down by a localizing strong electromagnetic field. To design the plasmonic nanostructures, we calculated the electromagnetic field distribution of Ag nanocube/Al2O3/NbN structure by finite-difference time-domain (FDTD). We observed a strong plasmonic resonance field which tightly confined in the Al2O3 layer between the Ag nanocube and NbN film at a resonant wavelength of 532 nm. In the end, we will also discuss the detailed working mechanism and the potential application of plasmon-enhanced photodetection in NbN superconducting photodetectors.