Nonideal Optical Cavity Structure of Superconducting Nanowire Single-Photon Detector

Optical cavity structure has been proven to be a crucial factor for obtaining high detection efficiency in superconducting nanowire single-photon detector (SNSPD). Practically, complicated fabrication processes may result in a nonideal optical cavity structure. The cross-sectional transmission electron microscope image of SNSPD fabricated in this study shows unexpected arc-shaped optical cavities, which could have originated due to the over-etching of SiO₂ layer while defining NbN nanowire. The effects of the arc-shaped optical cavity structure, such as the wavelength dependence of the optical absorption efficiency for different polarization, were analyzed by performing optical simulations using finite-difference time-domain method. The central wavelength of the device is found to exhibit a blue shift owing to the arced cavity structure. This effect is equivalent to the flat cavity with a reduced height. The results may give interesting reference for SNSPD design and fabrication.