  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
non-ideal optical cavity structure. The cross-sectional transmission electron
microscope (TEM) 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.
