Optical Modeling of Superconducting Nanowire Single Photon Detectors

Abstract

Superconducting nanowire single photon detectors (SNSPDs) can detect single photons or low levels of infrared light in applications that require high speed and low timing jitter, such as integrated circuit analysis. Most applications also require a high device detection efficiency (DDE), but the DDE of SNSPDs is limited by many factors. A good optical design with an integrated optical cavity and dielectric layers can increase the absorptance of 1550-nm light in the active area to over 90%. Therefore, optical modeling using the transfer matrix method was used to guide the design and fabrication of high-efficiency detectors with a measured DDE of over 70%. In addition, finite element analysis was used to simulate the effect of adding different types of optical antennas to SNSPD designs to increase their active area without compromising their speed, and the fabrication of antennas integrated with nanowires achieved sub-10 nm gaps between features. Thin films of niobium nitride, the starting material of the SNSPDs, were investigated using several techniques for thin film characterization, including x-ray diffraction, Auger electron spectroscopy and x-ray photoelectron spectroscopy. Optical setups based on reflectometry and transmittometry were built to determine the film thickness more accurately than deposition time for optical modeling and to provide feedback on the deposition conditions. The optical setups are able to provide reproducible and precise thickness measurements to within 0.1 nm.