Impedance model for the polarization-dependent optical absorption of superconducting single-photon detectors

TL;DR

This paper investigates the polarization-dependent optical absorption in superconducting NbN single-photon detectors, using measurements and an impedance model to optimize absorption efficiency across wavelengths.

Contribution

It introduces an optical-impedance model to describe polarization-dependent absorption and demonstrates how to maintain constant absorption by adjusting film parameters.

Findings

Polarization contrast increases with wavelength from 5% to 30%.

Maximum absorption is independent of filling factor for lossy NbN films.

Achieved ~70% absorption efficiency without optical cavity on Si or GaAs substrates.

Abstract

We measured the single-photon detection efficiency of NbN superconducting single photon detectors as a function of the polarization state of the incident light for different wavelengths in the range from 488 nm to 1550 nm. The polarization contrast varies from ~5% at 488 nm to ~30% at 1550 nm, in good agreement with numerical calculations. We use an optical-impedance model to describe the absorption for polarization parallel to the wires of the detector. For lossy NbN films, the absorption can be kept constant by keeping the product of layer thickness and filling factor constant. As a consequence, we find that the maximum possible absorption is independent of filling factor. By illuminating the detector through the substrate, an absorption efficiency of ~70% can be reached for a detector on Si or GaAs, without the need for an optical cavity.