Elucidating mechanism of optical cavities in superconducting strip single photon detectors using transmission line and impedance models

TL;DR

This paper uses transmission line and impedance models to clarify how optical cavities enhance absorption in superconducting single photon detectors, providing analytical formulas and design insights.

Contribution

It introduces analytical transmission line and impedance models to explain and optimize optical cavity effects in SSPDs, validated by numerical comparisons.

Findings

Analytical formulas for absorptance in SSPDs with optical cavities

Maximum absorptance occurs when input impedance matches medium impedance

Models applicable to other superconducting detectors like MKIDs and TESs

Abstract

We clarified the physical mechanism of superconducting strip single photon detectors (SSPDs) with optical cavities by using transmission line and impedance models. By introducing the transmission line model, we derived the analytical formulae for the absorptance of SSPDs with optical cavities. We compared the absorptance obtained from the analytical formulae for SSPDs with single-side, double-side, and dielectric multi-layer optical cavities against the results of numerical simulations. The comparison showed that the results were nearly identical. By introducing the impedance model, it was clearly shown that the SSPDs with optical cavities achieved the maximum absorptance when their input impedance of the SSPDs with optical cavities matched the impedance of the input medium. The design concepts proposed in this study are applicable to other superconducting detectors, such as microwave kinetic inductance detectors and transition-edge sensors.