Quasiparticle effective temperature in superconducting thin films illuminated at THz frequencies

TL;DR

This paper investigates the quasiparticle effective temperature in superconducting thin films under THz illumination, providing an analytical model that simplifies the inclusion of nonequilibrium effects in detector response calculations.

Contribution

It introduces an analytical expression relating absorbed power to quasiparticle effective temperature, validated against full nonequilibrium solutions and experimental data.

Findings

Good agreement between analytical model and full nonequilibrium solutions

Model accurately predicts frequency-dependent detector response

Experimental data for Ta KIDs at THz frequencies corroborate the model

Abstract

The response of superconducting pair-breaking detectors is dependent on the details of the quasiparticle distribution. In Kinetic Inductance Detectors (KIDs), where both pair breaking and non-pair breaking photons are absorbed simultaneously, calculating the detector response therefore requires knowledge of the often nonequilibrium distributions. The quasiparticle effective temperature provides a good approximation to these nonequilibrium distributions. We compare an analytical expression relating absorbed power and the quasiparticle effective temperature in superconducting thin films to full solutions for the nonequilibrium distributions, and find good agreement for a range of materials, absorbed powers, photon frequencies and temperatures typical of KIDs. This analytical expression allows inclusion of nonequilibrium effects in device models without solving for the detailed distributions. We also show our calculations of the frequency dependence of the detector response are in agreement with recent experimental measurements of the response of Ta KIDs at THz frequencies.