Broadband illumination of superconducting pair breaking photon detectors

TL;DR

This paper extends the understanding of superconducting pair breaking photon detectors by analyzing how broadband illumination affects quasiparticle distributions, showing that above gap absorption effects are additive while sub gap effects increase quasiparticle generation efficiency.

Contribution

It introduces a method to calculate the impact of broadband sources on quasiparticle distributions, revealing differences from monochromatic sources especially at sub gap frequencies.

Findings

Above gap broadband absorption effects are additive and do not alter distribution structure.

Distribution averaged quantities like quasiparticle generation efficiency match weighted averages over bandwidth.

Sub gap broadband absorption increases quasiparticle generation efficiency at higher powers.

Abstract

Understanding the detailed behaviour of superconducting pair breaking photon detectors such as Kinetic Inductance Detectors requires knowledge of the nonequilibrium quasiparticle energy distributions. We have previously calculated the steady state distributions resulting from uniform absorption of monochromatic sub gap and above gap frequency radiation by thin films. In this work, we use the same methods to calculate the effect of illumination by broadband sources, such as thermal radiation from astrophysical phenomena or from the readout system. Absorption of photons at multiple above gap frequencies is shown to not change the structure of the quasiparticle energy distribution close to the superconducting gap. Hence for typical absorbed powers, we find the effects of absorption of broadband pair breaking radiation can simply be considered as the sum of the effects of absorption of many monochromatic sources. Distribution averaged quantities, like quasiparticle generation effciency $\eta$, match exactly a weighted average over the bandwidth of the source of calculations assuming a monochromatic source. For sub gap frequencies, however, distributing the absorbed power across multiple frequencies does change the low energy quasiparticle distribution. For moderate and high absorbed powers, this results in a significantly larger $\eta$ - a higher number of excess quasiparticles for a broadband source compared to a monochromatic source of equal total absorbed power. Typically in KIDs the microwave power absorbed has a very narrow bandwidth, but in devices with broad resonance characteristics (low quality factors), this increase in $\eta$ may be measurable.