TLS and Quasiparticle Loss in Thin-Film Aluminum CPW Resonators: A Modified Model and Design Implications

TL;DR

This paper investigates loss mechanisms in aluminum CPW resonators used in superconducting detectors, presenting measurements, a modified loss model, and implications for device design.

Contribution

It introduces a modified loss model to better describe low-temperature, low-power behavior of superconducting resonators, informing improved device design.

Findings

Resonators achieved quality factors of approximately 3.64-8.57×10^{-8}.

Enhanced TLS loss suppression observed at intermediate-to-high powers.

Deviations from standard TLS loss model at temperatures below 60 mK.

Abstract

As superconducting kinetic inductance detectors (KIDs) continue to grow in popularity for sensitive submillimeter detection and other applications, there is a drive to advance toward lower-loss devices. We present measurements of diagnostic thin-film aluminum coplanar waveguide (CPW) resonators designed to inform ongoing KID development at NASA Goddard Space Flight Center. The resonance frequencies span $f_0$ = 3.5-4 GHz and include quarter-wave and half-wave resonators with varying coupling capacitor designs. We present measurements of the device film properties and an analysis of the dominant mechanisms of loss in the resonators measured in a dark environment, demonstrating quality factors of $Q_i^{-1} \approx 3.64-8.57\ \times 10^{-8}$. We observe an enhanced level of suppression in the loss contributions from two-level systems (TLS) at intermediate-to-high read powers, and a regime at these powers and low temperatures where contributions from intrinsic processes $Q_i^{-1}$,other dominate the total loss. We also observe deviations from the standard TLS loss model at low powers and temperatures below 60 mK, and use a modified model to describe this behavior.