Resolving Power of Visible to Near-Infrared Hybrid $\beta$-Ta/NbTiN Kinetic Inductance Detectors

TL;DR

This paper investigates a hybrid superconducting KID design with a beta-Ta/NbTiN structure, demonstrating a resolving power limited to 4.9-5.9 in the visible to near-infrared range, and explores improvements for high-resolution photon detection.

Contribution

It introduces a novel hybrid beta-Ta/NbTiN KID design with micro-lenses and models phase saturation effects to enhance resolving power understanding.

Findings

Average intrinsic quality factor $Q_i$ of 4.3×10^5

Resolving power limited to 4.9-5.9 by phase saturation

Linear response coordinate system increases resolving power to 5.9 at 402 nm

Abstract

Kinetic Inductance Detectors (KIDs) are superconducting energy-resolving detectors, sensitive to single photons from the near-infrared to ultraviolet. We study a hybrid KID design consisting of a beta phase tantalum ($\beta$-Ta) inductor and a NbTiN interdigitated capacitor (IDC). The devices show an average intrinsic quality factor $Q_i$ of 4.3$\times10^5$ $\pm$ 1.3 $\times10^5$. To increase the power captured by the light sensitive inductor, we 3D-print an array of 150$\times$150 $\mu$m resin micro lenses on the backside of the sapphire substrate. The shape deviation between design and printed lenses is smaller than 1$\mu$m, and the alignment accuracy of this process is $\delta_x = +5.8 \pm 0.5$ $\mu$m and $\delta_y = +8.3 \pm 3.3$ $\mu$m. We measure a resolving power for 1545-402 nm that is limited to 4.9 by saturation in the KID's phase response. We can model the saturation in the phase response with the evolution of the number of quasiparticles generated by a photon event. An alternative coordinate system that has a linear response raises the resolving power to 5.9 at 402 nm. We verify the measured resolving power with a two-line measurement using a laser source and a monochromator. We discuss several improvements that can be made to the devices on a route towards KID arrays with high resolving powers.