Subgap kinetic inductance detector sensitive to 85-GHz radiation

TL;DR

This paper introduces granular aluminum-based subgap kinetic inductance detectors (SKIDs) for 80-90 GHz radiation, demonstrating high sensitivity at 300 mK and operating based on subgap excitations, avoiding lower temperature requirements.

Contribution

The paper presents a novel SKID design sensitive to 80-90 GHz, utilizing subgap excitations, with improved operational simplicity and intrinsic frequency selectivity.

Findings

Achieved noise equivalent power of 1.3×10⁻¹⁶ W/Hz^0.5

Best NEP of 2.6×10⁻¹⁷ W/Hz^0.5 at 50 fW illumination

Intrinsic frequency selectivity below the superconducting gap

Abstract

We have fabricated an array of subgap kinetic inductance detectors (SKIDs) made of granular aluminum ($T_c\sim$2~K) sensitive in the 80-90 GHz frequency band and operating at 300~mK. We measure a noise equivalent power of $1.3\times10^{-16}$~W/Hz$^{0.5}$ on average and $2.6\times10^{-17}$~W/Hz$^{0.5}$ at best, for an illuminating power of 50~fW per pixel. Even though the circuit design of SKIDs is identical to that of the kinetic inductance detectors (KIDs), the SKIDs operating principle is based on their sensitivity to subgap excitations. This detection scheme is advantageous because it avoids having to lower the operating temperature proportionally to the lowest detectable frequency. The SKIDs presented here are intrinsically selecting the 80-90 GHz frequency band, well below the superconducting spectral gap of the film, at approximately 180 GHz.