Membrane-less phonon trapping and resolution enhancement in optical microwave kinetic inductance detectors

TL;DR

This paper demonstrates that a simple bilayer design in membrane-less MKIDs can roughly double their resolving power by preventing high energy phonons from entering the device, thus enhancing photon energy resolution.

Contribution

The study introduces a novel membrane-less bilayer design that improves MKID resolution without increasing fabrication complexity.

Findings

Resolving power roughly doubled with bilayer design

High energy phonons are blocked from entering the additional layer

Modeling shows phonon state availability limits phonon entry

Abstract

Microwave Kinetic Inductance Detectors (MKIDs) sensitive to light in the ultraviolet to near-infrared wavelengths are superconducting micro-resonators that are capable of measuring photon arrival times to microsecond precision and estimating each photon's energy. The resolving power of non-membrane MKIDs has remained stubbornly around 10 at 1 $\mu$m despite significant improvements in the system noise. Here we show that the resolving power can be roughly doubled with a simple bilayer design without needing to place the device on a membrane, avoiding a significant increase in fabrication complexity. Based on modeling of the phonon propagation, we find that the majority of the improvement comes from the inability of high energy phonons to enter the additional layer due to the lack of available phonon states.