Membraneless Phonon Trapping and Resolution Enhancement in Optical Microwave Kinetic Inductance Detectors

Microwave kinetic inductance detectors (MKIDs) sensitive to light in the ultraviolet to near-infrared wavelengths are superconducting microresonators that are capable of measuring photon arrival times to microsecond precision and estimating each photon’s energy. The resolving power of nonmembrane MKIDs has remained stubbornly around 10 at $1\mu \mathrm{m}$ 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.

Figure 1

A microscope image of a hafnium optical kinetic inductance detector coupled to a coplanar waveguide feedline. The image has been given false color to highlight the functions of each part of the device. The dark areas are the bare sapphire substrate. Light is focused onto the $47\mu \mathrm{m}\times 34.7\mu \mathrm{m}$ inductor with a microlens, which allows arrays of these detectors to achieve near unity fill factors. An approximate scale bar is included for reference.

Figure 2

Top: plotted are the combined spectra for the single layer hafnium device at seven laser energies. The small low energy tail to each distribution is likely explained by quasiparticle diffusion into the insensitive capacitor. Bottom: The noise decomposition for this device is shown. The filled in green area represents the resolving powers achievable by reducing the phonon loss but keeping the same noise spectrum.

Figure 3

A schematic representation is shown of the phonon blocking layer (yellow) employed in this paper. The lack of available phonon states in the blocking layer prohibits high energy phonons (red and orange) generated during a photon absorption from escaping the detector material, allowing all of their energy to be measured. Lower energy phonons (blue) pass through the barrier freely. Similarly, the phonon blocking layer provides some protection to the detector from high energy, ionizing radiation absorbed in the substrate.

Figure 4

The combined spectra and noise decomposition for the bilayer device are shown, similar to that in Fig. 2. We see a large improvement in the resolving power which is explained by significantly less athermal phonon escape.