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The KIDs are alright: improving uniformity, resolving power, and quantum efficiency of microwave kinetic inductance detector arrays for optical applications

Will H. Clay

Published Jun 4, 2026• 0 citations
OpenAlex

Abstract

MKIDs are a class of superconducting detector with high sensitivity, straightforward scalability, and inherent energy resolution. The applications for Optical MKIDs include astronomy, dark matter detection, photonic quantum computing, and biological process imaging. To motivate greater use in these applications, improvements to MKID yield and uniformity at the array scale, detection efficiency, dark counts, and resolving power are required. This dissertation details a variety of detector designs towards these ends. First, the performance of several styles of phonon blocking architectures is discussed: superconducting bilayers, polymer suspended inductors, and air-suspended inductors. Next, the characterization of the dark count rate of a large-format MKID array is presented, with some investigation of the sources of this background. Finally, a large-format array is presented which made improvements to the yield, resonator quality factor, and quantum efficiency of the previous state-of-the-art optical MKID camera. This includes the first incorporation of anti-reflection coatings on an MKID array of this size. These explorations will help inform further development of optical MKID arrays for a variety of science goals.

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MKIDs are a class of superconducting detector with high sensitivity, straightforward scalability, and inherent energy resolution. The applications for Optical MKIDs include astronomy, dark matter detection, photonic quantum computing, and biological process imaging. To motivate greater use in these applications, improvements to MKID yield and uniformity at the array scale, detection efficiency, dark counts, and resolving power are required. This dissertation details a variety of detector designs towards these ends. First, the performance of several styles of phonon blocking architectures is discussed: superconducting bilayers, polymer suspended inductors, and air-suspended inductors. Next, the characterization of the dark count rate of a large-format MKID array is presented, with some investigation of the sources of this background. Finally, a large-format array is presented which made improvements to the yield, resonator quality factor, and quantum efficiency of the previous state-of-the-art optical MKID camera. This includes the first incorporation of anti-reflection coatings on an MKID array of this size. These explorations will help inform further development of optical MKID arrays for a variety of science goals.
Abstract Microwave Kinetic Inductance Detectors (MKIDs) combine significant advantages for photon detection like single photon counting, single pixel energy resolution, vanishing dark counts and µs time resolution with a simple design and the feasibility to scale up into the megapixel range. But high quality MKID fabrication remains challenging as established superconductors tend to either have intrinsic disadvantages, are challenging to deposit or require very low operating temperatures. As alternating stacks of thin Ti and TiN films have shown very impressive results for far-IR and sub-mm MKIDs, they promise significant improvements for UV, visible to near-IR MKIDs as well, especially as they are comparably easy to fabricate and control. In this paper, we present our ongoing project to adapt proximity coupled superconducting films for photon counting MKIDs. Some of the main advantages of Ti/TiN multilayers are their good control of critical temperature (Tc) and their great homogeneity of Tc even over large wafers, promising improved pixel yield especially for large arrays. We demonstrate the effect different temperatures during fabrication have on the detector performance and discuss excess phase noise observed caused by surface oxidization of exposed Si. Our first prototypes achieved photon energy resolving powers of up to 3.1 but turned out to be much too insensitive. As the work presented is still in progress, we also discuss further improvements planned for the near future.
Microwave Kinetic Inductance Detectors (MKIDs), just like a planar resonance cavity resonating at a microwave frequency, are emerging as a kind of high-sensitivity detector suitable for large format arrays at terahertz (THz) wavelengths. There are two types of MKIDs, namely distributed (or antenna-coupled) MKIDs and lumped-element MKIDs (aka LeKIDs). Various superconducting thin films (such as Al, TiN, NbTiN, NbN, and Nb) have been investigated for MKIDs. They do work so long as the detected photon energy exceeds their energy gap, but their response and noise behaviors are yet to be fully understood. Here we report on the design, fabrication, and characterization of distributed and lumped-element MKIDs made of NbTiN superconducting film. Detailed simulation and measurement results will be presented.
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 μm 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.
We report on the design and performance of microwave kinetic inductance detectors (MKIDs) sensitive to single photons in the optical to near-infrared range using hafnium as the sensor material. Our test device had a superconducting transition temperature of 395 mK and a room temperature normal state resistivity of 97 μΩ cm with RRR=1.6. Resonators on the device displayed internal quality factors of around 200 000. Similar to the analysis of MKIDs made from other highly resistive superconductors, we find that modeling the temperature response of the detector requires an extra broadening parameter in the superconducting density of states. Finally, we show that this material and design is compatible with a full-array fabrication process, which resulted in pixels with decay times of about 40 μs and resolving powers of ∼9 at 800 nm.
Abstract We report on the experimental investigation of optical coupling for superconducting microresonators known as microwave kinetic inductance detectors (MKIDs) in the visible and near-infrared bands. MKIDs are photon-counting, time and energy-resolving detectors that still suffer from a poor quantum efficiency. To improve this efficiency, we propose to add a superconducting reflective layer below the absorbing part of the detector separated by a transparent Al2O3 layer with a quarter-wavelength thickness optimized around a single wavelength λ = 405 nm. We have first fabricated samples patterned from stoichiometric TiN ( T c ∼ 4 K), one with the full optical stack, one without for reference and one with a partial optical stack in order to characterize the noise influence of each layer individually. We observe that the full optical stack geometry has the most impact on the resonator’s noise and quality factors. A second design was fabricated to characterize the optical response to short pulses of the optical stack and we show from both the frequential noise and optical response that a strong signature of TLS is still present in the optical stack sample. We have finally obtained single-photon response with the optical stack using a more sensitive tri-layer TiN/Ti/TiN absorber ( T c ∼ 1.3 K) for which a maximum energy resolving power of R = E / Δ E ∼ 1.3 was achieved using 405 nm laser pulses at 225 mK. The quality factors of both the reference and optical stack samples are similar but the frequency noise is still a tenfold higher for the optical stack sample which degrades the energy-resolving power of the detector.
In the next decades millimeter and sub-mm astronomy requires large format imaging arrays and broad-band spectrometers to complement the high spatial and spectral resolution of the Atacama Large Millimeter/submillimeter Array. The desired sensors for these instruments should have a background limited sensitivity and a high optical efficiency and enable arrays thousands of pixels in size. Hybrid microwave kinetic inductance detectors consisting of NbTiN and Al have shown to satisfy these requirements. We present the second generation hybrid NbTiN-Al MKIDs, which are photon noise limited in both phase and amplitude readout for loading levels P850GHz < 10 fW. Thanks to the increased responsivity, the photon noise level achieved in phase allows us to simultaneously read out approximately 8000 pixels using state-of-the-art electronics. In addition, the choice of superconducting materials and the use of a Si lens in combination with a planar antenna gives these resonators the flexibility to operate within the frequency range 0:09 < v < 1:1 THz. Given these specifications, hybrid NbTiN-Al MKIDs will enable astronomically usable kilopixel arrays for sub-mm imaging and moderate resolution spectroscopy.