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TEION KOGAKU (Journal of Cryogenics and Superconductivity Society of Japan)

High Sensitivity Photon Sensors Relying on Superconducting Micro Resonators

Masato Naruse, Tomohiko Ando, Ritsuki KUBOTA, Masahiro KUWATA, Tohru Taino, Hiroaki Myoren

Published May 20, 2019• 0 citations
Publisher

Abstract

This paper reports how the superconducting micro-resonators work as high-sensitivity photon detectors, from millimeter-wave signals to high-energy particles. The resonators are called microwave kinetic inductance detectors and are suitable for use in a large format array. The resonators are easily fabricated applying a few photolithographic processes, and the resonator yield is more than 90%. Hundreds of superconducting resonators can be biased with a single readout line, enabling them to be read out together with frequency multiplexing. We introduce the recent progress of scientific projects relying on this technique, and summarize our results for millimeter-wave astronomy and a high-energy particle detection system. The device was made using epitaxial aluminum on a silicon wafer and its noise was as low as the background limit, approximately 6 × 10−18 W/Hz1/2 in the millimeter-wave range. For high-energy particle detection, we investigated two materials: niobium and a hightemperature superconductor, YBa2Cu3O7-δ. The niobium-based device detected the alpha line (5.4 MeV) events. The pulse decay time was approximately 5 μs and energy resolution was approximately 1 MeV.

DOI
10.2221/jcsj.54.174

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This paper reports how the superconducting micro-resonators work as high-sensitivity photon detectors, from millimeter-wave signals to high-energy particles. The resonators are called microwave kinetic inductance detectors and are suitable for use in a large format array. The resonators are easily fabricated applying a few photolithographic processes, and the resonator yield is more than 90%. Hundreds of superconducting resonators can be biased with a single readout line, enabling them to be read out together with frequency multiplexing. We introduce the recent progress of scientific projects relying on this technique, and summarize our results for millimeter-wave astronomy and a high-energy particle detection system. The device was made using epitaxial aluminum on a silicon wafer and its noise was as low as the background limit, approximately 6 × 10−18 W/Hz1/2 in the millimeter-wave range. For high-energy particle detection, we investigated two materials: niobium and a hightemperature superconductor, YBa2Cu3O7-δ. The niobium-based device detected the alpha line (5.4 MeV) events. The pulse decay time was approximately 5 μs and energy resolution was approximately 1 MeV.
Superconducting microwave resonators have the potential to revolutionize submillimeter and far-infrared astronomy, and with it our understanding of the universe. The field of low-temperature detector technology has reached a point where extremely sensitive devices like transition-edge sensors are now capable of detecting radiation limited by the background noise of the universe. However, the size of these detector arrays are limited to only a few thousand pixels. This is because of the cost and complexity of fabricating large-scale arrays of these detectors that can reach up to 10 lithographic levels on chip, and the complicated SQUID-based multiplexing circuitry and wiring for readout of each detector. In order to make substantial progress, next- generation ground-based telescopes such as CCAT or future space telescopes require focal planes with large-scale detector arrays of 104–106 pixels. Arrays using microwave kinetic inductance detectors (MKID) are a potential solution. These arrays can be easily made with a single layer of superconducting metal film deposited on a silicon substrate and pattered using conventional optical lithography. Furthermore, MKIDs are inherently multiplexable in the frequency domain, allowing ∼ 103 detectors to be read out using a single coaxial transmission line and cryogenic amplifier, drastically reducing cost and complexity. An MKID uses the change in the microwave surface impedance of a superconducting thin-film microresonator to detect photons. Absorption of photons in the superconductor breaks Cooper pairs into quasiparticles, changing the complex surface impedance, which results in a perturbation of resonator frequency and quality factor. For excitation and readout, the resonator is weakly coupled to a transmission line. The complex amplitude of a microwave probe signal tuned on-resonance and transmitted on the feedline past the resonator is perturbed as photons are absorbed in the superconductor. The perturbation can be detected using a cryogenic amplifier and subsequent homodyne mixing at room temperature. In an array of MKIDs, all the resonators are coupled to a shared feedline and are tuned to slightly different frequencies. They can be read out simultaneously using a comb of frequencies generated and measured using digital techniques. This thesis documents an effort to demonstrate the basic operation of ∼ 256 pixel arrays of lumped-element MKIDs made from superconducting TiNx on silicon. The resonators are designed and simulated for optimum operation. Various properties of the resonators and arrays are measured and compared to theoretical expectations. A particularly exciting observation is the extremely high quality factors (∼ 3 × 107) of our TiNx resonators which is essential for ultra-high sensitivity. The arrays are tightly packed both in space and in frequency which is desirable for larger full-size arrays. However, this can cause a serious problem in terms of microwave crosstalk between neighboring pixels. We show that by properly designing the resonator geometry, crosstalk can be eliminated; this is supported by our measurement results. We also tackle the problem of excess frequency noise in MKIDs. Intrinsic noise in the form of an excess resonance frequency jitter exists in planar superconducting resonators that are made on dielectric substrates. We conclusively show that this noise is due to fluctuations of the resonator capacitance. In turn, the capacitance fluctuations are thought to be driven by two-level system (TLS) fluctuators in a thin layer on the surface of the device. With a modified resonator design we demonstrate with measurements that this noise can be substantially reduced. An optimized version of this resonator was designed for the multiwavelength submillimeter kinetic inductance camera (MUSIC) instrument for the Caltech Submillimeter Observatory.
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.
Microwave Kinetic Inductance Detectors (MKIDs) are superconducting microresonators useful as detectors for sub/millimeter and far-infrared cameras, as well as instruments in optical and x-ray wavelengths. A probe sine wave at the resonance frequency can detect a change in frequency and microwave loss due to submillimeter optical load as a phase or amplitude change. These detectors may be coupled to light through on-chip, phased-array antennas. A single antenna may feed several detectors with different bandpasses defined by on-chip lumped-element filters. The resonators are easily frequency multiplexed by tuning the resonance frequencies, so hundreds of detectors can be read out while coupled to a single feedline. The deployment and operation of ground-based, background-limited detectors in close-packed arrays present significant challenges in both readout strategy and array design. The initial designs consisted of simple coplanar waveguide microwave resonators with aluminum absorptive sections. The first change has been to substitute the capacitive portion of the resonator with a large interdigitated capacitor (IDC) to reduce its noise contribution. While their low-noise characteristics have been demonstrated, these detectors had not yet been tested on array-scale devices. We present results from arrays of detectors designed for sub/millimeter imaging, tested for their optical response characteristics. In particular, we address challenges of microwave inter-resonator coupling, out-of-band resonator optical sensitivity, and noise reduction in the latest device designs. We show the results of stepped-impedance filters in preventing direct pickup of wide-band radiation, and new frequency spacing to limit crosstalk between resonators. We show direct comparison between olderdesign coplanar waveguide resonators and new IDC resonators under optical load, demonstrating the noise reduction and comparable responsivity. In addition, we show results of the probe signal readout power that maximizes signal-to-noise.
We demonstrate microwave kinetic inductance detectors (MKIDs) whose sensitivity is limited by photon noise at signal frequencies as low as 50 mHz. The subhertz part of the detection spectrum is important for contemporary millimeter-wave science, yet photon noise below 1 Hz in MKIDs has not previously been unambiguously demonstrated. These feedhorn-coupled detectors are sensitive over a 40% fractional bandwidth centered on λ = 2 mm, are dual-polarization-sensitive, and consist of lumped-element superconducting resonators fabricated from a hybrid of stoichiometric TiN and Al films together with an amorphous-Si passivation layer. When observing a 7 K, 3.8 pW thermal load and employing no signal modulation scheme, the measured noise spectrum is white down to 50 mHz and has an amplitude consistent with photon noise. These results have implications for the development of large-format, polarization-sensitive, millimeter-wave imagers.
Superconducting nanowires are widely used as sensitive single photon detectors with wide spectral coverage and high timing resolution. We describe a demonstration of an array of dc-biased superconducting nanowire single photon detectors read out with a microwave multiplexing circuit. In this design, each individual nanowire is part of a resonant LC circuit where the inductance is dominated by the kinetic inductance of the nanowire. The circuit also contains two parallel plate capacitors, one of them is in parallel with the inductor and the other is coupled to a microwave transmission line that carries the signals to a cryogenic low-noise amplifier. All of the nanowires are connected via resistors to a single dc bias line that enables the nanowires to be current biased close to their critical current. When a photon hits a nanowire, it creates a normal hotspot that produces a voltage pulse across the LC circuit. This pulse rings down at the resonant frequency of the LC circuit over a time period that is fixed by the quality factor. We present measurements of an array of these devices and an evaluation of their performance in terms of frequency and time response.
Kinetic inductance detectors (KIDs) provide an attractive solution to the production of large detector arrays for use in ground and space based 200μm astronomy. KIDs work by measuring the change in quasi-particle density upon photon absorption in a high Q superconducting resonator. A change in quasi-particle density is measured by a shift in phase of a microwave probe signal of frequency equal to that of the resonant frequency of the KID. Such detectors have a fundamental noise limit owing to the quasi-particle recombination rate, which, in a KID fabricated from a high quality Niobium film can give sensitivities of 10-18W√Hz at 1K. Constructing KIDs of varying resonant frequencies coupled to a single transmission line provides a multiplexed detector array with simple low temperature electronics. Here we discuss the theoretical requirements for both ground and space based 200μm cameras with various radiation coupling schemes for this wavelength range using distributed and lumped element high Q resonators.