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Millimeter, Submillimeter, and Far-Infrared Detectors and Instrumentation for Astronomy XI

Millimeter-wave kinetic inductance detectors with no 1/f noise

Johannes Hubmayr, Jason E. Austermann, Bradley Dober, Gene C Hilton, Jiansong Gao, M. Malnou, Joel N. Ullom, Michael R. Vissers, Jordan Wheeler

Published Dec 12, 2020• 1 citation
Publisher

Abstract

We demonstrate microwave kinetic inductance detectors (MKIDs) whose sensitivity is limited by photon noise at 50 mHz signal frequencies. The sub-Hz part of the detection spectrum is important for millimeter-wave instruments, yet photon noise below 1 Hz in MKIDs has not previously been demonstrated unambiguously. Devices are feedhorn-coupled, direct-absorber-type and consist of lumped-element superconducting resonators fabricated from a hybrid of stoichiometric TiN, Al, and an amorphous-Si passivation layer. Viewing a 7K thermal load and a passband centered on 2mm, the device noise spectrum is white down to 50 mHz and has an amplitude consistent with photon noise. We will present these measurements and discuss the multiple design choices that lead to the result.

DOI
10.1117/12.2562870

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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.

•Nov 24, 2021•0 citations
We demonstrate microwave kinetic inductance detectors (MKIDs) whose sensitivity is limited by photon noise at 50 mHz signal frequencies. The sub-Hz part of the detection spectrum is important for millimeter-wave instruments, yet photon noise below 1 Hz in MKIDs has not previously been demonstrated unambiguously. Devices are feedhorn-coupled, direct-absorber-type and consist of lumped-element superconducting resonators fabricated from a hybrid of stoichiometric TiN, Al, and an amorphous-Si passivation layer. Viewing a 7K thermal load and a passband centered on 2mm, the device noise spectrum is white down to 50 mHz and has an amplitude consistent with photon noise. We will present these measurements and discuss the multiple design choices that lead to the result.
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.
We demonstrate photon-noise limited performance at sub-millimeter wavelengths in feedhorn-coupled, microwave kinetic inductance detectors made of a TiN/Ti/TiN trilayer superconducting film, tuned to have a transition temperature of 1.4 K. Micro-machining of the silicon-on-insulator wafer backside creates a quarter-wavelength backshort optimized for efficient coupling at 250 μm. Using frequency read out and when viewing a variable temperature blackbody source, we measure device noise consistent with photon noise when the incident optical power is >0.5 pW, corresponding to noise equivalent powers >3×10−17 W/Hz. This sensitivity makes these devices suitable for broadband photometric applications at these wavelengths.
Context. Microwave kinetic inductance detectors (MKIDs) are superconducting detectors that have the potential to contribute to the construction of over 10 000 pixel cameras required for mm and submm astronomical observations over the next decade. One of the issues related to its application is two-level-system (TLS) noise that creates a 1/ f term in the power spectrum density (PSD) of the detectors. A hybrid-type MKID has been proposed for TLS noise suppression without affecting the MKID performance. In this paper, we present a systematic method to model a hybrid-type MKID for mitigating TLS noise while achieving high sensitivity. Aims. Previous studies have shown that TLS noise in MKIDs depends on the sampling frequency, internal power, device geometry, and temperature. The strength of the electrical field distribution along the resonator line also affects the TLS noise properties. Based on this knowledge, we constructed a noise model of a hybrid-type MKID. Methods. Using this model, we designed MKIDs for the cosmic microwave background radiation experiment, GroundBIRD. We evaluated the 1/ f noise properties and NET of the fabricated devices using the on-site measurements. Based on the NET measurements, we can confirm that the white noise of fabricated detectors is limited by atmospheric photon noise. From the 1/ f noise measurements, we also confirm that it is possible to reach an atmospheric photon noise limit down to a low sampling frequency (i.e., of a few Hz) at the typical observation conditions from Teide observatory. Results. We confirm the design methodology we propose for hybrid-type MKID works well in optimizing the TLS model to the hybrid-type MKID, as evidenced by performance verification measurements carried out in the laboratory and on site.
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 (MKID) are superconducting broadband direct detectors. If a superconducting thin film patterned into a resonator is illuminated with photons, Cooper pairs break up and the kinetic inductance of the film changes. This shifts the resonance frequency downwards and the dissipation of the resonator increases, which can be detected in the phase and amplitude of a transmitted signal. \n \nIn this thesis, a waveguide coupled MKID is designed and simulated. 350 GHz radiation is coupled into a copper waveguide via a horn antenna. A waveguide probe leads the signal onto a 9 µm silicon membrane where it is absorbed in a 40 nm superconducting aluminum film which is the inductive part of a superconducting resonator. The capacitor of the resonator is made of niobium. Two versions of the device are developed, a transmission line resonator and a lumped element resonator design. The new design will enable an accurate understanding of the optical sensitivity of MKIDs and allow comparison of different resonator materials. Since two different resonator types are used, the direct comparison of transmission line and lumped resonators will also be possible. \n \nA measurement setup is developed including an ADR cryostat to cool MKID devices down to below 40 mK. An analog readout system using either a vector network analyzer or an IQ mixer is designed and built. The necessary software to control the ADR, the temperature monitoring, the readout signal generation and detection and the signal acquisition of the setup is also developed. \n \nThe waveguide coupled MKID design is fabricated in our in-house microfabrication laboratory and measured in the new setup. In my measurements, the devices do not show a resonance feature. The VNA and DC measurements are analyzed to determine a plausible hypothesis of the failure. The devices are diagnosed for critical issues during fabrication which will be addressed in a future iteration. \n \nTo verify the measurement setup, a test device of the IRAM NIKA 2 MKID camera is measured and the complete analysis pipeline is exercised. The quasi-particle lifetime in the aluminum thin films is measured and a dark and optical noise equivalent power are determined. The results are within the uncertainty comparable to measurements of IRAM themselves. \n \nA test chip designed in my master thesis that contains eight NbTiN resonators is measured in the new setup and non-linear effects under strong readout power are studied. Parametric amplification and oscillation of a resonator are shown qualitatively.
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.