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arXiv:1408.3314 · Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Performance of hybrid NbTiN-Al microwave kinetic inductance detectors as direct detectors for sub-millimeter astronomy

R. M. J. Janssen, J. J. A. Baselmans, Akira Endo, L. Ferrari, S. J. C. Yates, Andrey M. Baryshev, T. M. Klapwijk

Published Jul 23, 2014• 14 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1117/12.2055537

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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 present results from completely lithographic antenna-coupled Microwave Kinetic Inductance detectors (MKIDs). MKIDs are superconducting resonators with resonant frequency and quality-factor which are highly sensitive to changes in the density of the quasiparticle population which occurs when photons above the superconducting gap energy are absorbed. The resonators are coupled to submm light through on-chip phased-array slot antennas. Each planar antenna consists of an array of N long slots which are fed along their length at M points. The resulting N*M feed points are combined in-phase using a binary summing tree made of low-loss superconducting microstrip lines. Due to its large size, the resulting planar antenna produces a narrow beam pattern and can therefore be used without additional optical coupling elements such as feedhorns or substrate lenses. The output of the antenna is a single superconducting thin-film microstrip line which can be efficiently coupled to one (or more) kinetic inductance coplanar waveguide resonators to produce a single (or multi-color) pixel in an imaging focal plane array, using in-line lumped element lithographic band-pass filters. Such highly integrated architectures can be easily fabricated on a single substrate, and many detectors can be frequency multiplexed through coupling to a single feedline. Microwave readout provides a lot of bandwidth per detector, allowing a large number of pixels to be read using a single cryogenic microwave amplifier and warm readout electronics.
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.
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.
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 in both phase and amplitude readout in microwave kinetic inductance detectors (MKIDs) consisting of NbTiN and Al, down to 100 fW of optical power. We simulate the far field beam pattern of the lens-antenna system used to couple radiation into the MKID and derive an aperture efficiency of 75%. This is close to the theoretical maximum of 80% for a single-moded detector. The beam patterns are verified by a detailed analysis of the optical coupling within our measurement setup.