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Superconductor Science and Technology

Noise equivalent power of a NbTiN-Al-based kinetic inductance detector for terahertz sensing

Runfeng Su, Lili Shi, Tiege Zhou, Bo-Zhi Yao, Jingbo Wu, Xuecou Tu, Xiaoqing Jia, Lin Kang, Biaobing Jin, Huabing Wang, Jian Chen, Peiheng Wu

Published Apr 1, 2022• 9 citations
Publisher

Abstract

Abstract We have designed and fabricated a superconducting hybrid NbTiN-Al kinetic inductance detector (KID) over a broad band centered at 4.3 THz. The detector sensitivity is characterized by changing the bath temperature in a dark environment. Both amplitude and phase readouts are performed. The measured electrical noise equivalent power for phase readout is about 2.6 × 10−18 W · Hz−0.5 at 200 mK. This ultra-sensitive KID, which enables large arrays with frequency domain multiplexing, is promising for imaging instruments in space-based far-infrared observatories.

DOI
10.1088/1361-6668/ac6348

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Abstract We have designed and fabricated a superconducting hybrid NbTiN-Al kinetic inductance detector (KID) over a broad band centered at 4.3 THz. The detector sensitivity is characterized by changing the bath temperature in a dark environment. Both amplitude and phase readouts are performed. The measured electrical noise equivalent power for phase readout is about 2.6 × 10−18 W · Hz−0.5 at 200 mK. This ultra-sensitive KID, which enables large arrays with frequency domain multiplexing, is promising for imaging instruments in space-based far-infrared observatories.
Future far-infrared space observatories equipped with cryogenic telescopes require ultra-sensitive detectors to enable measurements limited only by the sky background. We present the design, fabrication, and characterization of a superconducting kinetic inductance detector (KID) at 4.3 THz. The lumped-element KID comprises a tantalum interdigitated capacitor and a 100-nm-wide aluminum-line inductor connected in parallel. The inductor acts also as a low-volume absorber. We achieved photon-noise-limited performance with a noise equivalent power of 1.9 × 10−19 W/Hz referenced to absorbed blackbody power at a modulation frequency of 300 Hz. Under optical loading above 1.83 fW, the detector remains stable with white-noise spectra extending down to ∼100 mHz.
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.
We have measured and compared the response of hybrid NbTiN-Al Microwave Kinetic Inductance Detectors (MKIDs) to changes in bath temperature and illumination by sub-mm radiation. We show that these two stimulants have an equivalent effect on the resonance feature of hybrid MKIDs. We determine an electrical noise equivalent power (NEP) from the measured temperature responsivity, quasiparticle recombination time, superconducting transition temperature, and noise spectrum, all of which can be measured in a dark environment. For the two hybrid NbTiN-Al MKIDs studied in detail, the electrical NEP is within a factor of two of the optical NEP, which is measured directly using a blackbody source.
Abstract We studied and improved the performance of a terahertz (THz) direct detector based on the superconducting NbN hot electron bolometer (HEB) integrated with microwave readout for imaging arrays and other applications. A 6 nm thick NbN microbridge at the feed point of a twin-slot antenna is embedded into a high- Q quarter-wavelength coplanar waveguide (CPW) resonator made from a 350 nm thick NbN film which is weakly coupled to a CPW feedline. THz direct detection was performed at a signal frequency of 0.66 THz at a bath temperature of 4.2 K. When operating at a low input microwave power level, the device acting as a microwave kinetic inductance detector with a kinetic inductance fraction of 0.45 changes the surface impedance of the resonator in response to incident THz radiation. The estimated optical noise equivalent power ( NEP ) is ∼0.23 pW · Hz −0.5 which shows about one order of magnitude improvement compared to that of the traditional voltage-biased NbN HEBs. The improved performance of the THz direct detectors with lower NEP and easy readout makes them promising in imaging with a large number of the array pixels.
Microwave kinetic inductance detectors (MKIDs) promise high sensitivity, combined with device simplicity and intrinsic multiplexibility. We have measured the response and noise equivalent power (NEP) of an all-aluminium, antenna coupled MKID at 1.54 THz, both as a function of radiation power and as a function of readout power. Eight optical filters and a box-in-box configuration at the bath temperature, allow a large range of optical powers by varying the temperature of a black body source. In the range of 80 aW-700 fW the detector is photon-noise limited, with an optical efficiency of 48±8% for one polarisation, which is limited by the antenna design. At lower optical powers we reveal a saturation level in the NEP of 3.8×10-19W/Hz1/2, competitive with other detectors for this purpose and the lowest optical NEP so far for MKIDs. The noise over the entire power range is limited by photon- and generation-recombination noise, the fundamental noise sources for these detectors.