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

Superconducting quantum capacitance detectors at terahertz waveband

Jian Chen, Lili Shi, Tianyuan Chi, Jingbo Wu, Peiheng Wu

Published Jun 14, 2024• 0 citations
Publisher

Abstract

Single photon detectors (SPDs) at terahertz (THz) waveband are necessary for the THz applications. The superconducting quantum capacitance detectors (QCDs) are proved to be ultra-sensitive at THz waveband. 1.5 THz single photon detection has been achieved. Besides their sensitivity, large array of QCDs has also developed. Therefore, it is convenient of QCDs to achieve multiplexing through a microwave feedline like MKIDs. A QCD consists of a high quality factor (Q) microwave superconducting resonators and a single cooper-pair box (SCB). The microwave resonators are fabricated using tantalum (Ta) or niobium (Nb). The SCB is made of aluminum (Al). The ground electrode of the SCB is designed to be an absorber at THz waveband. The absorber is an inductive absorber consisting of orthogonal Al nanowires. Finally, we succeeded in measuring the periodic quantum capacitance signals. However, the responsivity is relatively low due to its weak coupling to the resonators of SCB. In the future, we need to optimize the performance of the detectors. QCDs are a prospect candidate of the next-generation astronomy observation and so on.

DOI
10.1117/12.3020906

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Single photon detectors (SPDs) at terahertz (THz) waveband are necessary for the THz applications. The superconducting quantum capacitance detectors (QCDs) are proved to be ultra-sensitive at THz waveband. 1.5 THz single photon detection has been achieved. Besides their sensitivity, large array of QCDs has also developed. Therefore, it is convenient of QCDs to achieve multiplexing through a microwave feedline like MKIDs. A QCD consists of a high quality factor (Q) microwave superconducting resonators and a single cooper-pair box (SCB). The microwave resonators are fabricated using tantalum (Ta) or niobium (Nb). The SCB is made of aluminum (Al). The ground electrode of the SCB is designed to be an absorber at THz waveband. The absorber is an inductive absorber consisting of orthogonal Al nanowires. Finally, we succeeded in measuring the periodic quantum capacitance signals. However, the responsivity is relatively low due to its weak coupling to the resonators of SCB. In the future, we need to optimize the performance of the detectors. QCDs are a prospect candidate of the next-generation astronomy observation and so on.
Superconducting quantum capacitance detectors (QCDs) have emerged as among the most sensitive terahertz (THz) photon detectors, positioning them as promising detector components for next-generation cryogenic telescopes. However, their limited dynamic range (DR, ∼32 dB) and low saturation power (∼0.1 fW) have hindered their broader applications, such as in THz astronomy, high-resolution spectroscopy, and microscopic imaging. Here, we propose a mechanism that enables controllable adjustment of the sensitivity and DR of THz QCDs, supported by both theoretical modeling and experimental validation. We developed a QCD design that features a fork-shaped gate capacitor and a high-efficiency grid absorber. The resulting devices exhibit a noise equivalent power of 5.0 × 10−20 W/Hz1/2 and a DR of 40 dB, providing a favorable balance between sensitivity and DR and advancing the practical application of QCDs in future THz applications.
The terahertz regime, as a last radio window, remains to be fully explored, and astronomical and atmospheric observations in this regime are scientifically important. Like other frequency regimes, developing high-sensitivity detectors (coherent and incoherent) is of particular significance for both ground-based and space-borne facilities. As the coherent detector of choice below 1.4 THz, superconductor-insulator-superconductor (SIS) heterodyne mixers have achieved as high a sensitivity as five times the quantum limit around 1.4 THz. It is, however, still a challenge to developing SIS mixers at frequencies beyond 1.4 THz with considerable transmission loss in superconducting circuits due to the Cooper-pair breaking by energetic photons and increased many difficulties in designing and fabricating. So far, superconducting hot electron bolometer (HEB) mixers have been the most sensitive heterodyne detectors at frequencies above 1.5 THz, and successfully used to detect molecular spectral lines up to 2.5 THz from ground-based and space telescopes. Although spiral-antenna coupled NbN HEB mixers show a good sensitivity in the whole THz frequency range, the directly measured spectral response with Fourier transform spectrometer falls quickly as frequency increases, especially above 3 THz. The terahertz band is also of particular importance to observe astronomical objects such as cosmic microwave background, early distant objects, cold objects and dusty objects. Aiming at such objects, we develop a terahertz imaging array system by combining advanced superconducting detectors such as transition edge sensor (TES) and microwave kinetic inductance detectors (MKIDs), thus the system has a frequency band centred at 350 m, an operational temperature of 0.3 K, and a sensitivity reaching background limit performance for ground-based applications. In addition, it is expected to have some breakthroughs in ultra-sensitive superconducting TES and MKID, low noise multi-channel readout and multiplexing, efficient terahertz-wave coupling technology, and large-scale array system integration. The developed terahertz imaging array system will serve as the next-generation instrument of Dome A 5 m terahertz telescope, conducting a 350 m-band legacy survey for studying the planets, stars, galaxies and cosmology. Besides the application in astronomy, the developed terahertz imaging array system can also be applied to some areas requiring rapid detection such as security, deep space exploration, and biomedical imaging. In this paper, we mainly introduce the superconducting detectors developed at Purple Mountain Observatory and those for international collaborative projects.
We propose a sensitive new detector based on Cooper pair breaking in a superconductor. The quantum capacitor detector (QCD) exploits the extraordinary sensitivity of superconducting single-electron devices to the presence of quasiparticles generated by pair-breaking photons. This concept would enable single-photon detection at far-IR and sub-millimeter frequencies with detector sensitivities that exceed that of transition-edge-sensor bolometers (TES), kinetic inductance detectors (KID), and superconducting tunnel junction detectors (STJ). The detectors we propose are based on the single Cooper pair box (SCB), a mesoscopic superconducting device that has been successfully developed at JPL for applications in quantum computing. This concept allows for frequency multiplexing of a large number of pixels using a single RF line, and does not require individual bias of each pixel. The QCD is ideal for the sensitive spectrographs considered for upcoming cold space telescopes, such as BLISS for SPICA in the coming decade, and for the more ambitious instruments for the SAFIR / CALISTO and SPIRIT / SPECS missions envisioned for the 2020 decade. These missions require large detector arrays (> 10,000 elements) which are limited by astrophysical background noise, corresponding to a noise-equivalent power (NEP) as low as 2x10-20 W / Hz1/2. Given its intrinsic response time, the QCD could also be used for energy-resolved visible photon detection, with estimated energy resolution > 100, enabling imaging low-resolution spectroscopy with an array of detectors.
We present a proposed design for a pair-breaking photodetector for far-infrared and submillimeter radiation. Antenna-coupled radiation generates quasiparticles in a superconducting absorber, the density of which are measured using a single Cooper-pair box. Readout is performed using an electromagnetic oscillator or a microwave resonator, which is well suited for frequency multiplexing in large arrays. Theoretical limits to detector sensitivity are discussed and modeled, with predicted sensitivities on the order of ${10}^{\ensuremath{-}21}\text{ }\text{W}/\sqrt{\text{Hz}}$. We anticipate that this detector can be used to address key scientific goals in far-infrared and submillimeter astronomy.