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Applied Physics Letters

Characterization of a tantalum-aluminum-based kinetic inductance detector for far-infrared space applications

Runfeng Su, Junhua Chen, Rui Tan, Yuwei Zhu, Tianyuan Chi, Siming Zang, Jingbo Wu, Xuecou Tu, Mei Yu, Jian Chen, Peiheng Wu

Published Oct 20, 2025• 2 citations
Publisher

Abstract

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.

DOI
10.1063/5.0288366

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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.
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.
As a newly developed pair-breaking superconducting detector, microwave kinetic inductance detectors are simple to integrate in the frequency domain and have already been used in astronomical detection and array imaging at the (sub)millimeter and optical wavelengths. For these applications, the dark noise level of kinetic inductance detector is one of the key performance indicators. Herein a noise power spectrum analysis method is introduced in detail, which can accurately and effectively analyze the noise spectrum of kinetic inductance detector in a wide frequency range. This method can well balance the noise spectrum resolution and variance performance, by taking the noise data at the resonance frequency with two sampling rates and setting the appropriate frequency resolutions for different frequency bands. This method is used to characterize and compare the noise of aluminum (Al) kinetic inductance detectors made from two different micromachining processes. We deposite a 25-nm-thick aluminum film on high-resistivity silicon substrate for one device, while place one silicon nitride (SiNx) film on the top and one on the bottom of the aluminum film for another device. It is found that the frequency noise of the device with two silicon nitride films is about 25% to 50% of the bare aluminum device. Using this double silicon nitride film fabrication technique, we further fabricate a few groups of lumped-element aluminum kinetic inductance detectors with various inductor and interdigitated capacitor designs. We investigate the noise properties of these devices at different microwave driven power and bath temperatures, and the experimental results show typical two-level system noise behaviors. Our work provides a standard method to characterize the noise power spectrum of kinetic inductor detector, and also paves the way to developing low-noise aluminum kinetic inductance detectors for terahertz imaging, photon-counting and energy-resolving applications.
Abstract The Cooper-pair breaking efficiency quantifies the effectiveness with which incident energy, such as that delivered by photons or phonons, is converted into quasiparticles—a process central to the response of superconducting pair-breaking detectors. We measure the pair-breaking efficiency in two far-infrared kinetic inductance detectors (KIDs) operating at 4.3 THz and 15 THz by comparing photon noise and thermal generation-recombination noise. Both KIDs are lumped-element devices consisting of a tantalum interdigitated capacitor in parallel with an aluminum inductor meandered line, which also acts as a low-volume absorber. The measured pair-breaking efficiencies are 0.42 and 0.07 for the 4.3 THz and 15 THz devices, respectively. The extremely low efficiency at 15 THz, a frequency well above the aluminum Debye frequency, is mainly attributed to rapid anharmonic decay of phonons, with additional phonon relaxation in the tantalum layer further reducing the available phonon population for pair-breaking. Furthermore, a relationship between the pair-breaking efficiency and detector resolving power is derived.
Focal plane arrays consisting of low-noise, polarisation-sensitive detectors have made possible the pioneering advances in the study of the cosmic microwave background (CMB). To make further progress, the next generation of CMB experiments (e.g. CMB-S4) will require a substantial increase in the number of detectors compared to current instruments. Arrays of kinetic inductance detectors (KIDs) provide a possible path to realising such large-format arrays owing to their intrinsic multiplexing advantage and relative cryogenic simplicity. In this paper, we report on the design of a variant of the traditional KID design: the antenna-coupled lumped-element KID. A polarisation-sensitive twin-slot antenna placed behind an optimised hemispherical lens couples power onto a thin-film superconducting microstrip line. The power is then guided into the inductive section of an aluminium KID, where it is absorbed and modifies both the resonant frequency and quality factor of the KID. We present the various aspects of the design and preliminary results from the first set of seven-element prototype arrays and compare to the expected modelled performance.
Kinetic inductance detectors (KIDs) provide a promising solution to the problem of producing large format arrays of ultra sensitive detectors for astronomy. Traditionally KIDs have been constructed from superconducting quarter-wavelength or half-wavelength resonator elements capacitively coupled to a co-planar feed line. Photons are detected by measuring the change in quasi-particle density caused by the splitting of Cooper pairs in the superconducting resonant element. This change in quasi-particle density alters the kinetic inductance, and hence the resonant frequency of the resonant element. This arrangement requires the quasi-particles generated by photon absorption to be concentrated at positions of high current density in the resonator. This is usually achieved through antenna coupling or quasi-particle trapping. For these detectors to work at wavelengths shorter than around 500 mum where antenna coupling can introduce a significant loss of efficiency, then a direct absorption method needs to be considered [3]. One solution to this problem is the Lumped Element KID (LEKID), which shows no current variation along its length and can be arranged into a photon absorbing area coupled to free space and therefore requiring no antennas or quasi-particle trapping. This paper outlines the basic concept of the LEKID, along with theoretical performance and experimental data for these devices.
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.