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Superconductor Science and Technology

Terahertz response of NbN-based microwave kinetic inductance detectors with rewound spiral resonator

Seiichiro Ariyoshi, Kensuke Nakajima, Atsushi SAITO, Tohru Taino, Chiko Otani, Hironao YAMADA, Shigetoshi Ohshima, Jongsuck Bae, S Tanaka

Published Feb 2, 2016• 8 citations
Publisher

Abstract

Abstract We have developed microwave kinetic inductance detectors (MKIDs) consisting of dual-purpose rewound spirals (spiral-MKIDs) made of a metal nitride superconductor for terahertz imaging applications. An NbN-based spiral-MKID array with 25 pixels was fabricated, and the fundamental performance of the terahertz receptivity was evaluated at the equilibrium temperature of a conventional 4 He refrigerator (≃3 K). The microwave transmission property of the array showed the 25 resonance lines, equally separated by frequency, in the microwave range from 2.70 to 2.95 GHz. To evaluate its optical performance as a terahertz detector, we measured the spiral-MKID’s spectral response, response time, and noise equivalent power (NEP), and confirmed that it revealed a broad spectral response from 1 to 9 THz and an effective response time of approximately 80 μ s. The noise-equivalent power was estimated to be .

DOI
10.1088/0953-2048/29/3/035012

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Abstract We have developed microwave kinetic inductance detectors (MKIDs) consisting of dual-purpose rewound spirals (spiral-MKIDs) made of a metal nitride superconductor for terahertz imaging applications. An NbN-based spiral-MKID array with 25 pixels was fabricated, and the fundamental performance of the terahertz receptivity was evaluated at the equilibrium temperature of a conventional 4 He refrigerator (≃3 K). The microwave transmission property of the array showed the 25 resonance lines, equally separated by frequency, in the microwave range from 2.70 to 2.95 GHz. To evaluate its optical performance as a terahertz detector, we measured the spiral-MKID’s spectral response, response time, and noise equivalent power (NEP), and confirmed that it revealed a broad spectral response from 1 to 9 THz and an effective response time of approximately 80 μ s. The noise-equivalent power was estimated to be .
We designed and fabricated a microwave kinetic inductance detector (MKID) using a niobium nitride (NbN) symmetric spiral resonator array. Previously we revealed that a rewound spiral structure works as not only a high-Q half-wavelength resonator but also as a broadband terahertz antenna. We conducted simulations for a 9 resonator array assuming NbN as the superconducting material and sapphire as the dielectric substrate, and obtained a maximum attenuation of over 30 dB and unloaded quality factors of over 2 × 10 5 for frequencies between 4.4 and 4.9 GHz. We fabricated the 9 resonator array MKID using NbN thin film deposited on an m-sapphire substrate by using dc magnetron sputtering. We observed half-wavelength resonances of around 4.5 GHz at 4 K. We measured the optical response of the MKID. The frequency shift was 0.5 MHz when illuminated with 650 nm photons.
We propose a novel microwave kinetic inductance detector (MKID) named spiral-MKID typified by a rewound spiral strip structure suitable for terahertz applications. The superconducting rewound strip works as a high-Q half wavelength microwave resonator as well as a broadband terahertz antenna. The spiral-MKIDs fabricated with NbN films reveal loaded quality factors of the order of 104 at 3 K. NbN is one of the promising materials suitable for working on a convenient 4K-refrigerator. We also discuss the expected noise equivalent power of the optimized detector.
We propose a YBCO-based microwave kinetic inductance detector with rewound strip structure (spiral-MKID) suitable for imaging applications. The superconducting rewound strip acts as a microwave resonator and broadband antenna. A 25-pixel MKID array with a width of 10 μm and slightly varying length of each of the resonators was fabricated on a 10 × 10 mm 2 -MgO substrate. Microwave resonance characteristics of the array were evaluated by measuring one of the scattering-matrix elements, S 21 , using a vector network analyser. The 25 resonance dips were clearly observed around 5 GHz; the frequency spacing between adjacent pixels was 13 MHz, with a standard deviation of 5 MHz. Each resonance frequency was shifted by 170 MHz, owing to the temperature change from 11 K to 50 K. We showed that the YBCO-based spiral-MKIDs with 200 nm-thick films have loaded quality factors of 1,200 at 11 K and 700 at 50 K. We irradiated visible light to a single pixel on the array. The obtained noise equivalent power was on the order of 10 −9 W/Hz 1/2 , while the response time was smaller than 30 ms at 13 K. The responsivity of the array at 70 K was six times larger than that at 13 K.
We analysed, fabricated, and characterised niobium nitride (NbN)-based microwave kinetic inductance detectors (MKIDs) with a rewound strip structure (spiral-MKIDs). To control resonance characteristics precisely, the linewidth ( w ) dependence of rewound spiral resonators varying between w = 10, 20, and 40 μm was analysed using an electromagnetic field simulator; the resonance frequencies ( f 0 's) were obtained as 4.42, 4.58, and 5.15 GHz respectively, proving that the inductance decreases as the w increases. To verify the simulation results experimentally, NbN-based MKID arrays with different linewidths were fabricated on 10 × 10 mm 2 sapphire substrates, and cooled down to 3 K. The microwave characteristics with a vector network analyser, and f 0 's were found to be 4.11, 4.23, and 4.85 GHz for the MKIDs with w = 10, 20, and 40 μm, respectively. These values are in agreement with the simulation results within 7.6% accuracy. The other resonance characteristics such as the scattering-matrix element ( S 21 ) and loaded quality factor ( Q L ) showed the similar w dependence with the simulation. Spiral-MKIDs with narrower w is expected to have better detector performance owing to shallower S 21 and higher Q L .
Abstract We designed, fabricated, and characterized microwave transmission properties with rewound strip structures for YBa2Cu3O7 (YBCO)-based kinetic inductance detectors (KIDs). The superconducting rewound strip serves as a microwave resonator and as a broadband terahertz-wave antenna. To predict the microwave resonance characteristics before fabrication, the line-width (w) and space (s) dependence of the spiral resonators were analyzed using an electromagnetic simulator; the resonance frequency increased, and the quality factor decreased with increasing w and s from 10 to 40 μm. YBCO-based KID arrays with different w (10 and 40 μm) were fabricated on 10 mm-square MgO substrates, cooled to 3 K using a 4He refrigerator, and evaluated using a vector network analyzer to verify the result of the simulation experimentally. The measured resonance frequency ratio of 1.11 times (5.04 → 5.59 GHz) agreed with the simulated ones of 1.10 times (4.84 → 5.33 GHz) between w = 10 and 40 μm. The other resonance characteristics, such as transmission coefficient and quality factor, have a similar w dependence with the simulation.
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.