We investigated the relationship between loaded quality factor (QL) and responsivity of microwave kinetic inductance detectors (MKIDs) using a dual-function spiral strip (DFSS). Three types of DFSSs with different QL values were designed, and the S21properties were simulated when kinetic inductance of a superconductor was changed by using a 2.5-dimentional simulator. The simulated results showed that a very high QLvalue was not necessarily required. The resonator frequencies of the 25-MKID array using a better type of DFSS (QL~ 3500) were adjusted to be separated by 5 MHz around 2.0 GHz. The 25-MKID array were fabricated using 20-nm thick niobium nitride films deposited on r-plane sapphire substrates. We clearly observed 25 half-wavelength resonances around 2.0 GHz at 2.245 K. The optical responses were also confirmed using hot and cold radiation. The dependence of the responsivity on the QLindicated that the QLvalue of approximately 2000 could obtain higher responsivity.
We investigated the relationship between loaded quality factor (QL) and responsivity of microwave kinetic inductance detectors (MKIDs) using a dual-function spiral strip (DFSS). Three types of DFSSs with different QL values were designed, and the S21properties were simulated when kinetic inductance of a superconductor was changed by using a 2.5-dimentional simulator. The simulated results showed that a very high QLvalue was not necessarily required. The resonator frequencies of the 25-MKID array using a better type of DFSS (QL~ 3500) were adjusted to be separated by 5 MHz around 2.0 GHz. The 25-MKID array were fabricated using 20-nm thick niobium nitride films deposited on r-plane sapphire substrates. We clearly observed 25 half-wavelength resonances around 2.0 GHz at 2.245 K. The optical responses were also confirmed using hot and cold radiation. The dependence of the responsivity on the QLindicated that the QLvalue of approximately 2000 could obtain higher responsivity.
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 .
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 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.
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 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.
Microwave crosstalk between rewound-spiral resonators was estimated for the optimal design of a microwave kinetic inductance detectors (MKIDs) array. An electromagnetic simulator (sonnet EM) was used to calculate the frequency responses. First, we analyzed the microwave interactions by changing the geometrical distance, Drr, between the centers of two resonators and the frequency interval, Δf, placed in the horizontal direction and the vertical direction. To analyze the microwave crosstalk, we changed the kinetic inductance, Lk, at one resonator, and we estimated the resonant frequencies. Our simulated results reveal that the microwave crosstalk was less than 0.22% placed on the horizontal and vertical directions with Drr= 1000 μm and Δf = 9 MHz. We then used these elements to design a 25-element spiral-MKIDs array and fabricated it by using an NbN thin film deposited on an r-sapphire substrate. The crosstalk was estimated using a vector network analyzer and a signal generator for pumping to the single element. In the measured results, 25 resonant dips were clearly observed in the frequency range between 2.70 and 2.95 GHz at 3.45 K. Furthermore, we could estimate the crosstalk between the resonators and found it to be less than 10-4% in both horizontal and vertical patterns.