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arXiv:1709.02997 · Applied Physics Letters

Superconducting noise bolometer with microwave bias and readout for array applications

Artem A. Kuzmin, Alexei D. Semenov, S. V. Shitov, Michael Merker, S. Wünsch, Alexey Vladimirovich Ustinov, M. Siegel

Published Jul 24, 2017• 8 citations
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Abstract

We present a superconducting noise bolometer for terahertz radiation, which is suitable for large-format arrays. It is based on an antenna-coupled superconducting micro-bridge embedded in a high-quality factor superconducting resonator for a microwave bias and readout with frequency-division multiplexing in the GHz range. The micro-bridge is kept below its critical temperature and biased with a microwave current of slightly lower amplitude than the critical current of the micro-bridge. The response of the detector is the rate of superconducting fluctuations, which depends exponentially on the concentration of quasiparticles in the micro-bridge. Excess quasiparticles are generated by an incident THz signal. Since the quasiparticle lifetime increases exponentially at lower operation temperature, the noise equivalent power rapidly decreases. This approach allows for large arrays of noise bolometers operating above 1 K with sensitivity, limited by 300-K background noise. Moreover, the response of the bolometer always dominates the noise of the readout due to relatively large amplitude of the bias current. We performed a feasibility study on a proof-of-concept device with a 1.0 × 0.5 μm2 micro-bridge from a 9-nm thin Nb film on a sapphire substrate. Having a critical temperature of 5.8 K, it operates at 4.2 K and is biased at the frequency 5.6 GHz. For the quasioptical input at 0.65 THz, we measured the noise equivalent power ≈3 × 10−12 W/Hz1/2, which is close to expectations for this particular device in the noise-response regime.

DOI
10.1063/1.4995981

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We present a superconducting noise bolometer for terahertz radiation, which is suitable for large-format arrays. It is based on an antenna-coupled superconducting micro-bridge embedded in a high-quality factor superconducting resonator for a microwave bias and readout with frequency-division multiplexing in the GHz range. The micro-bridge is kept below its critical temperature and biased with a microwave current of slightly lower amplitude than the critical current of the micro-bridge. The response of the detector is the rate of superconducting fluctuations, which depends exponentially on the concentration of quasiparticles in the micro-bridge. Excess quasiparticles are generated by an incident THz signal. Since the quasiparticle lifetime increases exponentially at lower operation temperature, the noise equivalent power rapidly decreases. This approach allows for large arrays of noise bolometers operating above 1 K with sensitivity, limited by 300-K background noise. Moreover, the response of the bolometer always dominates the noise of the readout due to relatively large amplitude of the bias current. We performed a feasibility study on a proof-of-concept device with a 1.0 × 0.5 μm2 micro-bridge from a 9-nm thin Nb film on a sapphire substrate. Having a critical temperature of 5.8 K, it operates at 4.2 K and is biased at the frequency 5.6 GHz. For the quasioptical input at 0.65 THz, we measured the noise equivalent power ≈3 × 10−12 W/Hz1/2, which is close to expectations for this particular device in the noise-response regime.
Terahertz (THz) direct detectors based on superconducting niobium nitride (NbN) hot electron bolometers (HEBs) with microwave (MW) biasing are studied. The MW is used to bias the HEB to the optimum point and to readout the impedance changes caused by the incident THz signals. Compared with the thermal biasing method, this method would be more promising in large scale array with simple readout. The used NbN HEB has an excellent performance as heterodyne detector with the double sideband noise temperature (TN) of 403 K working at 4.2 K and 0.65 THz. As a result, the noise equivalent power of 1.5 pW/Hz1/2 and the response time of 64 ps are obtained for the direct detectors based on the NbN HEBs and working at 4.2 K and 0.65 THz.
Superconducting hot electron bolometers (HEBs) are widely used as terahertz (THz) heterodyne detectors (mixers) and becoming the most advantageous candidate for low noise receiver at frequencies above 1.2 THz. However, there is still lack of accurate theoretical modeling which explains the complex mechanism of different aspects of superconducting HEBs. These difficulties do not prevent researchers around the world from making experimental progress. In the last few years, superconducting HEBs are also used as THz direct detectors to develop the array for the applications in biology, medicine and security systems. Recently, a noise equivalent power (NEP) of 0.2 pW/√Hz at 2.5 THz is reported for the superconducting HEBs with thermal biasing. Instead of thermal one, microwave (MW) which was first used to stabilize the HEB mixer is also applied to bias the device as a direct detector. Furthermore, THz can be used to bias the HEBs, if THz sources can be provided easy. Although MW biasing can improve sensitivity of the superconducting HEB direct detectors, little work has been done to compare the responsivity and NEP among the THz direct detectors with thermal, THz and MW biasing. To find a best biasing method, we have studied the similarities and differences of the direct detectors based on the superconducting HEBs with three different biasing methods. The fabricated NbN HEB detectors consist of a complementary logarithmic-spiral antenna made of gold and an NbN film (bridge) connecting across the antenna’s inner terminals. We have fabricated the niobium nitride (NbN) superconducting HEB mixers with the system double sideband (DSB) noise temperature better than 8 times of the quantum limit: hf/kB (where h is the Planck constant, kB is the Boltzmann constant and f is the operating frequency) at the frequencies higher than 1 THz. Here, the mixer chips with the system DSB noise temperature of about 400 K and the intermediate frequency (IF) gain bandwidth (GBW) of larger than 5 GHz at 4.2 K and 0.65 THz have been used. As the direct detectors, the current responsivity of 290 A/W and 105 A/W are obtained for MW and THz biasing, which about one order higher than that of thermal biasing. The NEP of 2.7 pW/√Hz is obtained with MW biasing, which can be expected to be improved in the future.
Our new detector and readout concept brings together TES (Transition Edge Sensor) and MKID (Microwave Kinetic Inductance Detector) technologies and exploits the idea of a microwave-induced superconducting transition in a small thin-film microbridge. The superconducting transition of the bridge manifests itself as variation in the Q -factor of niobium resonators at 5-8 GHz, somewhat similar to MKID operation. We present data showing the potential for developing this concept into multipixel detector arrays. Single-pixel sensitivity was measured at 4.5 K for an input band of 600-700 GHz using a prototype 10-nm-thick Nb bridge of size 1 μm × 500 nm. Radiation from human skin was detected with a resolution better than 1 K/rtHz, which is encouraging for terahertz imaging applications. To further improve device sensitivity, we are also developing Hf-based devices that operate near 0.35 K. Details about the physics and stability of these devices are discussed.
We propose a new detection technique based on radio-frequency (RF) bias and readout of an antenna-coupled superconducting nanobolometer. This approach is suitable for Frequency-Division-Multiplexing (FDM) readout of large arrays using broadband low-noise RF amplifier. We call this new detector RFTES. This feasibility study was made on demonstrator devices which are made in all-Nb technology and operate at 4.2 K. The studied RFTES devices consist of an antenna-coupled superconducting nanobolometer made of ultrathin niobium films with transition temperature Tc = 5.2 K. The 0.65-THz antenna and nanobolometer are embedded as a load into a GHz-range coplanar niobium resonator (Tc = 8.9 K, Q = 4000). To heat the superconducting Nb nanobolometer close to the Tc, the RF power at resonator frequency f = 5.8 GHz is applied via a transmission line which is weakly coupled (-11 dB) to the loaded resonator. The THz-antenna of RFTES was placed in the focus of a sapphire immersion lens inside a He4-cryostat equipped with an optical window and a semiconductor RF amplifier. We have demonstrated optical response of the RFTES to THz radiation. The demonstrator receiver system employing the RFTES device showed an optical Noise-Equivalent Power (NEP) 1e-14 W/sqrt(Hz) at 4.2 K.
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.
Terahertz (THz) direct detectors based on superconducting niobium nitride hot electron bolometers with thermal, microwave and THz biasing methods have been studied at a frequency of 0.65 THz and at an operating temperature of 4.2 K. A low noise amplifier at room temperature is used to read out the current change of the detectors modulated by the chopping frequency at 732 Hz. The noise spectral density, current responsivity and noise equivalent power (NEP) have been measured and compared respectively. The detectors with the microwave and THz biasing methods have about one order higher current responsivity than that of the thermal one. With the microwave biasing, an optical NEP at the order of pW/√Hz which shows no deterioration is obtained by choosing the microwave frequency and power appropriately.