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IEEE International Superconductive Electronics Conference (ISEC)

Terahertz Power Detectors based on Superconducting HEBs with Microwave Readout

Runfeng Su, J. Chen, Peiheng Wu, Yuankai Zhang, Xuecou Tu, Xiaoqing Jia, C. H. Zhang, Lin Kang, Biao Jin, Weiwei Xu, Huabing Wang

Published Jul 1, 2019• 0 citations
Publisher

Abstract

The properties of terahertz (THz) power detectors based on Superconducting NbN hot electron bolometer (HEB) with microwave (MW) readout are reported here. Features of relaxation oscillations probed with MW when the HEB is not pumped, pumped by THz source or heated by raising the bath temperature are studies. The periodic pulse in the reflected MW signals both in time and frequency domains show that the relaxation oscillation frequency increases with the bias voltage, incident THz power and/or bath temperature. The frequency count forms a quadratic polynomial fit to the bias voltage, a linear fit to the incident THz power and an exponential fit to the bath temperature. Based on the above results, incident THz power between 8-90 nW is measured using the HEB.

DOI
10.1109/isec46533.2019.8990949

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The properties of terahertz (THz) power detectors based on Superconducting NbN hot electron bolometer (HEB) with microwave (MW) readout are reported here. Features of relaxation oscillations probed with MW when the HEB is not pumped, pumped by THz source or heated by raising the bath temperature are studies. The periodic pulse in the reflected MW signals both in time and frequency domains show that the relaxation oscillation frequency increases with the bias voltage, incident THz power and/or bath temperature. The frequency count forms a quadratic polynomial fit to the bias voltage, a linear fit to the incident THz power and an exponential fit to the bath temperature. Based on the above results, incident THz power between 8-90 nW is measured using the HEB.
Abstract Relaxation oscillations related to terahertz (THz) power detection in superconducting niobium nitride hot electron bolometers (HEBs) have been studied using microwaves as a probe. The reflected microwave signal, which is related to the impedance changes of the HEB irradiated by an incident THz signal, can be used to characterize relaxation oscillations. The frequency of relaxation oscillation switching between the superconducting and resistive states increases linearly with incident THz power but nonlinearly with bias voltage or bath temperature. The observed strong periodic pulse trains of relaxation oscillations have a maximum duty cycle of ∼50%. These strong switching pulses indicate that DC Joule heating, THz radiation or thermal energy is absorbed in the bi-stable region of the HEB intermittently. In addition to relaxation oscillations, we also see weaker oscillations when the device is in the resistive state, which we call intrinsic oscillations. These have a constant frequency at the order of 10 kHz which is independent of external stimuli. The weak periodic pulse trains at a constant frequency can be divided into mainly two types—one is the pulse train with a small duty cycle and the other is the quasi-square wave. Measurement of these oscillations in HEBs has some practical applications. Digital THz power measurement with frequencies below and above the gap frequency can be made based on the linearity between relaxation oscillation frequency and incident THz power. The relaxation oscillation frequency can be used to estimate the maximum speed of a THz direct detection system especially for a multi-pixel HEB array. In particular, extra noise induced by relaxation oscillations and intrinsic oscillations can be determined for the HEB operating as a THz direct detector.
A new class of microwave-operated THz power detectors based on the NbN hot-electron-bolometer (HEB) mixer is proposed. The injected microwave signal (~1 GHz) serves the dual purpose of pumping the HEB element and enabling the read-out of the internal state of the device. A cryogenic amplifier amplifies the reflected microwave signal from the device and a homodyne scheme recovers the effects of the incident THz radiation. Two modes of operation have been identified, depending on the level of incident radiation. For weak signals, we use a chopper to chop the incident radiation against a black body reference and a lock-in amplifier to perform synchronous detection of the homodyne readout. The voltage measured is proportional to the incident power, and we estimate an optical noise equivalent power of ~5pW/ √Hz at 0.83 THz. At higher signal levels, the homodyne circuit recovers the stream of steady relaxation oscillation pulses from the HEB device. The frequency of these pulses is in the MHz frequency range and bears a linear relationship with the incident THz radiation over an input power range of ~15 dB. A digital frequency counter is used to measure THz power. The applicable power range is between 1 nW and 1 μW.
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.
Terahertz (THz) direct detectors based on superconducting niobium nitride (NbN) hot electron bolometers (HEBs) and biased by a simple microwave (MW) source have been studied. The frequency and power of the MW are selected by measuring the MW responses of the current–voltage ( I–V ) curves and resistance–temperature ( R–T ) curves of the NbN HEBs. The non-uniform absorption theory is used to explain the current jumps in the I–V curves and the resistance jumps in the R–T curves. Compared to the thermal biasing, the MW biasing method can improve the sensitivity, make the readout system much easier and consumes less liquid helium, which is important for long lasting experiments. The noise equivalent power (NEP) of 1.6 pW Hz −1/2 and the response time of 86 ps are obtained for the detectors working at 4.2 K and 0.65 THz.
Terahertz (THz) direct detectors based on superconducting niobium nitride (NbN) hot electron bolometers (HEBs) with thermal, microwave (MW) and THz biasing have been studied at 0.65 THz and 4.2 K in detail. The current responsivity and noise equivalent power (NEP) have been measured and compared respectively. The detectors with the MW and THz biasing have about one order higher current responsivity than that of the thermal one. With the MW biasing, an NEP at the order of pW/√Hz is obtained by choosing the MW frequency and power appropriately.
Terahertz (THz) direct detectors based on superconducting niobium nitride (NbN) hot electron bolometers (HEBs) with microwave (MW) injection have been studied. Compared to the thermal biasing method, the injected MW is used here to bias the HEB to the optimum point. MW can be expected to be used as the easy read out for the detecting array. The used NbN HEB has an excellent performance as heterodyne detector with the noise temperature of 403 K working at 4.2 K and 0.65 THz. As the result, the noise equivalent power (NEP) of 1.4 pW/Hz1/2and the response time of 64 ps have been obtained for the direct detectors with MW injection.