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Superconductor Science and Technology

Appropriate microwave frequency selection for biasing superconducting hot electron bolometers as terahertz direct detectors

Shoulu Jiang, X F Li, Xiaoqing Jia, Lin Kang, Biaobing Jin, Weiwei Xu, Jian Chen, Peiheng Wu

Published Feb 22, 2017• 28 citations
Publisher

Abstract

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.

DOI
10.1088/1361-6668/aa621b

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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 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) 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.
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.
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.
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.
In this paper, we report on the terahertz (THz) direct detection performance of two superconducting niobium nitride (NbN) hot-electron bolometers (HEBs) of different micro-bridge sizes. At the optimum bias point, the two HEB devices show similar thermal conductance (~80nW/K). The measured responsivity coincides well with the electrical responsivity calculated from the measured current-voltage (IV) curves at different bath temperatures. The noise equivalent power (NEP) of the two HEB devices is found to be about 10-12W/√Hz.