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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Direct detection behavior of a superconducting hot electron bolometer measured by Fourier transform spectrometer

W. Miao, W. Zhang, Kangmin Zhou, H. Lin Z, Qi Yao, Y. Delorme, R. Lefèvre, S. C. Shi

Published Nov 3, 2010• 1 citation
Publisher

Abstract

In this paper, the direct detection behaviors of a superconducting hot electron bolometer integrated with a log spiral antenna are investigated by using Fourier Transform Spectrometer (FTS). We find the response of the bolometer to a modulated signal can be detected by a lock-in amplifier not only from the DC bias current, but also from the output noise power at the IF port of the HEB. We attribute the response in output noise power to Johnson noise and thermal fluctuation noise. Both the current response and the output noise power response measured at different bias voltages can be explained by one dimensional distributed hot spot model. In addition, the frequency response of the hot electron bolometer measured from the response in DC bias current is in good agreement with that in IF output noise power.

DOI
10.1117/12.871759

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In this paper, the direct detection behaviors of a superconducting hot electron bolometer integrated with a log spiral antenna are investigated by using Fourier Transform Spectrometer (FTS). We find the response of the bolometer to a modulated signal can be detected by a lock-in amplifier not only from the DC bias current, but also from the output noise power at the IF port of the HEB. We attribute the response in output noise power to Johnson noise and thermal fluctuation noise. Both the current response and the output noise power response measured at different bias voltages can be explained by one dimensional distributed hot spot model. In addition, the frequency response of the hot electron bolometer measured from the response in DC bias current is in good agreement with that in IF output noise power.
The heterodyne mixing and direct detection performance of a superconducting NbN hot-electron bolometer (HEB) integrated with a log-spiral antenna have been thoroughly characterized. The corrected receiver noise temperature and IF gain bandwidth are approximately 800 K at 0.5 THz, 750 K at 0.85 THz and 3 GHz at its optimum bias point. In addition, both the receiver noise temperature and IF gain bandwidth were found insensitive to the bath temperature, while the bias point was fixed, in good agreement with those simulated with the hot-spot model taking account of the HEB's current-dependent resistive transition. Furthermore, the HEB's frequency response was measured by a Fourier transform spectrometer at different bias points and bath temperatures. The estimated noise equivalent power (NEP) was close to 10-13W/Hz0.5around the HEB's transition temperature.
In this paper, we report the investigation of direct detection behavior of a superconducting niobium nitride (NbN) hot electron bolometer (HEB) based on noise thermometry. The HEB consists of a microbridge made from 5.5 nm thick NbN film and a twin slot antenna for radiation coupling. In the microbridge, hot electron system is generated after radiation pumping and it creates extra noise at RF frequencies. We measured the response of the noise at 0.5-1.8 GHz and the noise equivalent power (NEP) of the superconducting NbN HEB at different bias voltages. The measurement result shows that the superconducting NbN HEB has a noise equivalent power of 1.24×10-12W/Hz1/2at its optimal bias point. We also compared the noise equivalent power measured by noise thermometry at two different readout bandwidths. The measured results are found to be in good agreement with the theoretical one.
Measured spectral response of spiral-antenna coupled superconducting hot electron bolometers (HEBs) often drops dramatically at frequencies that are still within the frequency range of interest (e.g., ~ 5 THz). This is inconsistent with the implied low receiver noise temperatures from the same measurements. To understand this discrepancy, we exhaustively test and calibrate the thermal sources used in Fourier transform spectrometer measurements. We first investigate the absolute emission spectrum of high-pressure Hg arc lamp, then measure the spectral response of two spiral-antenna coupled NbN HEBs with a Martin-Puplett interferometer as spectrometer and 77 K blackbody as broadband signal source. The measured absolute emission spectrum of Hg arc lamp is proportional to frequency, corresponding to an equivalent blackbody temperature of 4000 K at 1 THz, 1500 K at 3 THz, and 800 K at 5 THz, respectively. Measured spectral response of spiral-antenna coupled NbN HEBs, corrected for air absorption, is nearly flat in the frequency range of 0.5-4 THz, consistent with simulated coupling efficiency between HEB and spiral-antenna. These results explain the discrepancy, and prove that spiral-antenna coupled superconducting NbN HEBs work well in a wide frequency range. In addition, this calibration method and these results are broadly applicable to other quasi-optical THz receivers.
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.
A good understanding of input radio-frequency (RF) noise contribution is of particular importance to the development of high-sensitivity superconducting hot electron bolometer (HEB) receivers. In terms of simulation results based on the hot spot model, we found that the output noise of HEB mixers is nearly linearly decreased with the mixer's bath temperature when the HEB mixer is operated at its optimum DC-bias regime under appropriate local-oscillator signal pumping, and the effect of this decrease on the receiver output power can be well corrected. Thus, we used an intersecting lines technique to measure the input RF noise temperature of a quasi-optical superconducting NbN HEB receiver, and the measured input RF noise temperature of the HEB receiver is around 300 K at 850 GHz. For comparison, we also evaluated the losses and equivalent noise temperatures of the quasi-optical components in the RF signal path. The evaluated input RF noise temperature is found to be smaller than the measured one.