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IEEE Transactions on Applied Superconductivity

A Microwave-Operated Hot-Electron-Bolometric Power Detector for Terahertz Radiation

Edward Tong, Andrey Trifonov, Alexander S. Shurakov, R. Blundell, Gregory N Gol'tsman

Published Jan 1, 2014• 9 citations
Publisher

Abstract

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.

DOI
10.1109/tasc.2014.2377670

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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.
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.
The performance of a NbN based phonon-cooled Hot Electron Bolometric (HEB) quasioptical mixer is investigated in the 0.65-3.12 THz frequency range. The device is made from a 3 nm thick NbN film on high resistivity Si and integrated with a planar spiral antenna on the same substrate. The in-plane dimensions of the bolometer strip are 0.2/spl times/2 /spl mu/m. The best results of the DSB noise temperature at 1.5 GHz IF frequency obtained with one device are: 1300 K at 650 GHz, 4700 K at 2.5 THz and 10000 K at 3.12 THz. The measurements were performed at 4.5 K ambient temperature. The amount of local oscillator (LO) power absorbed in the bolometer is about 100 nW. The mixer is linear to within 1 dB compression up to the signal level 10 dB below that of the LO. The intrinsic single sideband conversion gain measured at 650 GHz is -9 dB, the total conversion gain is -14 dB.
The performance of a NbN based phonon-cooled Hot Electron Bolometric (HEB) quasioptical mixer is investigated in the 0.65-3.12 THz frequency range. The device is made from a 3 nm thick NbN film on high resistivity Si and integrated with a planar spiral antenna on the same substrate. The in-plane dimensions of the bolometer strip are 0.2/spl times/2 /spl mu/m. The results of the DSB noire temperature are: 1300 K at 650 GHz, 4700 K at 2.5 TBz and 10000 K at 3.12 THz. The RF bandwidth of the receiver is at least 2.5 THz. The amount of LO power absorbed in the bolometer is about 100 nW. The mixer is linear to within 1 dB compression up to the signal level 10 dB below that of the LO. The intrinsic single sideband conversion gain is measured to be -9 dB, the total conversion gain -14 dB.
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 (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.