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International Conference on Infrared, Millimeter, and Terahertz Waves (IRMMW-THz)

Superconducting high-Tc hot electron bolometers used as THz mixers: Predicted performance by hot spot modeling

Romain G. Ladret, Alain Kreisler, Annick F. Degardin

Published Sep 1, 2013• 1 citation
Publisher

Abstract

High-TChot electron bolometers (HEB) are promising THz mixers due to their expected wide bandwidth, large mixing gain and low intrinsic noise. In this paper, we have simulated the DC characteristics, conversion loss and noise of YBaCuO HEBs with a hot spot model usually dedicated to low-TCdevices.
DOI
10.1109/irmmw-thz.2013.6665541

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High-TCsuperconductor (YBaCuO) hot electron bolometers (HEB) are promising THz mixers due to the large predicted bandwidth and the low local oscillator (LO) power requirement. Recently, we introduced the hot-spot model, initially dedicated to low-TCNbN devices, to predict YBaCuO mixer performance. Major improvements to the model have been introduced: local description of the power absorption along the HEB constriction, THz dependence of the resistive transition, impedance matching between the THz antenna and the constriction. Mixer noise temperature TNand conversion gain G were predicted up to 2.5 THz. For instance, TN= 2700 K (DSB) and G = -13 dB are obtained with 9 μW of LO power at 1 THz.

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IEEE Transactions on Applied Superconductivity•Jan 1, 2014•7 citations
High-TCsuperconductor (YBaCuO) hot electron bolometers (HEB) are promising THz mixers due to the large predicted bandwidth and the low local oscillator (LO) power requirement. Recently, we introduced the hot-spot model, initially dedicated to low-TCNbN devices, to predict YBaCuO mixer performance. Major improvements to the model have been introduced: local description of the power absorption along the HEB constriction, THz dependence of the resistive transition, impedance matching between the THz antenna and the constriction. Mixer noise temperature TNand conversion gain G were predicted up to 2.5 THz. For instance, TN= 2700 K (DSB) and G = -13 dB are obtained with 9 μW of LO power at 1 THz.
High- T C hot electron bolometers (HEB) are promising THz mixers due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. We have simulated the characteristics of YBaCuO HEBs with a hot spot model usually dedicated to low- T C phonon cooled devices. With respect to e.g., NbN, the high- T C specificities are mainly arising from the large values of YBaCuO phonon thermal conductivity, and the film to MgO substrate phonon escape time. The consequent effects on the electron temperature profiles along the YBaCuO constriction and the current voltage DC characteristics are considered. The conversion gain G and noise temperature T N were computed for two constriction dimensions. For a 100 nm long × 100 nm wide × 10 nm thick constriction at 9 microwatt local oscillator (LO) power, the expected double sideband (DSB) T N = 1520 K ( G = −13.7 dB). For a larger (but more realistic according to YBaCuO aging effects) 400 nm long × 400 nm wide × 35 nm thick constriction, minimum T N = 1210 K DSB at 35 microwatt LO power ( G = −13.1 dB).
Hot electron bolometers (HEB) made from high-TC superconducting YBa2Cu3O7–x (YBCO) oxide nano-constrictions are promising THz mixers, due to their expected wide bandwidth, large mixing gain, and low intrinsic noise. The challenge for YBCO resides, however, in the chemical reactivity of the material and the related aging effects. In this paper, we model and simulate the frequency dependent performance of YBCO HEBs operating as THz mixers. We recall first the main hypotheses of our hot spot model taking into account both the RF frequency effects in the YBCO superconducting transition and the nano-constriction impedance at THz frequencies. The predicted performance up to 4 THz is given in terms of double sideband noise temperature TDSB and conversion gain G. At 2.5 THz for instance, TDSB ≅ 1000 K and G ≅ − 6 dB could be achieved at 12.5 μW local oscillator power. We then consider a standoff target detection scheme and examine the feasibility with YBCO devices. For instance, detection at 3 m through cotton cloth in passive imaging mode could be readily achieved in moderate humidity conditions with 10 K resolution.
Superconducting hot electron bolometer (HEB) mixers are the most sensitive heterodyne detectors at frequencies beyond 1 THz, and are used for many astronomical observations. HEB, consisting of a narrow and short superconducting thin film patch connected to an antenna via contact pads, is brought into a highly sensitive operating regime by applying a DC current and a certain amount of local-oscillator power. Potentially, superconducting HEB mixers are capable of exhibiting high sensitivity for frequencies up to several tens of THz, as their output noise and conversion gain are nearly frequency independent according to the mixing mechanism. Moreover, the understanding of mixing mechanism has evolved over time from lumped-element model to distributed hot-spot model. In this review, we provide an overview of theoretical and experimental studies on HEBs and their applications, especially in astronomical observations on ground, airborne and space telescopes.
In this paper, we investigate the critical temperature dependence of the performance of niobium nitride superconducting hot electron bolometer (HEB) mixers. The input noise temperature and intermediate frequency (IF) noise bandwidth of superconducting HEB mixers of different critical temperatures Tc(between 6 and 12 K) are simulated with a hot spot model. Simulation results show that the mixer input noise temperature is the lowest at Tcapproximately equal to 7-9.5 K and the IF noise bandwidth increases with Tc. We also measure three superconducting HEB mixers of different critical temperatures (i.e., 7.5, 8.8, and 10.3 K) at 0.85 and 1.3 THz. An agreement between measurement and prediction is observed.
To investigate the THz mixing performance of YBCO hot electron bolometers (HEB), the influence of the local oscillator (LO) radiofrequency (RF) radiation has been considered in detail. As opposed to the usual hot spot modeling approach, where the LO power is assumed to be uniformly distributed over the HEB constriction length, a uniform RF current has been assumed. The local electron temperature-as obtained by solving the coupled electron and phonon thermal reservoir equations-could then be used to determine the local YBCO complex resistivity, hence the locally dissipated LO power. Besides, the use of a modified two-fluid description allowed to determine the RF-dependent temperature shift (~-1% THz-1) and broadening (~+20% THz-1) of the resistive transition. Finally, the impedance matching to the THz antenna was considered. For a typical constriction, the conversion loss and noise temperature TDSBwere computed, with TDSB× exp(0.32f) behavior up to 2.5 THz.