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

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.

Total loss measurement and simulation in a REBCO coated conductor carrying DC current in perpendicular AC magnetic field at various temperatures

“…Other typical features of dynamic resistance were also observed from figure 3: (a) at each operating temperature, the R dyn increases with growing DC current levels and magnetic field amplitudes; (b) flux-flow resistance appears at the cases of high reduced currents as shown in figures 3(g) and (h) at 77 K and 70 K. All the features are consistent with our previous R dyn measurement results [22][23][24][25][26].…”

“…where a is the half-width of the conductor, I c0 is the self-field critical current of the conductor, B a is the amplitude of the applied magnetic field. The B th is the threshold field, which has been proved to agree well with experimental results using equation (8) in the case of i ⩽ 0.2, and equation ( 9) for i > 0.2 [23][24][25][26]:…”

“…However, the µ 0 H m value which gives maximum value of the Q m0 /(µ 0 H m ) 2 curve increases with decreasing temperature. We attribute it to the temperature-dependent effective penetration field [12,26,43], which is proportional to the critical current density of the superconductor [12,44]. With decreasing temperature, the measured Q m0 /(µ 0 H m ) 2 values become larger at high magnetic fields, which is due to the increase of critical current of the coated conductor at lower temperature.…”

“…Three temperature sensors (PT 100) were used to monitor the operating temperature. At each given temperature, the total loss of the REBCO coated conductor was measured using electrical methods: calibration-free method for magnetization loss component [11][12][13] and four-probe method for dynamic resistance/dynamic loss component [22][23][24][25][26]. Calibration-free method uses a pick-up coil which is co-wound with the AC magnet.…”

Recent progress of superconducting electronics in China

“…工 作于太赫兹频段的超导超材料是一种有效的超导 太赫兹调控器件. 目前超导超材料的工作温度已经 由液氦温区提高到液氮温区 [173] , 并实现了超过 80%的调制深度和接近1 MHz的调制速度, 表现 出良好的调制性能 [174,175] . [176] , 另外在高温超导YBCO双晶 结阵列混频器上实现了高达154次的谐波混频 [177] .…”