The self-heating in long superconducting microbridges made from thin NbN films deposited on top of high silicon mesa structures was studied by analyzing the hysteresis current density jH. We observed a more than twofold decrease of jH with increase in the ratio of the height of the Si mesa, h, to the width of the microbridge, W, from 0 to 24. We describe our experimental results using one-dimensional thermal balance equations taking into account disordered matter in our thin NbN films and limitations imposed on the phonon mean free path by the width of the Si mesa. In the framework of this model we obtain a good agreement between theory and experiment over a wide temperature range from 4.2 K up to the critical temperature TC for all h/W ratios.

Abstract-We report the time-domain analysis of fast pulses emitted by a quantum cascade laser (QCL) operating at ~3.1 THz using superconducting THz detectors made from either NbN or YBa 2 Cu 3 O 7- (YBCO) thin films. The ultra-fast response from these detectors allows resolution of emission features occurring on a nanosecond time-scale, which is not possible with commercially available Ge or InSb bolometers owing to their much larger time constants. We demonstrate that the timedependent emission can be strongly affected by relatively small variations in the driving pulse. The QCL output power-current relationship was determined, based on correlation of the timedependent emission of radiation with current flow in the QCL, under different QCL bias conditions. We show that this relationship differs from that obtained using bolometric detectors that respond only to the integrated pulse energy. The linearity of the detectors, and their agreement with measurements using a Ge bolometer, was also established by studying the QCL emission as a function of bias voltage and excitation pulse length. This measurement scheme could be readily applied to the study of ultra-fast modulation and mode-locking of THz-QCLs.

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EH (2013) Time-resolved measurement of pulse-to-pulse heating effects in a terahertz quantum cascade laser using an NbN superconducting detector. Applied Physics Letters, 103 (6). 061120-1 -061120-4 (4).http://dx.doi.org/10.1063/1.4818584Time-resolved measurement of pulse-to-pulse heating effects in a terahertz quantum cascade laser using an NbN superconducting detector Joule heating causes significant degradation in the power emitted from terahertz-frequency quantum-cascade lasers (THz QCLs). However, to date, it has not been possible to characterize the thermal equilibration time of these devices, since THz power degradation over sub-millisecond time-scales cannot be resolved using conventional bolometric or pyroelectric detectors. In this letter, we use a superconducting antenna-coupled niobium nitride detector to measure the emission from a THz QCL with a nanosecond-scale time-resolution. The emitted THz power is shown to decay more rapidly at higher heat-sink temperatures, and in steadystate the power reduces as the repetition rate of the driving pulses increases. The pulse-to-pulse variation in active-region temperature is inferred by comparing the THz signals with those obtained from low duty-cycle measurements. A thermal resistance of 8.2 ± 0.6 K/W is determined, which is in good agreement with earlier measurements, and we calculate a 370 ± 90-µs bulk heat-storage time, which corresponds to the simulated heat capacity of the device substrate.

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