Transient Analysis of THz-QCL Pulses Using NbN and YBCO Superconducting Detectors

Abstract

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.

Transient analysis of substrate heating effects in a terahertz quantum cascade laser using an ultrafast NbN superconducting detector

“…A 3.1-THz QCL, based on a resonant-phonon (RP) design scheme [4] was processed into a semi-insulating surfaceplasmon waveguide with dimensions 10×140×850 μm 3 and was mounted in a helium-cooled cryostat at 15-K heat-sink temperature. The QCL was driven by trains of 500-ns-long current pulses, with repetition rates, f rep , between 100 and 500 kHz, giving pulse-separations in the range 9.5-1.5 μs respectively.…”

“…We use an ultrafast antenna-coupled NbN superconducting detector with a response-time of ~30 ps [3] to observe the transient degradation in THz power from a QCL directly and determine the time-resolved heating in the substrate.…”

Time-resolved measurement of pulse-to-pulse heating effects in a terahertz quantum cascade laser using an NbN superconducting detector

“…The THz radiation emitted by the QCL was collimated and focused onto an NbN detector, within a separate cryostat, as described in Ref. 13. The intrinsic response time of the detector was around 30 ps, 10,11 and a time-resolution of 1.1 ns was estimated using the method in Ref.…”

“…The detector response was confirmed to be linear with respect to both the instantaneous incident THz power, and the integrated power (i.e., the energy) over a THz pulse. 13 Initially, the pulse-repetition rate was fixed at 500 kHz and the heat-sink temperature, T H , was varied between 15 and 45 K. and 45 K, acquired over the first 20 µs of the 1-ms-long pulse-train. A baseline shifting algorithm was used to compensate for ac-coupling effects in the detector.…”

“…Although QCLs do not exhibit relaxation oscillations 15 the intrapulse power responds to driving-current transients, which are principally caused by impedance mismatch. 13 An initial spike in THz power [label (i) in Fig. 1(a)] is caused by a ripple on the rising-edge of the current pulse briefly exceeding the lasing threshold.…”