Terahertz frequency quantum cascade lasers (THz-QCLs) are compact solid-state sources of coherent THz radiation over the frequency range 1–5 THz. Due to the high output power, narrow linewidths and broad range of emission frequencies, QCLs have numerous potential applications as radiation sources for imaging and spectroscopy within biomedical or security scenarios, or as local oscillators in heterodyne mixing schemes. Although THz-QCLs are known to exhibit sub-nanosecond dynamic behaviour, transient laser emission on these time-scales cannot be resolved by commercially-available semiconducting Ge or InSb bolometers, owing to the large detection time-constants. We present a time-domain analysis of fast pulses emitted by a quantum cascade laser operating at ~3.1 THz using superconducting THz detectors made from either NbN or YBa(2)Cu(3)O(7-δ) (YBCO) thin films. The QCL emission was studied as a function of the QCL bias voltage and the excitation pulse length. The ultra-fast response of superconducting NbN and YBCO detectors allows the resolution of nanosecond-time-scale dynamic laser effects. This enables to determine the QCL transfer function (output power vs. QCL drive current), based on experimentally studied correlations of the time-dependent emission of radiation with current flow in the QCL, under different QCL bias conditions. From the transfer function, specific QCL parameters were determined including the threshold and cut-off currents. We show that this transfer function differs from that obtained using semiconducting bolometric detectors which respond only to the integrated pulse energy.
CORE (COnnecting REpositories)
CORE (COnnecting REpositories)