Time-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 steady-state 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.