Measurement of active region temperature in THz quantum cascade lasers by micro-photocurrent spectroscopy

To accurately monitor the actual temperature of a terahertz quantum cascade laser (THz-QCL) under operating conditions, this study proposes a method utilizing micro-photocurrent spectroscopy to determine the active region temperature. Micro-photocurrent spectra of THz-QCL devices with cavity lengths of 0.5 mm and 1 mm are measured at various bias voltages. The results demonstrate that the peak of the photocurrent spectrum exhibits a linear shift with the bias voltage below 10 V, while presenting a sharp red shift as the voltage increases further. Additionally, micro-photocurrent spectra are investigated at multiple locations on the device facet. Further analysis of the temperature effect and quantum confinement Stark effect on the bandgap change reveals the temperature distributions of the THz-QCL. It indicates that at 15 V, the temperature gradient of the active region along the material growth direction is approximately 0.4 K/μm. The proposed method, based on photocurrent spectroscopy, achieves about 3 K resolution for temperature measurement of THz-QCL, facilitating the optimization of device thermal management and failure analysis.