Carrier scattering from terahertz quantum cascade laser obtained by near-infrared pump-probe spectroscopy

In this study, a near-infrared pump-probe spectroscopy method was established to obtain the intersubband and intrasubband carrier scattering processes in the conduction band of the active region of terahertz quantum cascade laser (THz-QCL). The photoreflectance (PR) spectrum results suggest that the THz-QCL with an uncorroded contact layer demonstrated a strong surface electric field at the band edge and that the THz-QCL with a corroded contact layer presented a distinct positive–negative structure in its PR spectrum at the band edge. With an increasing energy level position, the intrasubband scattering time gradually decreased, while the intersubband scattering time gradually increased, as revealed by pump-probe experimental results related to the laser energy levels in the conduction band of THz-QCL. The relaxation curves of the carriers were fitted by a double exponential function. The intrasubband and intersubband scattering times for the upper laser level were 1 and 17 ps, respectively, while those for the lower laser level were 0.3 and 28 ps, respectively.