Ultrafast population dynamics in electrically modulated terahertz quantum cascade lasers

The ultrafast population dynamics in electrically modulated three-well terahertz quantum cascade lasers (QCLs) is studied by using the self-consistent Bloch–Poisson equations. In the modulation process, the non-equilibrium oscillations of the population inversion are found before the population equilibrium recovers. The equilibrium formation time increases nonlinearly with the period number. This phenomenon stems from the non-uniform distribution of the electric potential. In different periods, different responses to the electrical modulation are also explored. An in-depth understanding of electron transport in the cascade structure is obtained. Finally, we demonstrate the feasibility of a modulation frequency up to gigahertz in terahertz QCLs.