Terahertz generation in quantum-cascade lasers through down-conversion optical nonlinearity: Design and modeling

In this paper, we propose an electrically driven optically excited laser structure for terahertz (THz) generation in quantum cascade lasers (QCLs) through frequency down-conversion optical nonlinearity. In our distinct design, both the pump and THz radiations are generated simultaneously by optical and electronic intersubband transitions (ISTs) between the quantized states within the conduction band of the same active region of a QCL. The modeling of laser bandstructure is achieved using a self-consistent solution of Schrödinger–Poisson equations, and the detailed analysis of electronic transport of the proposed structure is performed using the density matrix approach and energy-density balance equation that self-consistently consider the optical, electrical and thermal interactions in the structure. Additionally, the waveguide loss for the THz optical mode is considered using the bulk Drude model. Using a stationary solution of the density matrix equations, an expression for the THz intensity gain is derived as a function of pump field intensity, carrier distributions and quantum coherence contributions. Using the presented model, we analyze the steady-state and transient characteristics of the device for two different heatsink temperatures, 220 and 300 K. Simulation results show that the proposed structure has a potential to generate room-temperature THz radiation in QCLs. •The design of quantum cascade lasers (QCLs) with integrated nonlinear optical element is proposed for terahertz generation.•The bandstructure of the laser is calculated using a self-consistent solution of Schrödinger–Poisson system.•The output characteristics of the structure is carried out using a density matrix approach and energy balance equation.•High terahertz power levels up to 14.5 mW and large nonlinear conversion efficiency of around 9.5% are obtained.•The results reveal that the proposed structure can be an alternative technique for terahertz generation in QCLs.