Optical and transport characteristics of quantum-cascade lasers with optimized second-harmonic generation

We present simulations of midinfrared quantum-cascade lasers (QCLs) with optimized second-harmonic generation (SHG). The optimized design was obtained utilizing techniques from supersymmetric quantum mechanics with both material-dependent effective mass and band nonparabolicity. Carrier transport and power output of the structure are analyzed by self-consistently solving rate equations for the carriers and photons. Nonunity pumping efficiency from one period of the QCL to the next is taken into account by including all relevant electron-electron and electron-longitudnal (LO) phonon scattering mechanisms between the injector/collector and active regions. Two-photon absorption processes are analyzed for the resonant cascading triple levels designed for enhancing SHG. Both sequential and simultaneous two-photon absorptions are included in the rate-equation model. The current-output characteristics for both structures are analyzed and compared. Stronger resonant tunneling in the optimized structure is manifested by enhanced negative differential resistance. Current-dependent linear optical output power is derived based on the steady-state photon populations in the active region. The second-harmonic-power is derived from the Maxwell equations with the phase mismatch included. Due to stronger coupling between lasing levels, the optimized structure has both higher linear and nonlinear output powers. Phase mismatch effect is significant for both structures leading to a substantial reduction of the linear-to-nonlinear conversion efficiency. The optimized structure can be fabricated through digitally grading the submonolayer alloys by molecular beam epitaxy technique.