Abstract:Current active region designs of terahertz Quantum Cascade Lasers (QCL) include chirped superlattice, bound-to-continuum transition and resonant phonon modes. The density matrix method is employed to study the properties of gain of THz QCL, especially the dependences of the QCL’s gain on the inter-subband transitions’ lifetimes. Based on the analysis, an active region design of THz QCL based on bound-to-continuum transition with resonant phonon scattering extraction is presented, employing the longitudinal optical phonon scattering to shorten the lifetime of the lower radiation level of QCL.