We present a microscopic many-particle theory for the dephasing of coherent intersubband excitations in semiconductor quantum wells including carrier-carrier and carrier-phonon scattering and light propagation effects. The contributions of many-particle processes are nonadditive and thus cannot be treated separately. It is shown that due to nondiagonal correlation contributions, scattering rates alone cannot be taken as a measure for the dephasing of the intersubband polarization. Surprisingly, radiative damping is found to be important even at moderate carrier densities. Calculated absorption spectra are in excellent agreement with experiments on a high-quality sample.

A many-body theory based on nonequilibrium Green functions, in which transport and optics are treated on a microscopic quantum mechanical basis, is used to compute gain and absorption in the optical and THz regimes in quantum cascade laser structures. The relative importance of Coulomb interactions for different intersubband transitions depends strongly on the spatial overlap of the wavefunctions and the specific nonequilibrium populations within the subbands. The magnitude of the Coulomb effects can be controlled by changing the operation bias.

In a recent Letter [Appl. Phys. Lett. 82, 1015(2003], Williams et al. reported the development of a terahertz quantum cascade laser operating at 3.4 THz or 14.2 meV. We have calculated and analyzed the gain spectra of the quantum cascade structure described in their work, and in addition to gain at the reported lasing energy of ≃ 14 meV, we have discovered substantial gain at a much lower energy of around 5 meV or just over 1 THz. This suggests an avenue for the development of a terahertz laser at this lower energy, or of a two-color terahertz laser.

We investigate lasing and transport properties of GaAs/Al x Ga 1−x As quantum cascade laser structures with varying injector doping density n e . Calculations from a nonequilibrium Green function theory show a linear dependence of the current density on n e in agreement with the experimental trend. We compare theoretical results for the threshold current density J th and laser emission energy as a function of n e with previously obtained experimental results.

We apply a quantum transport theory based on nonequilibrium Green's functions to quantum cascade laser (QCL) structures, treating simultaneously the transmission through the injector regions and the relaxation due to scattering in the active region. The quantum kinetic equations are solved self-consistently using self-energies for interface roughness and phonon scattering processes within the self-consistent Born approximation. In this way, we obtain the current density J, and the average electronic distribution f(E) at a given energy E, as a function of applied bias. As a test case, we apply the theory to a GaAs/Al_xGa_{1-x}As QCL structure reported in the literature. The theoretical results reproduce well reported voltage-current (V-I) measurements, and also demonstrate a population inversion at a bias that agrees well with the range of currents and fields at which lasing is observed.Comment: 10 pages, 3 figures. To be published in Physica E, proceedings MSS10 (2001