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

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We report gain calculations for a quantum cascade laser using a fully self-consistent quantum mechanical approach based on the theory of nonequilibrium Green functions. Both the absolute value of the gain as well as the spectral position at threshold are in excellent agreement with experimental findings for T = 77K. The gain strongly decreases with temperature.

The optical spectra of a quantum cascade laser structure under nonequilibrium conditions is studied using the technique of nonequilibrium Green functions. We focus on the impact of Coulomb corrections and their dependence on the operating conditions. The role of the static mean field due to the charge distribution by electrons and ioninzed donors is investigated.The development of the quantum cascade laser (QCL) from its original concept [1], and first experimental realization [2], has been dramatic in recent years. It has evolved to be a high performance midinfrared device [3], and a potential source of THz radiation [4]. A number of different architectures have been efficiently designed and simulated using semi-phenomenological approaches [5]. An alternative to those schemes constitutes the nonequilibrium Green's functions approach [6], which allows for a detailed study of microscopic phenomena and provides a starting point for predictive device simulations.The operation of a quantum cascade laser depends crucially on the energetic alignment of the quantized levels in the conduction band. These are mainly determined by the sequence of heterostructure layers and the bias applied. In addition, the electronic charge in combination with doping layers contributes to an additional mean field potential. The spatial distribution of the electrons depends strongly on the transport processes and is typically far from thermodynamic equilibrium under laser operating conditions.Next to the relocation of energy levels, the electron-electron interactions are known to have pronounced effects on the gain/absorption spectrum which has been thoroughly studied for intersubband optical absorption of quantum wells [7]. On the other hand, the gain spectra of a rather more complicated QCL structure [3] have been explained relatively well without those effects [6]. Recently we could show, that these many-body correction are indeed small for gain transitions in typical infrared QCLs [8]. These many-body effects depend essentially on the occupations of the subbands and on the detailed electronic wavefunctions, determining Coulomb matrix elements. Taking into account the interplay between quantum evolution and scattering, a nonequilibrium Keldysh Green's functions approach was used for this purpose. In contrast, simplifying approximations that are relatively successful for quantum wells fail in the more complex QCL scenario.Here we want to concentrate on the role of the static mean field for the QCL operation. This is routinely considered in the analysis and design of QCL's. However standard approaches, which do not solve the transport problem self-consistently, rely on simple estimates for the location of the electron charge density. Thus the relevance of the correct mean field for device performance is an important issue for sample design.Our calculations are based on the self-consistent solution of the quantum kinetic equations under operating steady-state conditions [6]. Absorption and gain are evaluated from the imaginary part of the