Microscopic model for intersubband gain from electrically pumped quantum-dot structures

Abstract

We study theoretically the performance of electrically pumped self-organized quantum dots as a gain material in the mid-IR range at room temperature. We analyze an AlGaAs/InGaAs based structure composed of dots-in-a-well sandwiched between two quantum wells. We numerically analyze a comprehensive model by combining a many-particle approach for electronic dynamics with a realistic modeling of the electronic states in the whole structure. We investigate the gain both for quasi-equilibrium conditions and current injection. Comparing different structures we find that steady-state gain can only be realized by an efficient extraction process, which prevents an accumulation of electrons in continuum states, that make the available scattering pathways through the quantum dot active region too fast to sustain inversion. The tradeoff between different extraction/injection pathways is discussed. Comparing the modal gain to a standard quantum-well structure as used in quantum cascade lasers, our calculations predict reduced threshold current densities of the quantum dot structure for comparable modal gain. Such a comparable modal gain can, however, only be achieved for an inhomogeneous broadening of a quantum-dot ensemble that is close to the lower limit achievable today using self-organized growth.

Mid-Infrared Quantum-Dot Quantum Cascade Laser: A Theoretical Feasibility Study

“…The model system of a QD structure as a gain material for QCLs that we investigate here was introduced in Reference [29], where the material composition and geometry have been optimized. Here, we give only a brief description.…”

“…Furthermore, our dynamical model is based on the semiconductor Bloch equations and contains Boltzmann-like electron-phonon and electron-electron scattering terms including polaronic effects with an effective quasi-particle broadening [29]. The latter is especially important for scattering where discrete QD levels are involved.…”

“…Among the Auger-like contributions we count the following processes: QW-assisted QD capture/emission (three QW states involved), QW-assisted QD scattering (transitions between two QW states and two QD states, respectively), pure QD-QW scattering (QD-QW transitions), and QD-assisted QD capture/emission (three QD states involved). More details on the calculation of the electronic structure, the Coulomb-or electron-phonon scattering matrix elements and the derivation of the scattering contributions can be found in Reference [29].…”

“…where Γ con = 0.65 is the confinement factor,hω 0 = 105 meV is the transition energy, n b = 3.4 (GaAs) the background refractive index of the host material, = 50 nm is the periodicity length of the structure, µ = 2.5 e nm the dipole moment,hγ d = 1 meV the polarization dephasing (see also Reference [29] for the choice of this quantity), n QD the QD density, and N the inversion of the optically active states. The inhomogeneous broadening, in contrast to the homogeneous broadening determined by the polarization dephasing, acts as an effective reduction Γ inh of the density n QD of QDs that are resonant with the optical field and is calculated assuming a Gaussian distributed level spacing of the QD ensemble.…”

“…In particular, we have proposed a dot-in-a-well QD structure (DWELL structure) sandwiched between QWs, i.e., a QW-QD-QW structure [29,30].…”