Jelena Radovanović

Optimization of resonant second- and third-order nonlinearities in step and continuously graded semiconductor quantum wells

Methods for systematic optimization of step-graded and continuously graded ternary alloy based quantum wells (QWs), in respect to second- or third-order intersubband nonlinear susceptibilities at resonance, are discussed. The use of these methods is examplified on the design of Al/sub x/Ga/sub 1-x/N and Al/sub x/Ga/sub 1-x/As-based QWs intended for resonant second harmonic or third harmonic generation with h/spl omega/=116 meV or h/spl omega/=240 meV pump photon energies, the objective being the largest susceptibility achievable with the chosen material. The obtained results exceed those previously reported in the literature.

Electron-phonon relaxation rates and optical gain in a quantum cascade laser in a magnetic field

We present a model for calculating the optical gain in a midinfrared GaAs∕AlGaAs quantum cascade laser in a magnetic field, based on solving the set of rate equations that describe the carrier density in each level, accounting for the optical- and acoustic-phonon scattering processes. The confinement caused by the magnetic field strongly modifies the lifetimes of electrons in the excited state and results in pronounced oscillations of the optical gain as a function of the field. Numerical results are presented for the structure designed to emit at λ∼11.4μm, with the magnetic field varying in the range of 10–60T. The effects of band nonparabolicity are also included.

Intersubband absorption in Pöschl–Teller-like semiconductor quantum wells

Abstract The bound–bound, bound–free and free–free intersubband optical transitions are considered in semiconductor quantum wells derived from the Poschl–Teller potential with a constant electron mass. To make this type of Hamiltonian effectively realizable in ternary (Al x Ga 1− x As) alloys, where the effective electron mass necessarily varies, just as does the potential, the coordinate transform method was employed to design variable-mass variable-potential structures realizable in Al x Ga 1− x As, with their Hamiltonians fully equivalent to the constant-mass Poschl–Teller case. Applications of this special QW profile for photodetectors operating in the infrared are analyzed and the results are compared to those obtained in the simple square wells.

Influence of the active region design on output characteristics of GaAs/AlGaAs quantum cascade lasers in a strong magnetic field

We describe the application of our computational model, developed for finding the optical gain in a mid-infrared quantum cascade laser subjected to a strong magnetic field, to two distinct λ ~ 9 µm GaAs-based structures. The additional carrier confinement induced by the field alters the transition rates for the optical- and acoustic-phonon scattering processes from the upper laser level, thus affecting the laser output properties, in particular the optical gain. Within this model, the gain is found by solving the system of rate equations, from which the carrier densities in each level are calculated. Numerical results are presented for magnetic fields between 10 and 60 T, and the band nonparabolicity is taken into account.

Phase-breaking effects in double-barrier resonant tunneling diodes with spin-orbit interaction

Several recent theoretical studies showed that the spin-orbit interaction in narrow gap InGaAs/InAlAs double-barrier resonant tunneling structures might yield a highly spin-polarized current in the ballistic limit. In this paper, a nonequilibrium Green’s function model is used to examine the effect of phase-breaking on the spin-dependent transport of carriers. The scattering is described as a local interaction with a bath of scatterers and treated in the self-consistent first Born approximation. Elastic and inelastic scatterers, with scattering strengths that cause a few millielectron volt broadening of quasibound states, have been found to significantly reduce the spin polarization. The magnitude of spin polarization has been found to be dominantly determined by the quasibound state broadening, while the interaction details are not significant.

Optimal design of GaN-AlGaN Bragg-confined structures for intersubband absorption in the near-infrared spectral range

A method is proposed for the design and optimization of structural parameters of GaN-AlGaN Bragg-confined structures with respect to peak intersubband absorption from the ground to the first excited state,1 /spl rarr/ 2 electronic transition, in the near infrared spectral range. An above-the-barrier bound state was used to extend the range of transition energies above the values available in conventional quantum wells. Intrinsic polarization fields and nonparabolicity effects were taken into account. The selection of optimal parameters, maximizing the absorption at wavelengths of 1.55 and 1.3 /spl mu/m, was performed by using a simulated annealing algorithm, and optimal structures with infinite superlattices as confinement regions were thus designed. These optimal parameters were then used to set realistic, finite structures with a small number of layers, the performance of which was re-evaluated by solving the Schrodinger-Poisson equation self-consistently for a few different levels and profiles of doping.

Analysis of tunneling times in absorptive and dispersive media

Tunneling times in absorptive and dispersive media are considered, as are the relations between them. Group delay and dwell time are used as the most appropriate tunneling time characterizations. A general expression that relates these two quantities, valid for all linear media (with both positive and negative index of refraction), is derived, but particular attention is given to negative index metamaterials. The example of a nonmagnetic, lossless medium with dispersive surroundings was chosen to illustrate the derivation of self-interference time. Existence of the Hartman effect and negative group delay in a certain range of frequencies, in metamaterials, is numerically verified.

Anisotropic spin-dependent electron tunneling in a triple-barrier resonant tunneling diode

The one-band envelope function approximation is used to investigate the spin-dependent tunneling of conduction band electrons in semiconductor heterostructures when both the bulk inversion asymmetry (BIA) and structure inversion asymmetry (SIA) are present. It is shown that under certain conditions the interplay between BIA and SIA may be used to induce a strong dependence of transmission probabilities on the direction of electrons lateral momenta thus offering means to improve the existing designs of nonmagnetic semiconductor spin filters.

Quantum-well profile optimization for maximal Stark effect and application to tunable infrared photodetectors

Two procedures are described for quantum-well shape tailoring, such that both the Stark effect and the intraband absorption on the 1→2 transition in a prescribed range of bias fields are maximized. One of them relies on the isospectral transform of the Hamiltonian, and delivers quantum wells with continuously graded composition. The other uses the simulated annealing algorithm and delivers globally optimal stepwise-graded wells with a preset number of layers. Numerical calculation is performed for wells based on the AlxGa1−xAs alloy.

Optimization of spin-filtering properties in diluted magnetic semiconductor heterostructures

We have performed structural parameter optimizations of asymmetric ZnSe∕Zn1−xMnxSe multilayer structures in magnetic and electric fields to maximize the spin polarization of the electron tunnel current. The optimization procedure was carried out by simulated annealing, with target functions set to obtain the best possible spin-polarization properties within the chosen range of bias voltages. The performance of the optimized structure is predicted to exceed that of the existing spin-diode designs.