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We have analyzed the spin-filtering effects of the electron current in asymmetric ZnSe/Zn1-xMnxSe multilayer structures, under the influence of both an external magnetic field and a bias voltage. In this type of semiconductor systems, conduction band electrons interact with 3d electrons of the magnetic Mn2+ ions. Because of this sp-d exchange interaction, an external magnetic field modulates the effective potential profile seen by spin-up and spin-down electrons, giving rise to a large Zeeman effect. It is found that the degree of spin polarization changes significantly when the electrical bias is switched from forward to reverse, thus the proposed structure displays obvious behavior of spin-filter diode. This originates from the effective “lifting” of the potential for spin-up electrons, which tunnel through actual potential barriers. Structural parameters optimization, with the goal of maximizing the spin-filtering coefficient, was performed by using simulated annealing algorithm. The described effect may be important for designing new tunable spin-based multifunctional semiconductor devices.

The optical gain in the active region of quantum cascade laser in an external magnetic field is analyzed. When the magnetic field is applied in the direction perpendicular to the plane of the layers, electron dispersion is broken into series of discrete Landau levels. This additional confinement strongly modifies the lifetime of electrons in the upper state of the laser transition, which is controlled by electron-phonon scattering. Landau levels are magnetically tuneable and, depending on their configuration, phonon emission is either inhibited or resonantly enhanced. This translates into a strong modulation of the population inversion, and consequently of the optical gain by varying the magnetic field. Numerical results are presented for a structure previously considered by Smirnov et al. [Phys. Rev B 66 (2002) 125317] which is designed to emit radiation at λ~11.4µm, with the magnetic field varied in the range 10-60T. The effects of band nonparabolicity are taken into account in this model.

A method is proposed for finding optimal AlGaN-based quantum well profiles with respect to specified intersubband optical properties, e.g. nonlinear optical susceptibilities at resonance. It is based on applying the isospectral transformations (supersymmetric and coordinate transform) to an initial Hamiltonian in order to generate a family of parameters of controlled potentials, accompanied by position-dependent effective mass. By varying those control parameters, one call change the values of dipole matrix elements relevant for a particular effect, while energy levels, once obtained in the initial potential, remain unchanged. The internal electrostatic field, originating from piezoelectric and spontaneous polarization in the nitride alloy, has been taken into account. The use of the method is illustrated by maximizing second order nonlinear optical susceptibilities at wavelengths in the near infrared spectral range, and optimal smooth structures are eventually discretized to make their fabrication easier.

The object of this paper is to analyze the electronic and optical properties of semiconductor nanostructures made of semiconductors with more than one significant minimum in the conduction band by applying the single-band envelope-function model in the entire Brillouin zone. We have developed a method of calculating the eigenvalues and eigenfunctions of the corresponding Hamiltonian and explored the possibility of forming a single-zone model for the entire Brillouin zone by taking the exact dependence of the conduction band energy on the wave vector E(k). Since the crystalline potential is periodic, an expansion of the electron energy band can be made in terms of vectors of the direct lattice. This method is applied to GaAs and Si quantum wells and quantum wires with square and rectangular cross sections of [001] and [110] orientation, and its accuracy is examined by comparing it with other numerically more complex and demanding methods.

A procedure is proposed for finding the globally optimal semiconductor quantum well profile in respect to the second order intersubband susceptibilities at resonance. It relies on the combined use of simulated annealing and variational calculus (i.e. the optimal control theory), and effectively performs a free variation of the quantum well (QW) profile, searching for the global optimum. The proposed method does not depend on the choice of input ("seed") potential and includes the variation of a continuous function instead of a set of scalar parameters, so it should lead to the globally optimal QW profile for a particular application, unconstrained to any particular class of functional forms.

The possibility of employing Bragg-confined semiconductor structures for resonant second and third order harmonic generation is considered in this paper. Advantage is taken of the existence of above-the-barrier bound states in order to extend the range of input photon energies above the values available in conventional Al x Ga 1-x As quantum wells. The optimization procedure is based on solving the system of nonlinear equations which involve the geometric parameters of the structure and bound state energies. Numerical results are given for asymmetric structures based on Al x Ga 1-x As, designed for resonant second harmonic generation (at incident photon energy ∞ω = 240 meV) and third harmonic generation (at ∞ω = 240 meV).