V. Milanović

Optically pumped terahertz laser based on intersubband transitions in a GaN∕AlGaN double quantum well

A design for a GaN∕AlGaN optically pumped terahertz laser emitting at 34 μm (ΔE∼36meV) is presented. This laser uses a simple three-level scheme where the depopulation of the lower laser level is achieved via resonant longitudinal-optical-phonon emission. The quasibound energies and associated wave functions are calculated with the intrinsic electric field induced by the piezoelectric and the spontaneous polarizations. The structures based on a double quantum well were simulated and the output characteristics extracted using a fully self-consistent rate equation model with all relevant scattering processes included. Both electron-longitudinal-optical phonon and electron-acoustic-phonon interactions were taken into account. The carrier distribution in subbands was assumed to be Fermi–Dirac-like, with electron temperature equal to the lattice temperature, but with different Fermi levels for each subband. A population inversion of 12% for a pumping flux Φ=1027cm−2s−1 at room temperature was calculated for th...

Designing strain-balanced GaN/AlGaN quantum well structures: Application to intersubband devices at 1.3 and 1.55 μm wavelengths

A criterion for strain balancing of wurtzite group-III nitride-based multilayer heterostructures is presented. Single and double strain-balanced GaN/AlGaN quantum well structures are considered with regard to their potential application in optoelectronic devices working at communication wavelengths. The results for realizable, strain-balanced structures are presented in the form of design diagrams that give both the intersubband transition energies and the dipole matrix elements in terms of the structural parameters. The optimal parameters for structures operating at λ∼1.3 and 1.55 μm were extracted and a basic proposal is given for a three level intersubband laser system emitting at 1.55μm and depopulating via resonant longitudinal optical (LO) phonons (ℏωLO≈90 meV).

Intraband absorption in InAs/GaAs quantum dot infrared photodetectors—effective mass versus k × p modelling

Theoretical modelling of the intraband absorption spectrum in InAs/GaAs quantum dot infrared photodetectors is performed for several typical structures reported in the literature. The calculations are performed within the framework of the two methods: a simple and so far widely used effective mass method with the values of conduction band offset and the effective mass modified to take account of the effects of strain and band mixing on average and the more realistic eight-band k × p method with the strain distribution taken into account via the continuum mechanical model. Both methods give qualitatively the same results; however, the peak positions obtained within the effective mass approach are blue shifted and the absorption cross sections are overestimated, compared to the more accurate k × p approach.

Optimization and magnetic-field tunability of quantum cascade laser for applications in trace gas detection and monitoring

We explore the possibilities of using advanced tools for global optimization, namely the genetic algorithm, to obtain structural parameters of gain-maximized quantum cascade laser (QCL) emitting at specified wavelengths, suitable for detection of pollutant gasses, such as SO2, HNO3, CH4 and NH3, in the ambient air. Upon completing this initial optimization stage, we introduce a strong external magnetic field perpendicular to the epitaxial layers, to fine tune the laser output properties. This magnetic field alters the electron energy spectrum by splitting the continuous energy subbands into discrete Landau levels whose arrangement influences the magnitude of the optical gain. In addition, strong effects of band nonparabolicity result in subtle changes in the lasing wavelength at magnetic fields which maximize the gain, thus allowing us to explore the prospects of multi-wavelength emission of the given structure, and achieving resonance with additional compounds, absorbing at wavelengths close to the original one. Numerical results are presented for GaAs/AlxGa1−xAs based QCL structures designed for operation in the mid-infrared part of the spectrum.

Application of the genetic algorithm to the optimized design of semimagnetic semiconductor-based spin-filters

We have further improved our procedure for the optimization of multilayer semiconductor nanostructures, based upon diluted magnetic semiconductors, developed previously with the goal of maximizing their spin-filtering properties. The new optimization scheme relies on the application of a modern class of evolutionary algorithms for global optimization, specifically the genetic algorithm. Its fitness function is set to select the best possible spin-polarization properties within the chosen range of bias voltages and with a fixed value of external magnetic field. Numerical calculations are presented for the ZnSe/Zn1−xMnxSe based semiconductor system, and the obtained results predict an enhanced spin-diode performance over the existing designs.

Electron transport in n-doped Si/SiGe quantum cascade structures

An electron transport model in n-Si/SiGe quantum cascade or superlattice structures is described. The model uses the electronic structure calculated within the effective-mass complex-energy framework, separately for perpendicular (Xz) and in-plane (Xxy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is then described via scattering between quantized states, using a rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbor periods. Acoustic phonon, optical phonon, alloy disorder, and interface roughness scattering are taken into account. The calculated current/voltage dependence and gain profiles are presented for two simple superlattice structures.

The absorption cross section for bound–free transitions in semiconductor quantum dots

Abstract The absorption cross sections for intraband transitions in spherical semiconductor quantum dots are considered, with the self-consistent effects taken into account. Electronic states are described by the effective mass Schrodinger equation, and the influence of accumulated electronic charge on the electronic structure is accounted for by the conventional numerical self-consistent procedure, i.e. by solving the Schrodinger and the Poisson equations iteratively. The self-consistent Hartree potential is found to push the wave function out of the dot “core” and thus may significantly influence the transition matrix elements, particularly in cases where the upper state is shallow or becomes a resonant state upon including the self-consistency.

Intersubband absorption at λ ∼ 1.3 μm in optimized GaN/AlGaN Bragg-confined structures

A method is developed for the analysis and extracting the optimal structural parameters of GaN/AlGaN Bragg-confined structures, in order to maximize the intersubband absorption on bound–above-the-barrier transitions at wavelengths in the near infrared spectral range. The “built-in” electrostatic field originating from piezoelectric and spontaneous polarization, and band nonparabolicity are taken into account. The optimal GaN/Al0.65Ga0.35N Bragg-confined structure designed for maximal absorption at λ=1.3 μm (950 meV) provides a fractional absorption of 1.5%, at 2×1012 cm−2 doping per active well.

Multiparameter optimization of optical nonlinearities in semiconductor quantum wells by supersymmetric quantum mechanics

Abstract The multiparameter procedure of semiconductor quantum well profile optimization, using the supersymmetric quantum mechanics, is described and explored. The method generates families of isospectral potentials that depend on a specified number of scalar parameters, which are then varied so to maximize the desired property of the system, in this case the nonlinear susceptibility χ 0 (2) which gives rise to the optical rectification. The merits and limits of the multiparameter procedure are discussed.

Enhanced modeling of band nonparabolicity with application to a mid-IR quantum cascade laser structure

We analyze the influence of conduction-band nonparabolicity on bound electronic states in the active region of a quantum cascade laser (QCL). Our model assumes expansion of the conduction-band dispersion relation up to a fourth order in wavevector and use of a suitable second boundary condition at the interface of two III-V semiconductor layers. Numerical results, obtained by the transfer matrix method, are presented for two mid-infrared GaAs/Al0.33Ga0.67As QCL active regions, and they are in very good agreement with experimental data found in the literature. Comparison with a different nonparabolicity model is presented for the example of a GaAs/Al0.38Ga0.62As-based mid-IR QCL. Calculations have also been carried out for one THz QCL structure to illustrate the possible application of the model in the terahertz part of the spectrum.