J. Radovanović

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.

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.

Exploring negative refraction conditions for quantum cascade semiconductor metamaterials in the terahertz spectral range

In order to avoid losses in metamaterial unit cells at frequencies of interest, caused by metallic inclusions, an active medium design has been proposed. As candidate structures for this active medium, we have chosen quantum cascade lasers because of their high output gain. Here we analyze and compare two quantum cascade structures that emit at 4.6 THz and 3.9 THz, respectively, placed under the influence of a strong magnetic field. We first solve the full system of rate equations for all relevant Landau levels, and obtain the necessary information about carrier distribution among the levels, after which we are able to evaluate the permittivity component along the growth direction of the structure. With these data one can determine the conditions under which negative refraction occurs, and calculate the values of the refractive index of the structure, as well as the range of frequencies at which the structure exhibits negative refraction for a predefined total electron sheet density.

Modeling of electron relaxation processes and the optical gain in a magnetic-field assisted THz quantum cascade laser

We present a detailed model for calculating the optical gain of a quantum cascade laser (QCL) that operates in the terahertz spectral range, when subjected to a strong magnetic field, as well as the total relaxation rates due to the emission of longitudinal-optical phonons and interface roughness scattering, as a function of the applied field. When the magnetic field is applied in the direction perpendicular to the plane of the layers, each energy state is split into a series of discrete Landau levels, which are magnetically tunable, and it is therefore possible to control the modulation of the population inversion and consequently the optical gain just by varying the magnetic field. In this model, the gain is obtained by solving the full system of rate equations, from which one can calculate the carrier density of each level. The simulations are performed on a system that comprises a two-well design QCL that operates at 4.6 THz, implemented in GaAs/Al0.15Ga0.85As. Numerical results are presented for magnetic field values from 1.5 T up to 20 T, while the band non-parabolicity is taken into account.

Scattering effects in resonant magnetotunneling in InAs-based heterostructures

Electron transport through an InGaAs resonant tunneling structure with Rashba spin-orbit interaction and magnetic field parallel to the growth direction was studied theoretically. A nonequilibrium Greens function model was used, wherein interface roughness and longitudinal optical phonon scattering are treated in the self-consistent first Born approximation. The model predicts the main features of the two-dimensional magnetopolaron density of states and the secondary peaks in the I-V curve due to both resonant elastic and inelastic scattering. The I-V curves were studied at magnetic fields around the magnetophonon resonance and the elastic and inelastic contributions identified. At these fields (5 to 7 T), the current spin polarization was found to be dominated by the Zeeman effect and significant even in the presence of scattering events.

A quantum transport model for the double-barrier nonmagnetic spin filter

A model for calculating the current and spin polarization in a double-barrier InGaAs resonant tunnelling structure is described with the aim to account for phase-breaking scattering. It is based on the nonequilibrium Greens function method with both elastic and inelastic (LO-phonon) scattering described within the self-consistent first Born approximation. It has been found that the maximum current spin polarization of around 0.4 in the ballistic limit decreases to around 0.1 for scattering transport with scattering-induced broadening of quasi-bound states of around 4meV.

Modeling of dwell time and group delay in dispersive and absorptive media

In this paper, a more general expression that describes the relationship between dwell time and group delay is derived. This expression is valid for all kinds of materials, including negative-index metamaterials (NIMs). An obstacle made of double-negative NIMs (DN-NIMs) and surrounded by a double-positive waveguide was used as a model. In the cases where the obstacle was made of left-handed materials and the surroundings were air, it has been shown that the dwell time and absorption have similar dependences on the incident wave frequency. On the other hand, group delay becomes negative in some cases. Numerical results show that an increase in the length of the obstacle leads to saturation of the dwell time and absorption, which is in accordance with the phenomenon known as the Hartman effect. Similar results were obtained for terahertz range of frequencies and for the dispersive waveguide. In this case, it is shown that there is a certain range of frequencies where group velocity is positive, whereas the phase velocity remains negative, i.e. the peak of the output pulse appears before the peak of the input pulse. Finally, the use of a model that considers an obstacle made of a lossless, non-magnetic metamaterial, with background permittivity equal to 1 and a dispersive waveguide, leads to the appearance of a new delay, called self-interference time.

Frequency conversion in a-GaN/AlGaN Bragg-confined structures for applications for solar cells

Solar cells are unable to absorb photons with energies lower than the constituent materials band-gap, which eliminates a significant part of the spectrum. The efficiency of the process may be increased by converting low-energy photon pairs, via nonlinear optical effects, into photons optimal for the solar cell. The converter structures consist of GaN/AlGaN super lattice series perturbed by asymmetric quantum wells which form the Bragg confined structures (BSCs). BCSs support additional bound electron states in the super lattice minigaps, including states above the barrier. Parameters of the structure are optimized for each photon pair, by using the genetic algorithm, to obtain a continual converter.

GaInAs/AlInAs quantum cascade laser design based on optimized second harmonic generation

In this work, we present an innovative procedure for the design and optimization of GaInAs/AlInAs quantum cascade laser (QCL) structures based on the use of the genetic algorithm. The purpose of the algorithm is to determine the set of design parameters that would enable the maximization of the second order nonlinear susceptibility, thus facilitating significant optical nonlinearities to take place. In our optimization model, we start from the existing design in which the active region consists of two coupled InGaAs quantum wells separated by an AlInAs barrier, and the active region levels form double resonant nonlinear cascades. Upon obtaining the optimized structure and evaluating its energies and wave functions, the output characteristics are calculated by applying the full self-consistent rate equation modeling of the electron transport in a periodic QCL structure. The results of the calculations predict a noticeable improvement of targeted properties of the optimized design, while at the same time the original design calculations show excellent agreement with experimental results. The described procedure is applicable to various active region designs and can be used for other wavelength ranges.