Quantum Cascade Laser Design for Tunable Output at Characteristic Wavelengths in the Mid-Infrared Spectral Range

We present a method for systematic optimization of quantum cascade laser active region, based on the use of the genetic algorithm. The method aims at obtaining a gain-maximized structure, designed to emit radiation at specified wavelengths suitable for direct absorption by pollutant gasses present in the ambient air. After the initial optimization stage, we introduce a strong external magnetic field to tune the laser output properties and to slightly modify the emission wavelength to match the absorption lines of additional compounds. The magnetic field is applied perpendicularly to the epitaxial layers, thus causing two dimensional continuous energy subbands to split into series of discrete Landau levels. This affects all the relevant relaxation processes in the structure and consequently the lifetime of carriers in the upper laser level. Furthermore, strong effects of band nonparabolicity result in subtle changes of the lasing wavelength at magnetic fields which maximize the gain, thus providing a path for fine tuning of the output radiation properties. Numerical results are presented for GaAs/Al(x)Ga(1-x)As based quantum cascade laser structures designed to emit at particular wavelengths in the mid infrared part of the spectrum.11th Annual Conference of the Materials-Research-Society-of-Serbia (YUCOMAT 2009), Aug 31-Sep 04, 2009, Herceg Novi, Montenegr

Time Delay in Thin Dielectric Slabs with Saturable Nonlinearity

Time delays for an intense transverse electric wave propagating through a slab with saturable nonlinearity are investigated. The nonlinearity is assumed in a form of the Vinetskii-Kukhtarev model, which is relevant for the slabs made of nonlinear photorefractive crystals, such as GaAs and LiNbO(3), which feature a saturable nonlinearity. The expressions for the group delay and the dwell time are derived and the relation between them is studied. It is shown that the difference between them has three different contributions. The first one corresponds to the self-interference associated with the dispersion of the medium surrounding the slab. The other two appear due to the nonlinearity of the slab and oblique incidence of the transverse electric wave. All the results are compared with the case of dielectric slabs with cubic (Kerr) nonlinearity.10th Annual Conference of the Materials-Research-Society-of-Serbia, Sep 08-12, 2008, Herceg Novi, Montenegr

Inter-Landau Level Scattering Processes in Magnetic Field Assisted THz Quantum Cascade Laser

We present a detailed analysis of GaAs/AlGaAs terahertz quantum cascade laser in the presence of an intense external magnetic field. One of the objectives in further development of THz quantum cascade laser is the realization of structures operating at higher temperatures. This is difficult to obtain as the operating photon emission energy is smaller than the longitudinal-optical phonon energy in the semiconductor material. With increased temperature, electrons in the upper radiative state gain sufficient in-plane energy to emit an longitudinal-optical phonon, which represents a non-radiative scattering and reduces the optical gain. By applying strong magnetic field, two-dimensional continuous energy subbands become split into series of discrete Landau levels, and at particular values of B it is possible to quench these non-radiative channels. Numerical simulations are performed on two-well design quantum cascade laser operating at 4.6 THz, implemented in CaAs/Al(0.15)Ga(0.85)As, and the magnetic field is perpendicular to the epitaxial layers. Strong oscillations of carrier lifetimes for the upper state of the laser transition, as a function of magnetic field are observed, which can be attributed to interface roughness scattering and longitudinal-optical phonon scattering between Landau levels.12th Annual YUCOMAT Conference, Sep 06-10, 2010, Herceg Novi, Montenegr

Modeling of tunneling times in anisotropic non-magnetic metamaterials

Tunneling times in an anisotropic structure composed of alternately placed layers of two linear, dispersive and absorptive materials are analyzed in this paper. For such a structure, a general expression for the relation between the two most common tunneling time definitions, namely the dwell time and the group delay, is obtained. Numerical calculations are performed for a new type of semiconductor metamaterial. The results show a peak position frequency mismatch between the dwell time and the group delay, and between the dwell time and the absorption.3rd International School and Conference on Photonics, Aug 29-Sep 02, 2011, Belgrade, Serbi

Optimization of nonlinear optical rectification in parabolic quantum wells using supersymmetric quantum mechanics

A systematic procedure is proposed for the optimized design of semiconductor quantum well structures that provide maximal values of the resonant optical rectification coefficient. It relies on employing supersymmetric quantum mechanics to vary a starting potential in the isospectral manner, via the Variation of a single scalar parameter, in search of the largest available value of the appropriate product of matrix elements (difference of permanent moment times, the transition dipole moment squared, in this instance). An example the design of the structure matched for the optical rectification of 10.6 mu m (CO2 laser) radiation is presented that starts from the truncated parabolic potential and delivers the rectification coefficient in excess of the best value reported in literature. The possibility of realization of optimized structures is also discussed

Optimization of cubic GaN/AlGaN quantum well-based structures for intersubband absorption in the infrared spectral range

A method is proposed for the optimization of structural parameters of GaN/AlGaN quantum wells and Bragg-confined structures, with respect to peak intersubband absorption from the ground to the first excited electronic state in the mid and near infrared spectral range. It is based on the application of the Genetic Algorithm and delivers globally optimal structures with a preset number of embedded layers. Simple rectangular quantum well profile is investigated for the maximal Stark effect and applications to tunable mid-infrared photodetectors. In case of Bragg-confined structures, an above the barrier bound state is used to extend the range of transition energies above the values available in conventional quantum wells. The effects of band nonparabolicity are taken into account. (C) 2013 Elsevier Ltd. All rights reserve

Delay times in a terahertz chiral metamaterial slab

We investigate the propagation of circularly polarized electromagnetic waves through a free-standing Omega particle chiral metamaterial slab and study the associated delay times: dwell time and group delay. Through this analysis, we observe different interactions of right- and left-circularly polarized waves in the terahertz frequency range (1-2.5 THz) with the resonant elements. As a consequence, the resonant-frequency group delay and dwell time of the right-circularly polarized wave are one or more orders of magnitude longer than those of the left. In addition, we compare the delay times obtained from rigorous numerical simulations with those calculated using the retrieved effective parameters. We find that the dwell time cannot be evaluated correctly from the effective medium approximation, while no such problems exist in the case of group delay

Influence of the geometry of terahertz chiral metamaterial on transmission group delays

Recently, group delay has been proven as a very convenient tool for control of terahertz electromagnetic signals used for filters, waveguides and polarization components. Here, we investigate the propagation of circularly polarised terahertz pulse via analysis of group delays in X particle chiral metamaterial. When varying the geometry of the X particle chiral metamaterial, significant modification of chiroptical effects- optical activity and circular dichroism can be observed. Through the analysis of group delays in the frequency region between 1 and 3 THz we conclude that the right circularly polarised wave is more sensitive to changes in the geometry than the left circularly polarised wave. This result implies that the right circularly polarised wave is responsible for variation of chiroptical effects in these structures and opens up a possibility for potential applications

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 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.15th Central European Workshop on Quantum Optics, May 29-Jun 03, 2008, Belgrade, Serbi