Resonantly enhanced bound-continuum intersubband second harmonic generation in optimized asymmetric semiconductor quantum wells

A systematic procedure applied to a step-asymmetric quantum well in order to maximize intersubband bound–continuum second-order susceptibility is described. The possibility is explored of obtaining resonantly enhanced nonlinear optical susceptibilities in quantum wells with two bound and a continuum resonance state as the dominant third state. This would significantly extend the range of input radiation photon energies that may be frequency doubled under resonance conditions in realistic structures. Calculation for the AlxGa1−xAs alloy based wells designed for pump photon energies in range of ℏω=200–300 meV indicate a perspective of employing continuum states in resonant second harmonic generation at higher photon energies

Laterally pumped GaAs/AlGaAs quantum wells as sources of broadband terahertz radiation

In this work we consider lateral current pumped GaAs/AlGaAs quantum wells as sources of incoherent terahertz radiation. The lateral field heats the electrons in a two-dimensional quantum layer and increases the population of higher subbands, hence also increasing the radiation power generated in spontaneous intersubband emission processes. Digitally graded quasi-parabolic and simple square quantum wells are considered, and the advantages of both types are discussed. Calculations at lattice temperatures of 77 K and 300 K, for electric fields up to 10 kV/cm, show that the optical output power of ~100−200 W/m2 may be achieved for the 7 THz source. The main peak of the spectrum, at 7 THz, of the quasi-parabolic quantum well exceeds the black body radiation at 300 K by approximately a factor of two and by two orders of magnitude at 77 K

Symmetry of k·p Hamiltonian in pyramidal InAs/GaAs quantum dots: Application to the calculation of electronic structure

A method for the calculation of the electronic structure of pyramidal self-assembled InAs/GaAs quantum dots is presented. The method is based on exploiting the C-4 symmetry of the 8-band k·p Hamiltonian with the strain taken into account via the continuum mechanical model. The operators representing symmetry group elements were represented in the plane wave basis and the group projectors were used to find the symmetry adapted basis in which the corresponding Hamiltonian matrix is block diagonal with four blocks of approximately equal size. The quantum number of total quasiangular momentum is introduced and the states are classified according to its value. Selection rules for interaction with electromagnetic field in the dipole approximation are derived. The method was applied to calculate electron and hole quasibound states in a periodic array of vertically stacked pyramidal self-assembled InAs/GaAs quantum dots for different values of the distance between the dots and external axial magnetic field. As the distance between the dots in an array is varied, an interesting effect of simultaneous change of ground hole state symmetry, type, and the sign of miniband effective mass is predicted. This effect is explained in terms of the change of biaxial strain. It is also found that the magnetic field splitting of Kramer's double degenerate states is most prominent for the first and second excited state in the conduction band and that the magnetic field can both separate otherwise overlapping minibands and concatenate otherwise nonoverlapping minibands

Quantum transport in semiconductor quantum dot superlattices: electron-phonon resonances and polaron effects

Electron transport in periodic quantum dot arrays in the presence of interactions with phonons was investigated using the formalism of nonequilibrium Green's functions. The self-consistent Born approximation was used to model the self-energies. Its validity was checked by comparison with the results obtained by direct diagonalization of the Hamiltonian of interacting electrons and longitudinal optical phonons. The nature of charge transport at electron -- phonon resonances was investigated in detail and contributions from scattering and coherent tunnelling to the current were identified. It was found that at larger values of the structure period the main peak in the current -- field characteristics exhibits a doublet structure which was shown to be a transport signature of polaron effects. At smaller values of the period, electron -- phonon resonances cause multiple peaks in the characteristics. A phenomenological model for treatment of nonuniformities of a realistic quantum dot ensemble was also introduced to estimate the influence of nonuniformities on current -- field characteristics

Towards automated design of quantum cascade lasers

We present an advanced technique for the design and optimization of GaAs/AlGaAs quantum cascade laser structures. It is based on the implementation of the simulated annealing algorithm with the purpose of determining a set of design parameters that satisfy predefined conditions, leading to an enhancement of the device output characteristics. Two important design aspects have been addressed: improved thermal behavior, achieved by the use of higher conduction band offset materials, and a more efficient extraction mechanism, realized via a ladder of three lower laser states, with subsequent pairs separated by the optical phonon energy. A detailed analysis of performance of the obtained structures is carried out within a full self-consistent rate equations model of the carrier dynamics. The latter uses wave functions calculated by the transfer matrix method, and evaluates all relevant carrier–phonon and carrier–carrier scattering rates from each quantized state to all others within the same and neighboring periods of the cascade. These values are then used to form a set of rate equations for the carrier density in each state, enabling further calculation of the current density and gain as a function of the applied field and temperature. This paper addresses the application of the described procedure to the design of lambda~9 µm GaAs-based mid-infrared quantum cascade lasers and presents the output characteristics of some of the designed optimized structures. © 2005 American Institute of Physic

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

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 χ0(2) which gives rise to the optical rectification. The merits and limits of the multiparameter procedure are discussed

Electronic structure and electron distribution in an inverse superatom calculated by self-consistent method

A full self-consistent procedure, applied to an inverse superatom strucuture is described. It is shown, both numerically and theoretically, that the electron concentration is large, but not maximal, the point of maximum being displaced off centre due to the fact that the second excited level has three times as many electrons as the ground level. Such an effect does not occur in classical quantum wells and superlattices. Moreover it is shown that the self-consistent treatment is necessary for an exact analysis of the energy band structure of the inverse superatom: solving the problem only by a trial rectangular potential gives an error of about 20%

The self-consistent calculation of discrete and continuous states in spherical semiconductor quantum dots

A self-consistent procedure for calculating the energy structure, wave functions, and charge distribution in spherically symmetric semiconductor quantum dots is presented that takes account of both bound and free-electron states. The Schrödinger and Poisson equations are solved iteratively while using the Morse-type parametrized potential to keep the charge neutrality in each iterative step. Numerical calculations performed for a GaAs-Al0.3Ga0.7As based quantum dot indicate that under realistic doping conditions bound states account for most of the charge accumulated in the dot. However, the self-consistent potential very significantly modifies the free-state wave functions and hence the bound-free transition matrix elements

The self-consistent electronic structure of spherical semiconductor quantum dots including bound and free states

A self-consistent procedure for calculating the energy structure, wave functions and charge distribution in spherically symmetric semiconductor quantum dots is presented, that takes account of both bound and free electron states. The Schrodinger and Poisson equation are solved iteratively while using the Morse-type parametrized potential to keep the charge neutrality in each iterative step. Numerical calculations performed for GaAs-Al0.3Ga0.7As based quantum dot indicate that bound states account for most of the charge accumulated in the dot, while including the free states is necessary only at larger doping levels to describe the depleted region outside the dot

Density matrix theory of transport and gain in quantum cascade lasers in a magnetic field

A density matrix theory of electron transport and optical gain in quantum cascade lasers in an external magnetic field is formulated. Starting from a general quantum kinetic treatment, we describe the intraperiod and interperiod electron dynamics at the non-Markovian, Markovian, and Boltzmann approximation levels. Interactions of electrons with longitudinal optical phonons and classical light fields are included in the present description. The non-Markovian calculation for a prototype structure reveals a significantly different gain spectra in terms of linewidth and additional polaronic features in comparison to the Markovian and Boltzmann ones. Despite strongly controversial interpretations of the origin of the transport processes in the non- Markovian or Markovian and the Boltzmann approaches, they yield comparable values of the current densities