Energy spectra of two-electron two-dimensional quantum dots confined by elliptical and bowl-like potentials

The laterally confining potential of quantum dots (QDs) fabricated in semiconductor heterostructures is approximated by an elliptical two-dimensional harmonic-oscillator well or a bowl-like circular well. The energy spectrum of two interacting electrons in these potentials is calculated in the effective-mass approximation as a function of dot size and characteristic frequency of the confining potential by the exact diagonalization method. Energy level crossover is displayed according to the ratio of the characteristic frequencies of the elliptical confinement potential along the y axis and that along the x axis. Investigating the rovibrational spectrum with pair-correlation function and conditional probability distribution, we could see the violation of circular symmetry. However, there are still some symmetries left in the elliptical QDs. When the QDs are confined by a "bowl-like" potential, the removal of the degeneracy in the energy levels of QDs is found. The distribution of energy levels is different for the different heights of the barriers. (C) 2003 American Institute of Physics.

Impact of polarization on quasi-two-dimensional exciton and barrier-width dependence of the exciton associated transition in wurtzite III-V nitride quantum wells

Excitonic states in AlxGa1-xN/GaN quantum wells (QWs) are studied within the framework of effective-mass theory. Spontaneous and piezoelectric polarizations are included and their impact on the excitonic states and optical properties are studied. We witnessed a significant blue shift in transition energy when the barrier width decreases and we attributed this to the redistribution of the built-in electric field between well layers and barrier layers. For the exciton the binding energies, we found in narrow QWs that there exists a critical value for barrier width, which demarcates the borderline for quantum confinement effect and the quantum confined Stark effect. Exciton and free carrier radiative lifetimes are estimated by simple argumentation. The calculated results suggest that there are efficient non-radiative mechanisms in narrow barrier QWs. (C) 2002 Elsevier Science Ltd. All rights reserved.

Blocking of the polaron effect and spin-split cyclotron resonance in a two-dimensional electron gas

Cyclotron resonance (CR) of high density GaAs quantum wells exhibits well-resolved spin splitting above the LO-phonon frequency. The spin-up and spin-down CR frequencies are reversed relative to the order expected from simple band nonparabolicity. We demonstrate that this is a consequence of the blocking of the polaron interaction which is a sensitive function of the filling of the Landau levels.

Binding energy of excitons bound to neutral donors in two-dimensional semiconductors

The binding energies of excitons bound to neutral donors in two-dimensional (2D) semiconductors within the spherical-effective-mass approximation, which are nondegenerate energy bands, have been calculated by a variational method for a relevant range of the effective electron-to-hole mass ratio sigma. The ratio of the binding energy of a 2D exciton bound to a neutral donor to that of a 2D neutral donor is found to be from 0.58 to 0.10. In the limit of vanishing sigma and large sigma, the results agree fairly well with previous experimental results. The results of this approach are compared with those of earlier theories.

Electronic states of a two-dimensional electron system in a lateral superlattice and a perpendicular magnetic field

The electronic state of a two-dimensional electron system (2DES) in the presence of a perpendicular uniform magnetic field and a lateral superlattice (LS) is investigated theoretically. A comparative study is made between a LS induced by a spatial electrostatic potential modulation (referred to as a PMLS) and that induced by a spatial magnetic-field modulation (referred ro asa MMLS). By utilizing a finite-temperature self-consistent Hartree-Fock approximation scheme; the dependence of the electronic state on different system parameters (e.g., the modulation period, the modulation strength, the effective electron-electron interaction strength, the averaged electron density, and the system temperature) is studied in detail. The inclusion of exchange effect is found to bring qualitative changes to the electronic state of a PMLS, leading generally to a nonuniform spin splitting, and consequently the behavior of the electronic state becomes similar to that of a MMLS. The Landau-level coupling is taken into account, and is found to introduce some interesting features not observed before. It is also found that, even in the regime of intermediate modulation strength, the density dependence of the spin splitting of energy levels, either for a PMLS or a MMLS, can be qualitatively understood within the picture of a 2DES in a perpendicular magnetic field with the modulation viewed as a perturbation. [S0163-1829(97)02248-0].

Fabrication and characterization of two-dimensional photonic crystal on silicon by efficient methods

Two-dimensional photonic crystals working in near infrared region are fabricated into silicon-on-insulator wafer by 248-nm deep UV lithography. We present an efficient way to measure the photonic crystal waveguide's light transmission spectra at given polarization states.

Two-dimensional photonic crystals with a large photonic band gap designed by a rapid genetic algorithm

We used Plane Wave Expansion Method and a Rapid Genetic Algorithm to design two-dimensional photonic crystals with a large absolute band gap. A filling fraction controlling operator and Fourier transform data storage mechanism had been integrated into the genetic operators to get desired photonic crystals effectively and efficiently. Starting from randomly generated photonic crystals, the proposed RGA evolved toward the best objectives and yielded a square lattice photonic crystal with the band gap (defined as the gap to mid-gap ratio) as large as 13.25%. Furthermore, the evolutionary objective was modified and resulted in a satisfactory PC for better application to slab system.

Anomalous shift of the beating nodes in illumination-controlled In1-xGaxAs/In1-yAlyAs two-dimensional electron gases with strong spin-orbit interaction

The beating patterns in the Shubnikov-de Haas oscillatory magnetoresistance originating from zero-field spin splitting of two-dimensional electron gases (2DEGs) in In0.52Al0.48As/InxGa1-xAs/In0.52Al0.48As quantum wells with silicon delta doped on the upper barrier layer have been investigated by means of magnetotransport measurements before and after illumination. Contrary to the expectation, after each illumination, the beating nodes induced by the zero-field spin-splitting effect shift to lower and lower magnetic field due to the decrease in the zero-field spin-splitting energy of the 2DEGs. The anomalous phenomenon of the shift of the beating nodes and the decrease in spin-orbit coupling constants after illumination cannot be explained by utilizing the previous linear Rashba model. It is suggested that the decrease in the zero-field spin-splitting energy and the spin-orbit coupling constant arise from the nonlinear Rashba spin splitting.

Characterization and analysis of two-dimensional GaAs-based photonic crystal nanocavities at room temperature

This paper describes the design and fabrication process of a two-dimensional GaAs-based photonic crystal nanocavity and analyzes the optical characterization of cavity modes at room temperature. Single InAs/InGaAs quantum dots (QDs) layer was embedded in a GaAs waveguide layer grown on an Al0.7Ga0.3As layer and GaAs substrate. The patterning of the structure and the membrane release were achieved by using electron-beam lithography, reaction ion etching, inductively coupled plasma etching and selective wet etching. The micro-luminescence spectrum is recorded from the fabricated nanocavities, and it is found that some high-order cavity modes are clearly observed besides the lowest-order resonant mode is exhibited in spite of much high rate of nonradiative recombination. The variance of resonant modes is also discussed as a function of r/a ratio and will be used in techniques aimed to improve the probability of achieving spectral coupling of a single QD to a cavity mode.

Short range scattering mechanism of type-II GaSb/GaAs quantum dots on the transport properties of two-dimensional electron gas

We have studied the scattering process of AlGaAs/GaAs two-dimensional electron gas with the nearby embedded GaSb/GaAs type-II quantum dots (QDs) at low temperature. Quantum Hall effect and Shubnikov-de Haas oscillation were performed to measure the electron density n(2D), the transport lifetime tau(t) and the quantum lifetime tau(q) under various biased gate voltage. By comparing measured results of QDs sample with that of reference sample without embedded QDs, mobilities (transport mobility mu(t) and quantum mobility mu(q)) dominated by GaSb QDs scattering were extracted as functions of n(2D). It was found that the ratios of tau(t) to tau(q) were varying within the range of 1-4, implying the scattering mechanism belonging to the sort of short-range interaction. In the framework of Born approximation, a scattering model considering rectangular-shaped potential with constant barrier height was successfully applied to explain the transport experimental data. In addition, an oscillating ratio of tau(t)/tau(q) with the increasing n(2D) was predicted in the model.

Two-dimensional numerical simulations of shock waves in micro convergent-divergent nozzles

The steady two-dimensional Navier-Stokes equations with the slip wall boundary conditions were used to simulate the supersonic flow in micro convergent-divergent nozzles. It is observed that shock waves can take place inside or outside of the micronozzles under the earth environment. For the over-expanded flows, there is a boundary layer separation point, downstream of which a wave interface separates the viscous boundary layer with back air flow and the inviscid core flow. The oblique shock wave is followed by the bow shock and shock diamond. The viscous boundary layer thickness relative to the whole nozzle width on the exit plane is increased but attains the maximum value around of 0.5 and oscillates against this value with the continuous increasing of the nozzle upstream pressures. The viscous effect either changes the normal shock waves outside of the nozzle for the inviscid flow to the oblique shock waves inside the nozzle, or transfers the expansion jet flow without shock waves for the inviscid flow to the oblique shock waves outside of the nozzle. 

Phonon-limited mobility in two-dimensional semiconductors with spin-orbit coupling

The authors demonstrate that the Rashba spin-orbit interaction in low-dimensional semiconductors can enhance or reduce the electron-phonon scattering rate by as much as 25%. The underlying mechanism is that the electron-phonon scattering phase space for the upper (lower) Rashba band is significantly enhanced (suppressed) by the spin-orbit interaction. While the scattering time decreases for the upper level, the mobility of the level increases due to an additional term in the electron velocity. (C) 2007 American Institute of Physics.

Two-dimensional photovoltaic dark spatial solitons of partially spatially incoherent light

We report a successful experimental observation of two-dimensional photovoltaic dark solitons in an anisotropic crystal with partially spatially incoherent light beams. This kind of solitons results from the bulk photovoltaic effect, which depends on the direction of propagation of the optical beam and on the orientation of the intensity gradient, with respect to the principal axes of the crystal.

Two-dimensional photonic lattices in polymer-dispersed liquid crystal composites

The relationship between liquid crystal orientational ordering and optical diffraction properties is investigated for a two-dimensional square photonic lattice fabricated in a polymer-dispersed liquid crystal (PDLC) composite. Modifications of the nematic director field in the liquid crystal domains were induced by an external applied voltage and by heating over the nematic-isotropic (N-I) phase transition. They were studied by optical polarization microscopy and by analysing far-field optical diffraction patterns. The intensities of various diffraction orders (from the zeroth up to the eighth diffraction order) were monitored with a CCD camera, and their variations were correlated with the modifications of the director field.