Asymmetric Quantum Wells Research Articles

Interface optical phonon-assisted scattering rates in wurtzite nitride step quantum wells with strong built-in electric field

The interface optical (IO) phonon-assisted electron scattering rates in wurtzite GaN-based asymmetrical step quantum wells (QWs) are theoretically investigated by adopting the usual Fermi golden rule. Based on the dielectric continuum model and Loudon’s uniaxial crystal model, the analytical IO phonon states and their dispersive equation as well as the Fröhlich electron–phonon interaction Hamiltonian are derived. Taking into consideration the effects of strong built-in electric field (BEF) and the band nonparabolicity, the exact electronic eigen-states in the step QW are obtained with the aid of the two Airy functions. Numerical calculations on a wurtzite AlN/GaN/AlxGa1−xN/AlN step QW are performed. It is found that there are four branches of IO phonon modes in the asymmetric nitride step QWs, which is obviously different from the situation of symmetrical GaN/AlN single and coupling QWs. This is mainly ascribed to the asymmetry of the step QW structures studied here. The calculated results show that the intrasubband and intersubband scattering rates in wurtzite step QWs are one order of magnitude larger than those in GaAs-based step QWs, which is attributed to the larger electron–phonon coupling constants of GaN-based materials. The intrasubband scattering rates in wurtzite step QWs depend on the structural parameters in a similar manner as in cubic GaAs-based step QWs, but the intersubband scattering rates here show obviously different dependent behavior on the structural parameters. This is ascribed to the effects of the strong BEF and the band nonparabolicity. Moreover, the high-frequency IO modes play more important role to the total scattering rates than the low-frequency ones. And the contributions of the low-frequency IO modes could be neglected when analyzing the electron–phonon scattering processes in wurtzite step QWs.

Polaron effects on the third-harmonic generation in asymmetrical semi-exponential quantum wells

The polaron effects on the third-harmonic generation (THG) are investigated theoretically for electrons confined in asymmetrical semi-exponential quantum wells (ASEQWs). The analytic expression for the THG in ASEQWs is derived by using the compact-density-matrix approach and the iterative method. The numerical results are presented for GaAs/AlGaAs ASEQWs. The results demonstrate that polaron effects have an important influence on the THG. It is found that the THG is obviously increased when considering the electron-LO-interaction.

Polaron effects on the second-harmonic generation in asymmetrical semiexponential quantum wells

Polaron effects on the second-harmonic generation (SHG) in asymmetrical semiexponential quantum wells (ASEQWs) are investigated theoretically. By using the framework of the compact-density-matrix approach and iterative method, the analytical expression of the SHG coefficients in ASEQWs is presented. Numerical results are illustrated for a typical GaAs/AlGaAs. By considering the electron--LO--phonon interaction (ELOPI), the energy levels and the wave functions of an electron confined in ASEQWs are obtained. It is found that whether we consider the ELOPI or not, the incident photon frequency $\omega$ and parameters $U_0$ and $\sigma$ also affect the SHG coefficient obviously.

Recent Approaches for Broadening the Spectral Bandwidth in Resonant Cavity Optoelectronic Devices

Resonant cavity optoelectronic devices, such as vertical cavity surface emitting lasers (VCSELs), resonant cavity enhanced photodetectors (RCEPDs), and electroabsorption modulators (EAMs), show improved performance over their predecessors by placing the active device structure inside a resonant cavity. The effect of the optical cavity, which allows wavelength selectivity and enhancement of the optical field due to resonance, allows the devices to be made thinner and therefore faster, while simultaneously increasing the quantum efficiency at the resonant wavelengths. However, the narrow spectral bandwidth significantly reduces operating tolerances, which leads to severe problems in applications such as optical communication, imaging, and biosensing. Recently, in order to overcome such drawbacks and/or to accomplish multiple functionalities, several approaches for broadening the spectral bandwidth in resonant cavity optoelectronic devices have been extensively studied. This paper reviews the recent progress in techniques for wide spectral bandwidth that include a coupled microcavity, asymmetric tandem quantum wells, and high index contrast distributed Bragg-reflectors. This review will describe design guidelines for specific devices together with experimental considerations in practical applications.

Structural and optical properties of MBE‐grown asymmetric cubic GaN/AlxGa1–xN double quantum wells

The non‐resonant carrier transfer in asymmetric double quantum wells is studied. Asymmetric cubic GaN/AlxGa1−xN double quantum wells with Al content of x = 0.26 ± 0.03 were grown on 3C‐SiC (001) substrate by radio‐frequency plasma‐assisted molecular beam epitaxy. The barrier thickness d between a wide quantum well having 2.5 nm thickness and a narrow quantum well with width of 0.7 nm was varied from 1 to 15 nm. Furthermore, high resolution X‐ray diffraction reciprocal space maps around the (113) direction provided the Al content and revealed a partially strain in the AlxGa1−xN barriers and QWs. The coupling between the QWs was studied by interband photoluminescence spectroscopy at low temperatures. Four clearly distinguishable emission bands at 3.27 eV, 3.37 eV, 3.60 eV, and 3.74 eV are observed and could be assigned to the different layers. With decreasing barrier thickness d the photoluminescence intensity from the narrow QW is strongly reduced, indicating wave function redistribution from the narrow QW to the wide QW. The emission energies for the QWs are in good agreement with theoretical calculations using a Schrödinger–Poisson solver based on an effective mass model (nextnano3). The PL intensity ratio of the narrow QW to the wide QW for varied barrier thicknesses was calculated by exploiting rate equations, revealing a good agreement between theory and experiment.

Specific Features of Second Harmonic Generation of Femtosecond YB3+:KY(WO4)2 Laser Radiation in Heterostructures A3B5 with Asymmetric Quantum Wells

Second harmonic generation of femtosecond Yb3+:KY(WO4)2 laser radiation was investigated in semiconductor heterostructure grown at the singular GaAs substrate of orientation (001) and including a set of asymmetric quantum wells. Azimuth second harmonic dependence was investigated in transmission for the incidence angles of 10–70 degree, and in reflection for incidence angle of 45 degree by application of p- and s-polarized pumping radiation. Second harmonic signals from heterostructure were several times higher than those from the substrate at p-polarized pumping in transmission and for the cases of p- and s-polarized pumping in reflection. The case of s-polarized pumping radiation and p-polarized second harmonic radiation in reflection resulted in high isotropic contribution from the heterostructure to second harmonic; here, the azimuth dependencies from the heterostructure and from the substrate are essentially different

Voltage control of the spin-dependent interaction constants of dipolaritons and its application to optical parametric oscillators

A dipolariton is a voltage-controlled mixture of direct and indirect excitons in asymmetric double quantum wells coupled by resonant carrier tunneling, and a microcavity photon. We calculate the voltage dependence of the spin-singlet and spin-triplet interaction parameters ${\ensuremath{\alpha}}_{1}$ and ${\ensuremath{\alpha}}_{2}$. Both parameters can reach values 1 order of magnitude larger than that of exciton-polaritons thanks to the strong interaction between indirect excitons. We show that the variation of the indirect exciton fraction of the dipolaritons induces a change of sign of ${\ensuremath{\alpha}}_{2}$: the interaction passes from attractive to repulsive with increasing voltage. For large enough voltage, ${\ensuremath{\alpha}}_{2}$ becomes larger than ${\ensuremath{\alpha}}_{1}$, which in principle can lead to the formation of a circularly polarized dipolariton condensate. We propose an application of the ${\ensuremath{\alpha}}_{2}$ dependence to a voltage-controlled dipolaritonic optical parametric oscillator. The change of sign of ${\ensuremath{\alpha}}_{2}$ allows on-site control of the linear polarization degree of the optical signal and its on-demand inversion.

Nonlinear optical absorption via two-photon process in asymmetrical Gaussian potential quantum wells

In this paper, linear and nonlinear optical absorption spectrum in asymmetrical Gaussian potential quantum wells under the applied magnetic and electric fields are studied via investigating the phonon-assisted cyclotron resonance (PACR) effect. The results are calculated for GaAs and Ga1−xAlxAs materials. Our results show that the optical absorption behaviors and the half-width are significantly dependent on the height of the Gaussian potential, the well width, the magnetic field, and the temperature. It is also found that there is a clear monotonic behavior of the resonant peaks and the half-width as functions of the factors mentioned above in both one and two-photon absorption processes.

Controllable optical bistability and multistability in asymmetric double quantum wells via spontaneously generated coherence

We investigate the nonlinear optical phenomena of the optical bistability and multistability via spontaneously generated coherence in an asymmetric double quantum well structure coupled by a weak probe field and a controlling field. It is shown that the threshold and hysteresis cycle of the optical bistability can be conveniently controlled only by adjusting the intensity of the SGC or the controlling field. Moreover, switching between optical bistability and multistability can be achieved. These studies may have practical significance for the preparation of optical bistable switching device.

The effects of intense laser field and applied electric and magnetic fields on optical properties of an asymmetric quantum well

This paper presents the results of the theoretical study of the effects of non-resonant intense laser field and electric and magnetic fields on the optical properties (the linear and third-order nonlinear refractive index and absorption coefficients) in an asymmetric quantum well. The electric field and intense laser field are applied along the growth direction of the asymmetric quantum well and the magnetic field is oriented perpendicularly. To calculate the energy and the wave functions of the electron in the asymmetric quantum well, the effective mass approximation and the method of envelope wave function are used. The asymmetric quantum well is constructed by using different aluminium concentrations in both right and left barriers. The confinement in the quantum well is changed drastically by either the effect of electric and magnetic fields or by the application of intense laser field. The optical properties are calculated using the compact density matrix approach. The results show that the effect of the intense laser field competes with the effects of the electric and magnetic fields. Consequently, peak position shifts to lower photon energies due to the effect of the intense laser field and it shifts to higher photon energies by the effects of electric and magnetic fields. In general, it is found that the concentration of aluminum, electric and magnetic fields and intense laser field are external agents that modify the optical responses in the asymmetric quantum well.

High magnetic field studies of charged exciton localization in GaAs/AlxGa1−xAs quantum wells

We report on low temperature, polarization resolved, high magnetic field (up to 23 T) photoluminescence experiments on high mobility asymmetric GaAs quantum wells. At high magnetic fields, we detect two strong emission lines of the neutral and positively charged excitons (X and X+) and a series of weaker lines of the excitons bound to ionized acceptors (AX−). From polarization energy splittings of these lines, we determine the hole Landé factors (gh) of different complexes. For X and X+, gh initially grows with magnetic field and then saturates at gh = 0.88 and 1.55, respectively; for AX−'s, gh begins from a high value (from 6 to 11 at zero field) and decreases with the field growth. This contrasting behavior is traced to the structure of valence band Landau levels, calculated numerically in the Luttinger model, beyond axial approximation. This points to the coexistence (in the same well) of mobile X and X+ with localized and interface-pressed AX− states.

Transient and steady-state properties of asymmetric semiconductor quantum wells at telecom wavelength bands

The transient and steady-state dispersion and absorption properties of a three-subband asymmetric semiconductor quantum well system are investigated. In the steady-state regime, it is shown that by increasing the strength of Fano-interference as well as enhancement of energy splitting of two excited states the slope of dispersion changes from negative to positive which is corresponding to a switch between superluminal to subluminal light propagation. At the same time, the probe absorption reduces at telecommunication wavelength λ = 1550 nm. The influence of incoherent pumping fields on time-dependent susceptibility is then discussed. It is found that due to more transfer of population to the upper levels, increasing the rate of incoherent pump field leads to the reduction of probe absorption. In addition, it is realized that incoherent pumping has a major role in converting fast to slow propagation of light at long wavelength. We also introduce an extra controllability for the light pulse to be slow downed at Telecom wavelength just through the quantum interference arising from incoherent pumping fields. The obtained results may be practical in telecommunication applications.

A study of electric-field-induced second-harmonic generation in asymmetrical Gaussian potential quantum wells

Electric-field-induced second-harmonic generation in asymmetrical Gaussian potential quantum wells is investigated using the effective mass approximation employing the compact density matrix method and the iterative approach. Our results show that the absolute value, the real part and the imaginary part of second-harmonic generation are greatly affected by the height of the Gaussian potential quantum wells, the range of the Gaussian confinement potential and the applied electric field. The relationship between the absolute value and the imaginary part of second-harmonic generation together with the relationship between the absolute value and the real part of second-harmonic generation is studied. It is found that no matter how the height of the Gaussian potential quantum wells, the range of the Gaussian confinement potential and the applied electric field vary, the resonant peaks of the absolute value of second-harmonic generation do not originate from the imaginary part but from the real part.

Phase control of light propagation via Fano interference in asymmetric double quantum wells

We investigate the light propagation and dynamical control of a weak pulsed probe field in asymmetric double quantum wells via Fano interference, which is caused by tunneling from the excited subbands to the same continuum. Our results show that the system can produce anomalous and normal dispersion regions with negligible absorption by choosing appropriate coupling strength of the tunneling and the Fano interference. Interesting enough, the dispersion can be switched between normal and anomalous by adjusting the relative phase between the pulsed probe and coherent control fields owing to the existence of the perfectly Fano interference. Thus, the relative phase can be regarded as a switch to manipulate light propagation with subluminal or superluminal. The temporal and spatial dynamics of the pulsed probe field with hyperbolic secant envelope are analyzed.

Two-component fractional quantum Hall effect in the half-filled lowest Landau level in an asymmetric wide quantum well

We investigate theoretically the fractional quantum Hall effect at half filling in the lowest Landau level observed in asymmetric wide quantum wells. The asymmetry can be achieved by a potential bias applied between the two sides of the well. Within exact-diagonalization calculations in the spherical geometry, we find that the incompressible state is described in terms of a two-component wave function. Its overlap with the ground state can be optimized with the help of a rotation in the space of the pseudospin, which mimics the lowest two electronic subbands.

Effects of Asymmetric Quantum Wells on the Structural and Optical Properties of InGaN-Based Light-Emitting Diodes.

A metalorganic vapor phase epitaxy-grown InGaN/GaN multiple-quantum-well (MQW) with three graded-thickness wells (the first-grown well had the greatest width) near the n-GaN was used as the active layer of an LED. For LEDs with an asymmetric quantum well (AQW), high-resolution X-ray diffraction and transmission electron microscopic reveal that the modified MQWs with a reasonable crystalline quality were coherently strained on the underlying GaN epilayers without any relaxation. In addition, the slight increase of indium segregation in the LED with an AQW may be attributed to variations in indium contents experienced during epitaxial growth of the wide well-containing MQWs. By preventing the energetic electrons from accumulating at the topmost quantum well nearest the p-GaN, the presence of light intensity roll-off in the LED with an AQW is shifted to higher currents and the corresponding maximum light output power is increased with a ratio 7.9% higher than that of normal LEDs. Finally, similar emission wavelengths were observed in the electroluminescence spectra of both LEDs, suggesting that light emitted mostly from the top quantum wells (near the p-GaN) while the emissions from the AQW region were insignificant.

Polaron effects on nonlinear optical rectification in asymmetrical Gaussian potential quantum wells with applied electric fields

Polaron effects on nonlinear optical rectification in asymmetrical Gaussian potential quantum wells are studied by the effective mass approximation and the perturbation theory. The numerical results show that nonlinear optical rectification coefficients are strongly dependent on the barrier hight V0 of the Gaussian potential quantum wells, the range L of the confinement potential and the electric field F. Besides, the numerical results show that no matter how V0, L and F change, taking into consideration polaron effects, the optical rectification coefficients χ0(2) get greatly enhanced.

The effects of the electric and magnetic fields on the nonlinear optical properties in the step-like asymmetric quantum well

In the present work, total optical absorption coefficient (the linear and third-order nonlinear) and total refractive index change for transition between two first lower-lying electronic levels in the step-like GaAs/Ga1−xAlxAs quantum well under the electric and magnetic fields are investigated and also the effect of relaxation time on saturation is investigated. A compact density-matrix approach is applied to investigate optical properties. The obtained results show that both total absorption coefficient and refractive index change are sensitive to well dimensions more than external fields. With the increase of quantum well width, total absorption coefficient and refractive index change shift to lower photon energies (red shift), the magnitude of total refractive index increases significantly while total absorption coefficient is reduced. Furthermore, the electric and magnetic fields induce a blue-shift on absorption coefficient and refractive index change.

Magnetic field effect on electron spin dynamics in (110) GaAs quantum wells

We study the electron spin relaxation in both symmetric and asymmetric GaAs/AlGaAs quantum wells (QWs) grown on (110) substrates in an external magnetic field B applied along the QW normal. The spin polarization is induced by circularly polarized light and is detected using the time-resolved Kerr rotation technique. In the asymmetric structure, where a δ-doped layer on one side of the QW produces the Rashba contribution to the conduction-band spin–orbit splitting, the lifetime of electron spins aligned along the growth axis exhibits an anomalous dependence on B in the range 0 < B < 0.5 T; this results from the interplay between the Dresselhaus and Rashba effective fields which are perpendicular to each other. For larger magnetic fields, the spin lifetime increases, which is a consequence of the cyclotron motion of the electrons and is also observed in (001)-grown quantum wells. The experimental results are in agreement with the calculation of the spin lifetimes in (110)-grown asymmetric quantum wells described by the point group Cs, where the growth direction is not the principal axis of the spin-relaxation-rate tensor.

Hydrogenic donor in asymmetric [formula omitted] quantum wells

In this work we study the effects of barrier height ratio VL/VR on asymmetric AlxLGa1−xLAs/GaAs/AlxRGa1−xRAs quantum wells in absence of external influences. We use a variational method within the effective mass approximation to observe the effects of the barrier height ratio. It has been observed that the binding energy of the impurity, the expectation value of the electron–impurity distance along the z-direction, density of impurity states, and edge binding energy difference depend strongly on the barrier height ratio. Furthermore we present new curves that analyze the binding energy of asymmetric quantum wells as functions of barrier height ratio.