Asymmetric Quantum Wells Research Articles

The effect of temperature on strong-coupling magnetopolaron in an asymmetrical Gaussian potential quantum well

The influences of temperature and cyclotron frequency of a magnetic field on the ground state energy and mean number of phonons (MNP) of strong-coupling magnetopolarons in an asymmetric Gaussian potential quantum well(AGPQW) are researched by employing the linear-combination operator method and the unitary transformation. It was demonstrated through the numerical calculations that the ground state energy and the MNP increase with higher magnetic field cyclotron frequencies and temperature. In addition, increasing of the barrier of asymmetric Gaussian potential (AGP) causes the ground state energy to decrease while increasing the MNP of magnetopolarons.

Exciton optical absorption in asymmetric ZnO/ZnMgO double quantum wells with mixed phases**Project supported by the National Natural Science Foundation of China (Grant No. 61764012).

The optical absorption of exciton interstate transition in Zn1 − xlMgxlO/ZnO/Zn1 − xcMgxcO/ZnO/Zn1 − xrMgxrO asymmetric double quantum wells (ADQWs) with mixed phases of zinc-blende and wurtzite in Zn1 − xMgxO for 0.37 < x < 0.62 is discussed. The mixed phases are taken into account by our weight model of fitting. The states of excitons are obtained by a finite difference method and a variational procedure in consideration of built-in electric fields (BEFs) and the Hartree potential. The optical absorption coefficients (OACs) of exciton interstate transition are obtained by the density matrix method. The results show that Hartree potential bends the conduction and valence bands, whereas a BEF tilts the bands and the combined effect enforces electrons and holes to approach the opposite interfaces to decrease the Coulomb interaction effects between electrons and holes. Furthermore, the OACs indicate a transformation between direct and indirect excitons in zinc-blende ADQWs due to the quantum confinement effects. There are two kinds of peaks corresponding to wurtzite and zinc-blende structures respectively, and the OACs merge together under some special conditions. The computed result of exciton interband emission energy agrees well with a previous experiment. Our conclusions are helpful for further relative theoretical studies, experiments, and design of devices consisting of these quantum well structures.

Spin-locked transport in a two-dimensional electron gas

Spin-orbit interactions in solids have inspired innovative physics for spin-based technologies. One such example is the persistent spin helix, where spin-orbit interactions from the semiconductor lattice are balanced with those in asymmetric quantum wells, to create long-lived spin textures. Spin transport in the presence of the momentum-dependent spin-orbit interactions lead to Larmor precession and subsequent dephasing that challenges the design of current spin-based information processing devices. We demonstrate that external magnetic fields can be applied to overcome this issue for spin-polarized charge carriers transported by in-plane electric fields. A frame of reference picture is introduced to describe the emergence and dynamics of the polarization-locked spin-wave packet after optical excitation. Applying well-matched magnetic fields maintains the persistent spin-helix profile regardless of whether the frame of reference is in motion or not. Monte Carlo simulations allow this traveling persistent spin-helix concept to be extended to a proposed spin Hall-effect transistor to ease design requirements.

Optical properties of n-type asymmetric triple δ-doped quantum well under external fields

We present a theoretical investigation about the the influence of external electric, magnetic and non-resonant intense laser fields on intersubband-related second harmonics generation (SHG) and the nonlinear optical rectification (NOR) coefficients in n-type asymmetric triple δ-doped quantum wells (QWs). A particular design of asymmetric triple δ-doped QW with Lw = 200 Å width and, respectively, in the left side, central and right side, and doping concentrations is taken into account. For QWs under the combined effect of the external electric, magnetic and laser fields, the time-dependent wave equation is modified by using Kramers-Henneberger transformation and the dipole approximation. The subband energy spectra and the electronic wave functions are obtained by solving numerically the wave equation. The originality of this work can be presented as; (i) The results explain NOR and SHG characteristics of triple QW depending on external field effects in detail. The effects of the electric, magnetic and laser field on transition energies and NOR, SHG characteristics are presented detail. (ii) In addition to, the alternativeness to each other of the external fields is discussed by probing the features of SHG and NOR under the strong and weak regimes of external fields. (iii) These nonlinear optical responses to the external fields are compared, researching the optimum cases for these optical specifications. (iv) The control of SHG through the external fields in triple δ-doped QWs reveals to be easier and more precise.

Proposal of nonlinear measurement of tunneling in quantum wells with Fano interference

An enhanced absorption spectrum by Fano interference is proposed to measure the tunneling between a discrete state and a continuum in asymmetric quantum wells (QWs). Interestingly, the asymmetry of the enhanced absorption spectrum is sensitive to tunneling strength, which can be used to measure the tunneling in QWs. Further study shows that the asymmetry of a nonlinear absorption spectrum is greater than that of a linear absorption spectrum, indicating better probe sensitivity. The simulation results show that the probe sensitivity based on nonlinear absorption is approximately 10 times larger than that of linear absorption. In addition, the effects of control-field intensity and detuning on probe sensitivity are evaluated.

Highly efficient vortex four-wave mixing in asymmetric semiconductor quantum wells.

Orbital angular momentum (OAM) is an important property of vortex light, which provides a valuable tool to manipulate the light-matter interaction in the study of classical and quantum optics. Here we propose a scheme to generate vortex light fields via four-wave mixing (FWM) in asymmetric semiconductor quantum wells. By tailoring the probe-field and control-field detunings, we can effectively manipulate the helical phase and intensity of the FWM field. Particularly, when probe field and control field have identical detuning, we find that both the absorption and phase twist of the generated FWM field are significantly suppressed. Consequently, the highly efficient vortex FWM is realized, where the maximum conversion efficiency reaches around 50%. Our study provides a tool to transfer vortex wavefronts from input to output fields in an efficient way, which may find potential applications in solid-state quantum optics and quantum information processing.

Optical absorption in a CdS/CdSe/CdS asymmetric quantum well

We investigate the electronic and optical properties of a confined exciton in CdS/CdSe/CdS asymmetric quantum well taking into account the geometrical confinement in which the right well size for various values of barrier sizes is varied. The variation of energy difference between the subbands with the change in well width alters the degree of quantum confinement. It behaves as single quantum well as the barrier width becomes very thin. The absorption coefficient brings out the fact that the optical susceptibilities demonstrate a stronger dependence on the spatial confinement. The narrow barrier plays a major role in determining the opto-electronic properties.

Dual wavelength operation of the GaSb-based Y-branch distributed Bragg reflector lasers near 2.1 μm

The GaSb-based diode lasers operating simultaneously at two wavelengths near 2.1 μm have been designed and fabricated. The Y-branch devices used the 6th order distributed Bragg reflectors (DBR) to provide spectrally selective feedback. The laser active region contained two asymmetric quantum wells with allowed optical transitions between two lowest electron (e1 and e2) and top hole (hh1) subbands. The laser emission lines loosely matched the e1-hh1 and e2-hh1 optical transitions with separation corresponding to ∼3.1 THz. The 10 μm wide deeply etched straight ridge DBR lasers demonstrated stable single mode operation with more than 50 mW of output power.

Performance Enhancement of GeSn Transistor Laser with Symmetric and Asymmetric Multiple Quantum Well in the Base

The performance of a Multiple Quantum Well (MQW) Heterojunction Bipolar Transistor Laser (HBTL) has been studied using GeSn alloy. Both symmetric and asymmetric quantum wells have been considered. Main analysis is focused on finding the minority carrier concentration in the base, the base threshold current, light output power of the device and the values are compared with GeSn based Single Quantum Well and InGaAs based Multiple Quantum Well Transistor Laser.

Ground State Binding Energy of the Strong-coupling Polaron in Asymmetrical Semi-exponential Quantum Wells

Ground State Binding Energy of the Strong-coupling Polaron in Asymmetrical Semi-exponential Quantum Wells

Doping-Dependent Nonlinear Electron Mobility in GaAs|In-=SUB=-x-=/SUB=-Ga-=SUB=-1-x-=/SUB=-As Coupled Quantum-Well Pseudo-Morphic MODFET Structure

We analyze the asymmetric delta-doping dependence of nonlinear electron mobility &amp;mu; of GaAs|InxGa1-xAs double quantum-well pseudo-morphic modulation doped field-effect transistor structure. We solve the Schrodinger and Poisson's equations self-consistently to obtain the sub-band energy levels and wave functions. We consider scatterings due to the ionized impurities (IMP), alloy disorder (AL), and interface roughness (IR) to calculate &amp;mu; for a system having double sub-band occupancy, in which the inter-sub-band effects play an important role. Considering the doping concentrations in the barriers towards the substrate and surface sides as Nd1 and Nd2, respectively, we show that variation of Nd1 leads to a dip in &amp;mu; near Nd1=Nd2, at which the resonance of the sub-band states occurs. A similar dip in &amp;mu; as a function of Nd1 is also obtained at Nd1=Nd2 by keeping (Nd1+Nd2) unchanged. By increasing the central barrier width and well width, the dip in &amp;mu; becomes sharp. We note that even though the overall &amp;mu; is governed by the IMP- and AL-scatterings, the dip in &amp;mu; is mostly affected through substantial variation of the sub-band mobilities due to IR-scattering near the resonance. Our results of nonlinear electron mobility near the resonance of sub-band states can be utilized for the performance analysis of GaAs|InGaAs pseudo-morphic quantum-well field-effect transistors. Keywords: asymmetric double quantum wells, GaAs|InxGa1-xAs structures, nonlinear electron mobility, pseudo-morphic HEMT structures, resonance of sub-band states.

Influence of conduction-band non-parabolicity on terahertz intersubband Raman gain in GaAs/InGaAs step asymmetric quantum wells

The conduction electron states in step-like strained GaAs/InGaAs quantum wells are theoretically investigated under the effective mass approximation, taking into account the effects of band non-parabolicity. With such information, the intersubband three-level Raman gain is calculated looking to reveal a possible application of the studied systems as sources for THz Raman lasing. A group of high-gain intersubband transitions is identified, and the results are strongly dependent on a suitable geometric design in terms of potential well widths which can lead to values of the Raman gain between 200 and $$400\,\hbox {cm}^{-1}$$, the latter values being close to those previously reported in GaAs-based double asymmetric quantum wells. Secondary radiation frequencies are identified within the range of few tens of THz. It is found that the influence of band non-parabolicity causes a significant reduction of the Raman gain, in comparison with the values obtained neglecting such a phenomenon. Therefore, conduction-band non-parabolicity becomes a crucial element for the accurate quantitative description of the intersubband-related optical response in low-dimensional heterostructures involving small gap materials.

Multiwavelength GaN‐Based Surface‐Emitting Lasers and Their Design Principles

AbstractDual‐wavelength lasing operations are demonstrated in GaN‐based vertical‐cavity surface‐emitting lasers (VCSELs) comprising ingeniously designed asymmetric InGaN quantum wells (AS‐QWs). The dual laser modes show exact positive‐correlated polarization dependences with a high degree of polarization of up to 98%. By simply tuning the pump energy, the components and intensity of the laser outputs can be continuously changed, making wavelength selection and switching available for the GaN‐based VCSELs. Detailed theoretical analysis and experimental measurements show that the intensity of optical gain and the coupling between the active layer and optical field, namely the electron–photon interaction, as well as carrier tunneling and photon reabsorption play a crucial role in the multiwavelength lasing processes. Moreover, the design principles of the proposed AS‐QWs and multistacked size‐varied quantum dot (MS‐QD) active regions are elaborated to provide guidelines for controllable multiwavelength emissions in GaN‐based surface‐emitting lasers. These results not only provide better understanding of lasing in nitride‐based microcavity systems but also shed insight into the more fundamental issues of electron–photon coupling in such systems. Importantly, such controllable multiwavelength laser operations may extend nitride‐based VCSELs to previously inaccessible areas, for example, flip‐flop, ultrafast switches, and other functional devices such as Raman lasers and sensors.

The electronic states and the optical absorption for an asymmetrical quantum well applied with an external electric field

Electric field influences on the electronic states and the optical absorption in an asymmetrical quantum well with semiparabolic potential are investigated. The formula for the absorption coefficients in this asymmetrical quantum well with electric field are deduced by applying iterative method and density-matrix approach. The results are discussed with GaAs/AlGaAs materials and show that the external electric field has a significant effect on the electronic states and absorption coefficients of asymmetric quantum wells.

Optical absorption coefficients in asymmetric quantum well

The optical absorption coefficients considering electron-phonon interaction in asymmetrical quantum wells are theoretically studied. The result shows that the optical absorption coefficients depend strongly on the parameters of quantum well. Interestingly, the theoretical values of the optical absorption coefficients obviously increase with considering the electron-phonon interaction.

The effect of intense laser field on the nonlinear optical features in asymmetric multiple step and inverse V-shaped multiple step quantum wells

For asymmetric multiple step quantum wells (AMS QWs) and asymmetric inverse V-shaped multiple step quantum wells (AIVSMS QWs), the nonlinear optical features, such as the nonlinear optical rectification (NOR), the second harmonic generation (SHG) and the third harmonic generation (THG) coefficients are analyzed as according on the intense laser field (ILF). The found consequences indicate that the ILF parameter confirms a vital influence on the shape and height of the confined potential profile of both AMS QWs and AIVSMS QWs, and the alterations of the dipole moment matrix elements and the energy levels are adhered on the profile of the confined potential. According to the results, the potential profile and height of the AIVSMS QWs compared to AMS QWs were affected differently from ILF intensity. These results indicate that NOR, SHG, and THG coefficient of AMS QWs and AIVSMS QWs may be regulated in a preferred energy range and the magnitude of the resonance peak (RP) by tuning the ILF parameter. By varying the ILF parameter, it is feasible to classify blue or red shifts in RP locations of NOR, SHG and THG coefficients. Our results can be valuable in investigating new ways of manipulating the opto-electronic properties of semiconductor QW devices.Highlights•The electro-optical properties of AMS QWs and AIVSMS QWs vary by increasing ILF.•The change of AIVSMS QWs compared to AMS QWs is more affected by ILF.•NOR, SHG, and THG coefficients change with rising ILF.•RP spectrums show blue or red shifts as according to ILF.

Depending on the intense laser field of the nonlinear optical rectification, second and third harmonic generation in asymmetric parabolic-step and inverse parabolic-step quantum wells

The nonlinear optical rectification (NOR), the second harmonic generation (SHG) and the third harmonic generation (THG) coefficients in asymmetric parabolic-step quantum wells (APSQW) and asymmetric inverse parabolic-step quantum wells (AIPSQW) are considered as depending on the intense laser field (ILF). The found consequences indicate that ILF parameter approves a dynamic influence on the shape, width and height of the confined potential profile of both APSQW and AIPSQW, and the alterations of the dipole moment matrix elements and the energy levels (ELs) are followed on the profile of the confined potential. The potential profile and height of the AIPSQW compared to APSQW were affected differently from ILF intensity. With and without the ILF, the ELs in the APSQW are smaller than in the AIPSQW. These results designate that NOR, SHG, and THG coefficient of APSQW and AIPSQW may be controlled in a chosen energy range and the magnitude of the resonance peak (RP) by increasing ILF value. By modifying the ILF parameter, it is possible to classify red or blue shifts at the RP positions of the NOR, SHG and THG coefficients. We hope that our results will offer significant development of optical devices in many potential device applications for the appropriate selection of ILF values.

Effects of temperature and magnetic field on the ground state binding energy of the strong coupling magneto-polaron in an RbCl asymmetrical semi-exponential quantum well

The ground state binding energy (E[Formula: see text]) and the mean number of LO phonons (N) of the strong-coupling magneto-polaron (SCMP) in an asymmetrical semi-exponential quantum well (ASEQW) are studied theoretically. Temperature (T) effects on E[Formula: see text] and N are acquired with the quantum statistics theory (QST). By using the Lee-Low-Pines unitary transformation (LLPUT) and linear combination operation method (LCOM), the variations of E[Formula: see text] and N with T and [Formula: see text] of magnetic field are discussed. The investigated results indicate that both T and [Formula: see text] have great influence on E[Formula: see text] and N of LO phonons.

Dependence on well widths of total optical absorption coefficient of asymmetric triple GaAlAs/GaAs and GaInAs/GaAs quantum wells

Herein, the total optical absorption coefficient (TOAC) and the electronic characteristics of asymmetric triple Ga[Formula: see text]Al[Formula: see text]As/GaAs (A model) and Ga[Formula: see text]In[Formula: see text]As/GaAs (B model) quantum wells (QWs) have been examined related to the well widths (WWs). For TOAC, we first varied all WWs equally, and then we kept a WW constant and changed the other two WWs. The wavefunctions (WFs), the subband energies and the probability densities of asymmetric triple quantum wells (ATQW) with different QW shapes under effective mass approach were determined by the solution of Schrödinger equation. According to the results obtained, the major diversities of A and B models are the effective mass and the energy gap. The potential depth and the energy levels (ELs) of A model are continuously smaller than of B model. We see that WWs have a great influence on TOAC and the electronic features of ATQW. The inter-subband absorption spectrum designates blue/red shifts, when the energy spacing varies with WWs. These characteristics draw a convenient attention for the purpose of adjustable semiconductor devices. Hence, the alteration of this absorption spectrum, which could be fit for many optical modulators and infra-red optical device applications, may be efficiently realized by changing WWs. Highlights Energy spacing varies by increasing WWs. Due to the shape of the wells, the influence of each WW is quite different from the others. TOAC varies with rising WWs. The absorption spectrum illustrates red or blue shifts depending on WWs.

Optical characterization of laser-driven double Morse quantum wells

In this study, the first-order linear, third-order nonlinear, and total absorption coefficients for the intersubband transition between the two lower-lying electronic levels in both symmetric and asymmetric double Morse quantum wells under the non-resonant high-frequency intense laser field are investigated. The study takes into account the effects of the structure parameters. The results show that the electronic and also accordingly optical properties of the structures which we focus on can be adjustable to obtain a convenient response to certain studies or purposes by changing the applied external field and optical intensity as well as structure parameters.