Disk-shaped Quantum Dots Research Articles

Exciton quantum beats in GaAs/AlAs disk-shaped quantum dots

Using the renormalized wavefunction approach, we examined the excitonic quantum beat in a three-level system in disk-shaped quantum dots. The non-stationary electron wave function and the time-dependent exciton absorption intensity under the effect of an intensity pump laser that resonates with two excited electron states were also provided. Our numerical findings demonstrate that the time-dependent exciton absorption intensity takes the shape of a periodic oscillation when the quantum dot is subjected to a powerful resonant pump laser. This is clear evidence that quantum beats appears in the quantum disk. Additionally, the dot radius and the pump field detuning significantly affect the beat characteristics, such as frequency and amplitude.

Effect of Rashba spin-orbit coupling interaction on energy states of silicon disk-shaped quantum dot

Silicon quantum dots (QDs) are considered an excellent platform for spin qubits due to their weak spin-orbit interaction (SOI). Indeed, due to quantum confinement, novel spin properties arise from the SOI. In this work, we have studied the influence of the Rashba SOI and the confinement potential on the energy spectrum of an electron confined in a Silicon disk-shaped quantum dot, in the presence of an external magnetic field. The effects of the QD size, the confinement potential and the Rashba alpha coefficient on the energy levels are also studied. We used the effective mass approximation to determine the energy levels and their wave functions for different states. The results are presented as a function of the magnetic field in the presence and absence of SOI. We find that the energy levels of the electrons behave very differently depending on the magnetic field. The energy of all states changes with increasing magnetic field and each energy level splits into two and the energy difference between these two levels also increases with magnetic field, in the presence and absence of SOI. The energy levels are proportional to the Rashba alpha coefficient and inversely proportional to the radius of the QD.

Influence of the dispersion coefficient and dielectric constant ratio on the asymmetric Gaussian potential quantum dot qubit with electromagnetic field

Selecting the asymmetric Gaussian (AG) potential to describe the confinement of electron in a disk-shaped quantum dot (QD), the ground state and the first excited state energy and wave function of the system are derived by using the Lee $$\hbox {--}$$ Low $$\hbox {--}$$ Pines (LLP) Pekar transformation variational method, and the two-level structure required for a qubit is constructed. The influence of material parameters such as the dispersion coefficient, dielectric constant ratio and electron $$\hbox {--}$$ phonon coupling constant on the qubit properties of AG potential QD with the electromagnetic field are investigated. The results show that the electric field and magnetic field have opposite adjustment functions for the formation of qubit. The electric field is advantageous for the qubit survival and information storage, while magnetic field and electron $$\hbox {--}$$ phonon coupling are detrimental to the qubit survival and information storage, respectively. The decoherence time of the qubit increases with increasing magnetic field cyclotron frequency ‘from the turning point’. Applying an electric field, increasing the dielectric constant ratio, the dispersion coefficient and the electron $$\hbox {--}$$ phonon coupling constant of the materials are all beneficial to improve the coherence of the qubit.

Impurity-modulated optical response of a disc-shaped quantum dot subjected to laser radiation

We report the optical properties of a laser-driven two-dimensional quantum dot with a hydrogenic impurity exposed to a magnetic field. The effective mass approximation is used to bring out the effect of an intense laser field, resulting in dressed Coulomb and confinement potentials, on the one-electron energy levels and wave functions. The results obtained suggest that the action of the laser field leads to crucial changes in the energy spectra of the system and the impurity binding energy. Furthermore, the position and the height of the resonant peaks of the absorption coefficients and refractive index changes depend strongly not only on the geometrical confinement strength but also on the magnetic field and laser radiation through the “laser-dressed” potential.

Features of spin dynamics of magnetic ions and charge carriers in self-organized quantum dots CdSe/ZnMnSe

Self-organized disk-shaped quantum dots of CdSe embedded in diluted magnetic ZnMnSe barrier were studied by means of pump-probe time-resolved Kerr rotation (TRKR) technique at low temperature T = 7 K. In absence of the external magnetic field TRKR signal exhibits long-living spin dynamics with the decay time exceeding the period between laser pulses. Such spin dynamics is not typical for diluted magnetic semiconductors and nano-structures based on them and could be a trace of a bound magnetic polaron. Resonant spin amplification measured in transversal magnetic field up to 1 T shows the only one peak near B = 0. In B = 1 T the long-living non-precessing signal practically vanishes, while the precessing one appears with the Larmor frequency corresponding to the Mn2+ ions’ net spin precession around the magnetic field. It was found that the signal consists of three components with slightly different precession frequencies that could be due to the fine structure of the manganese spin sublevels occurring because of a stress in quantum dots.

Multiple spin-flip Raman scattering in CdSe/ZnMnSe quantum dots

Multiple ${\mathrm{Mn}}^{2+}$ spin-flip Raman scattering (SFRS) in Voigt geometry was observed in self-organized disk-shaped quantum dots (QDs) of $\mathrm{CdSe}/{\mathrm{Zn}}_{0.99}{\mathrm{Mn}}_{0.01}\mathrm{Se}$, where magnetic ions and QD carriers are spatially separated and therefore the exchange interaction between them is expected to be weak. Many lines (about ten) were observed in SFRS spectra, yet the overlapping of the hole wave function with ${\mathrm{Mn}}^{2+}$ ions is very small, in agreement with both the absence of observable Zeeman splitting of the photoluminescence line and the calculation. Interesting polarization properties of SFRS spectra were observed which could be affected by tilting the sample out of normal alignment and changing the temperature. These polarization properties were attributed to the selection rules in SFRS in Voigt geometry. It has been found that the theoretical model suggested by St\"uhler et al. [J. Cryst. Growth 159, 1001 (1996)] does not describe the SFRS spectra in systems with weak exchange interaction between charge carriers and magnetic ions. A qualitative model is suggested here for description of SFRS in such systems.

Second harmonic generation in a disk shaped quantum dot in the presence of spin-orbit interaction

The second harmonic generation (SHG) coefficients for a disk shaped quantum dot (DSQD) in the magnetic field are studied in the presence of spin-orbit interactions (SOI). The spin-orbit terms we have used in our calculations are both Rashba and Dresselhaus. We have shown that the presence of SOI modifies the SHG terms. In addition, it has been shown that SOI coupling terms influence the spectrum of DSQD resulting in defined changes in the harmonic generation.

Optical absorption in a disk-shaped quantum dot in the presence of an impurity

The linear and third order nonlinear optical absorption coefficients have been calculated in a two dimensional disk quantum dot. The confinement potential has been taken to be a combination of a parabolic and inverse squared part. The study has been performed in the presence of a perpendicular static magnetic field and a central or off-central impurity. The resulting Schrödinger equation has been solved by applying the variational method. It has been found that the presence of impurity causes a huge increase in the square of the transition matrix and in the absorption coefficients, in particular in the third order coefficient. Moreover, the asymmetry which results in the case of off-central impurity has been dealt with carefully by taking into consideration the transition matrices which vanish in other cases.

应变纤锌矿ZnO/MgxZn1-xO盘形量子点中的离子施主束缚激子态

应变纤锌矿ZnO/Mg_xZn_1-xO盘形量子点中的离子施主束缚激子态

Dynamics and relaxation ofspbiexcitons in disk-shaped quantum dots

We study the effects of intraband relaxation processes on optical manipulation protocols for $\mathit{sp}$ biexcitons hosted by CdTe disk-shaped quantum dots. The many-body states are calculated within the configuration interaction method starting from single-particle states given by the $k\ifmmode\cdot\else\textperiodcentered\fi{}p$ theory. The time-dependent occupations of relevant many-body states are extracted from the von Neumann-Lindblad equation for the density operator. We mainly investigate the generation of $\mathit{sp}$ biexcitons with two pulses of different polarizations ${\ensuremath{\sigma}}_{+}$ and ${\ensuremath{\sigma}}_{\ensuremath{-}}$. The fast hole relaxation processes prevent a high-fidelity controlled operation on $\mathit{sp}$ biexcitons and lead to the occupation of some transient states which can be optically probed. More importantly, the many-body structure of the transient states consists of two holes on the $s$ shell and antiparallel $\mathit{sp}$ triplet states for electrons. Our simulations show that these triplet states are more stable against decoherence as they can only be damaged through slow electron relaxation. The configuration mixing due to correlation effects is also discussed.

Competition Effects of Electric and Magnetic Fields on Impurity Binding Energy in a Disc-Shaped Quantum Dot in the Presence of Pressure and Temperature

We have theoretically studied the combination effects of the electric and magnetic fields on the binding energy of an on-center donor impurity in disc-shaped GaAs/Al0.3Ga0.7As quantum dots (QDs) with emphasis on the competition effects between the two fields under externally applied pressure and temperature. The electric field is applied along the radial direction of the QD, while the magnetic field is applied along the growth direction. The numerical method we employed in the present calculations is the potential morphing method in the framework of the effective mass approximation. Our results show that the two fields exhibit a competition effect on the donor binding energy, leading to an invariant binding energy as in the zero field case at a critical line. This line separates the region corresponding to the “red shift” of the donor binding energy from the one referring to the “blue shift” of the binding energy. Comparing to the magnetic field effect, increasing in the QD sizes or applied pressure is found to favour more its electric counterpart, enlarging the region satisfied by the “red shift” of the donor binding energy. The temperature effect appears to decrease the donor binding energy. However, the competition effects between the two fields retain the same when the temperature varies.

Population Transfer in Disk Shaped Quantum Dot in Delayed Half Cycle Pulses and Magnetic Field

Population Transfer in Disk Shaped Quantum Dot in Delayed Half Cycle Pulses and Magnetic Field

Optical nonlinearities associated to applied electric fields in parabolic two-dimensional quantum rings

The linear and nonlinear optical absorption as well as the linear and nonlinear corrections to the refractive index are calculated in a disc shaped quantum dot under the effect of an external magnetic field and parabolic and inverse square confining potentials. The exact solutions for the two-dimensional motion of the conduction band electrons are used as the basis for a perturbation-theory treatment of the effect of a static applied electric field. In general terms, the variation of one of the different potential energy parameters – for a fixed configuration of the remaining ones – leads to either blueshifts or redshifts of the resonant peaks as well as to distinct rates of change for their amplitudes.

On-center donor impurity-related nonlinear corrections to optical absorption and refractive index in a two-dimensional quantum ring

The donor impurity-related linear and nonlinear optical absorption as well as the linear and nonlinear corrections to the refractive index in a disc-shaped quantum dot under the effect of an external magnetic field and parabolic and inverse square confining potentials are calculated making use of the exact solutions for the two-dimensional motion of a conduction band electron in the system. It has been previously shown that this kind of structure may work well as a model for a quantum ring. In general terms, for a fixed confining configuration and the basic parameters typical of GaAs, the influence of an on-center donor impurity reflects in a blueshift of the resonant peak positions, together with a reduction in the peak amplitudes, affecting more significantly the nonlinear contributions. The results also show that the influence of the normally oriented magnetic field induces a blueshift in both the first and third order peaks of the calculated optical quantities. In addition, total peak amplitudes are shown to be growing functions of the magnetic field strength. The increase in the strength of the inverse square potential function enhances significantly the contribution from the nonlinear third-order terms in both the absorption and the relative correction to the refractive index.

Pressure and temperature effects on the third-order nonlinear optical properties in GaAs quantum dots

Abstract This work is used in the density matrix formalism and the effective mass approximation to study the third harmonic generation coefficient in a GaAs disc-shaped quantum dot with parabolic confinement potential. It is discussed the strong and weak confinement regime. The results show that the third harmonic generation coefficient is strongly dependent on the excitonic pair localization. The study is extended to consider effects such as hydrostatic pressure and temperature to show that it is possible to induce a blue-shift and/or red-shift on the resonant peaks of the third harmonic generation coefficient.

Large‐Scale Synthesis of High‐Quality Metal Sulfide Semiconductor Quantum Dots with Tunable Surface‐Plasmon Resonance Frequencies

High-quality CdS and Cu(7)S(4) quantum dots (QDs) were synthesized with N,N-dibutylthiourea (DBTU) as an organic sulfur source. In this method, nucleation and growth reactions were controlled simply by the heating rate of the reaction. The mild oxidation conditions gave monodisperse CdS QDs exhibiting pure band-edge emission with relatively high photoluminescence quantum yield. During the synthesis of Cu(7)S(4) QDs, the addition of dodecanethiol to the reaction system controlled the reaction rate to give monodisperse spherical or disk-shaped QDs. A hundred-gram scale of copper precursor could be used to generate the high-quality Cu(7)S(4) QDs, indicating that an industrial-scale reaction is achievable with our method. As observed in anisotropic noble-metal nanocrystals, larger disk-shaped Cu(7)S(4) QDs showed lower localized-surface-plasmon resonance energy in the near-infrared region. The disk-shaped Cu(7)S(4) QDs could be used effectively as templates to form cation-exchanged monodisperse disk-shaped CdS QDs.

Linear and nonlinear optical properties in a disk-shaped quantum dot with a parabolic potential plus a hyperbolic potential in a static magnetic field

Abstract Linear and nonlinear optical properties in a disk-shaped quantum dot (DSQD) with a parabolic potential plus a hyperbolic potential in a static magnetic field are theoretically investigated within the framework of the compact-density-matrix approach and iterative method. The energy levels and the wave functions of an electron are obtained by three kinds of approximation methods. It is found that optical absorption coefficients and refractive index changes are not only by the characteristic parameters of the hyperbolic potential and the confinement frequency, but also by the magnetic field.

Quantum disc plus inverse square potential. An analytical model for two‐dimensional quantum rings: Study of nonlinear optical properties

AbstractThe exact solutions for the two‐dimensional motion of a conduction band electron in a disc shaped quantum dot under the effect of an external magnetic field and parabolic and inverse square confining potentials are used to calculate the linear and nonlinear optical absorption as well as the linear and nonlinear corrections to the refractive index in the system. It is shown that this kind of structure may work well as a model for a quantum ring. Using the basic parameters typical of the GaAs, the results show that the influence of the normally oriented magnetic field induces a blueshift in both the first and third order peaks of the calculated optical quantities. In addition, total peak amplitudes are shown to be growing functions of the magnetic field strength. The increase in the strength of the inverse square potential function enhances significantly the contribution from the nonlinear third‐order terms in both the absorption and the relative correction to the refractive index.

Linear and nonlinear intersubband optical absorption in a disk-shaped quantum dot with a parabolic potential plus an inverse squared potential in a static magnetic field

The linear and nonlinear optical absorption in a disk-shaped quantum dot (DSQD) with parabolic potential plus an inverse squared potential in the presence of a static magnetic field are theoretically investigated within the framework of the compact-density-matrix approach and iterative method. The energy levels and the wave functions of an electron in the DSQD are obtained by using the effective mass approximation. Numerical calculations are presented for typical GaAs/AlAs DSQD. It is found that the optical absorption coefficients are strongly affected not only by a static magnetic field, but also by the strength of external field, the confinement frequency and the incident optical intensity.

Hydrostatic pressure and temperature effects of an exciton–donor complex in quantum dots

Using the matrix diagonalization method and the compact density-matrix approach, we studied the combined effects of hydrostatic pressure and temperature on the electronic and optical properties of an exciton–donor complex in a disc-shaped quantum dot. We have calculated the binding energy and the oscillator strength of the intersubband transition from the ground state into the first excited state as a function of the dot radius. Based on the computed energies and wave functions, the linear, third-order nonlinear and total optical absorption coefficients as well as the refractive index have been examined. We find that the ground state binding energy and the oscillator strength are strongly affected by the quantum dot radius, hydrostatic pressure and temperature. The results also show that the linear, third-order nonlinear and total absorption coefficients and refractive index changes strongly depend on temperature and hydrostatic pressure.