Effect of position-dependent effective mass on linear and nonlinear optical properties of a cubic quantum dot

In this paper, we first obtain an analytic relation for studying the position-dependent effective mass in a GaAs/Al_xGa_1−xAs cubic quantum dot. Then, the effect of position-dependent effective mass on the intersubband optical absorption coefficient and the refractive index change in the quantum dot are studied. Our numerical calculations are performed using both a constant effective mass and the position-dependent effective mass. We calculate the linear, nonlinear and total intersubband absorption coefficient and refractive index change as a function of the incident optical intensity and structural parameters such as dot length. The results obtained from the present work show that spatially varying electron effective mass plays an important role in the intersubband optical absorption coefficient and refractive index change in a cubic quantum dot.     

Electronic states and the resonant optical non-linearity of exciton in a narrow band InSb quantum dot

Binding energy, interband emission energy and the non-linear optical properties of exciton in an InSb/InGa_xSb_1−x quantum dot are computed as functions of dot radius and the Ga content. Optical properties are obtained using the compact density matrix approach. The dependence of non-linear optical processes on the dot sizes is investigated for different Ga concentrations. The linear, third order non-linear optical absorption coefficients, susceptibility values and the refractive index changes of the exciton are calculated for different concentrations of gallium content. It is found that gallium concentration has great influence on the optical properties of InSb/InGa_xSb_1−x dots.     

Study of optical properties in a cubic quantum dot

In this paper, the effect of hydrostatic pressure on both the intersubband optical absorption coefficients and the refractive index changes is studied for typical GaAs/Al _x  Ga_1?x As cubic quantum dot. We use analytical expressions for the linear and third-order nonlinear intersubband absorption coefficients and refractive index changes obtained by the compact-density matrix formalism. The linear, third-order nonlinear, and total intersubband absorption coefficients and refractive index changes are calculated at different pressures as a function of the photon energy with known values of box length (L), the incident optical intensity (I), and Al concentration (x). According to the results obtained from the present work, we have found that the pressure plays an important role in the intersubband optical absorption coefficient and refractive index changes in a cubic quantum dot.     

Nonlinear optical properties of a D system in a spherical quantum dot

The nonlinear optical properties of a D⁻ system confined in a spherical quantum dot represented by a Gaussian confining potential are studied. The great advantage of our methodology is that the model potential possesses the finite height and range. Calculations are carried out by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. We calculate the linear, third-order nonlinear and total optical absorption coefficients under the density matrix formalism. Numerical results for GaAs − Ga_1 − xAl_xAs QDs are presented. Our results show that the optical absorption coefficients in a spherical QD are much larger than their values for GaAs quantum wells. It is found that optical absorptions are strongly affected not only the confinement barrier height, dot radius, the electron-impurity interaction but also the position of the impurity.     

Interband absorption and exciton binding energy in an inverse parabolic quantum well under the magnetic field

We have investigated the effects of the magnetic field which is applied perpendicular to the growth direction of the well on the interband absorption and on the binding energy of the excitons in an GaAs/Ga1−xAlxAs inverse parabolic quantum well (IPQW) with different widths as well as different Al concentrations at the well center. The calculations were performed within the effective mass approximation, using a variational method. We observe that IPQW structure turns into parabolic quantum well with the inversion effect of the magnetic field and the effective band gap of the system can be modified by changing Al concentration at the well center, the strength of the magnetic field and well dimensions. This case directly influences the nature of electronic and optical properties in this structure.     

Conduction band non-parabolicity effect on the optical absorption coefficient and refractive index changes of spherical quantum dots

Optical absorption coefficients and refractive index changes associated with intersubband transition in typical GaAs/Al_xGa_1−xAs spherical quantum dots are theoretically investigated, both in the presence and in the absence of the conduction band non-parabolicity effect. In this regard, the effect of band non-parabolicity on the eigenvalues and eigenfunctions of the dot has been performed using the Luttinger-Kohn effective mass equation. Also, by means of the compact-density-matrix approach the linear and nonlinear optical absorption coefficients and refractive index changes have been calculated. The results show that magnitudes of these quantities are decreased and the peaks are shifted to the lower energies as the influence of band non-parabolicity is considered.     

Linear and nonlinear intersubband optical absorption in symmetric double semi-parabolic quantum wells

The linear and the nonlinear intersubband optical absorption in the symmetric double semi-parabolic quantum wells are investigated for typical GaAs/AlxGa1−xAs. Energy eigenvalues and eigenfunctions of an electron confined in finite potential double quantum wells are calculated by numerical methods from Schrödinger equation. Optical properties are obtained using the compact density matrix approach. In this work, the effects of the barrier width, the well width and the incident optical intensity on the optical properties of the symmetric double semi-parabolic quantum wells are investigated. Our results show that not only optical incident intensity but also structure parameters such as the barrier and the well width really affect the optical characteristics of these structures.     

First principles investigation of optical properties of zinc-blende AlxGa1−xAs1−yNy materials

We have performed a first-principle Full Potential Linearized Augmented Plane Waves calculation within the local density approximation (LDA) to the zinc-blende Al_xGa_1−xAs_1−yN_y to predict its optical properties as a function of N and Al mole fractions. The accurate calculations of electronic properties such as band structures and optical properties like refractive index, reflectivity and absorption coefficient of Al_xGa_1−xAs and Al_xGa_1−xAs_1−yN_y with x≤0.375 and y up to 4% are presented. Al_xGa_1−xAs on GaAs have a lattice mismatch less than 0.16% and the lattice constant of Al_xGa_1−xAs has a derivation parameter of 0.0113±0.0024. The band gap energies are calculated by LDA and the band anticrossing model using a matrix element of C_MN=2.32 and a N level of E_N=(1.625+0.069x) eV. The results show that Al_xGa_1−xAs can be very useful as a barrier layer in separate confinement heterostructure lasers and indicate that the best choice of x and y Al_xGa_1−xAs_1−yN_y could be an alternative to Al_xGa_1−xAs when utilized as active layers in quantum well lasers and high-efficiency solar cell structures.     

Study on the electronic structure and optical properties of different Al constituent Ga1−xAlxAs

Using quantum mechanics GASTEP software package based on the first principle density function theory, the electronic structure and optical properties of Ga_1−xAl_xAs at different Al constituent are calculated. Result shows that with the increase of Al constituent, the band gap of Ga_1−xAl_xAs increases and varies from direct band gap to indirect band gap; the absorption band edge and the absorption peak move to high-energy side; the static reflectivity decreases. With the increasing of the incident photon energy, Ga_1−xAl_xAs shows metal reflective properties in certain energy range. With the increasing of Al constituent, static dielectric constant decreases and the intersection of dielectric function and the x-axis move towards high-energy side; the peak of energy loss function move to low-energy side and the peak value reduces.     

Intensity-dependent nonlinear optical properties in a modulation-doped single quantum well

In the present work, the changes in the intersubband optical absorption coefficients and the refractive index in a modulation-doped quantum well have been investigated theoretically. Within the envelope function approach and the effective mass approximation, the electronic structure of the quantum well is calculated from the self-consistent numerical solution of the coupled Schrödinger-Poisson equations. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. The numerical results GaAs/Al_xGa_1−xAs are presented for typical modulation-doped quantum well system. The linear, third-order nonlinear and total absorption and refractive index changes depending on the doping concentration are investigated as a function of the incident optical intensity and structure parameters, such as quantum well width and stoichiometric ratio. The results show that the doping concentration, the structure parameters and the incident optical intensity have a great effect on the optical characteristics of these structures.     

Interface phonons in cylindrical quantum dot heterostructure

The top interface optical (TIO) and side interface optical (SIO) phonon modes of a cylindrical GaAs/ Al_xGa_1−xAs quantum dot are derived within the framework of dielectric continuum approximation. Results reveal that, in the case of taking the “two-mode” behavior of the Al_xGa_1−xAs material into account, there exist eight branches of TIO phonon modes and four branches of SIO phonon modes. The dispersion frequencies of TIO or SIO phonon modes sensitively depend on the Al mole fraction x$x$ in the Al_xGa_1−xAs material. With increasing wavevector q$q$ (κ$\kappa$), the frequency of each TIO (SIO) mode approaches one of the two frequency values of the single Al_xGa_1−xAs heterostructure.     

Band gap energy and bowing parameter of In-rich InAlN films grown by magnetron sputtering

The crystal structure, band gap energy and bowing parameter of In-rich In_xAl_1−xN (0.7 < x < 1.0) films grown by magnetron sputtering were investigated. Band gap energies of In_xAl_1−xN films were obtained from absorption spectra. Band gap tailing due to compositional fluctuation in the films was observed. The band gap of the as-grown InN measured by optical absorption method is 1.34 eV, which is larger than the reported 0.7 eV for pure InN prepared by molecular beam epitaxy (MBE) method. This could be explained by the Burstein-Moss effect under carrier concentration of 10²⁰ cm⁻³ of our sputtered films. The bowing parameter of 3.68 eV is obtained for our In_xAl_1−xN film which is consistent with the previous experimental reports and theoretical calculations.     

Pressure dependence of dielectric constant,elastic constants,and lattice parameters of the Y3(Ga,Al)5O12 host

Atomistic simulations were performed to investigate the lattice parameters, dielectric constant, and elastic constants of Y₃(Ga_xAl_5−x)O₁₂ (x = 1, 2, 3, 4, 5) structures. The calculated lattice parameters and elastic constants are in good agreement with those in available experimental results. The pressure dependence of all studied quantities was investigated. In general, a change in the behavior of all studied quantities is found when the Ga concentration becomes more than that of the aluminum (Al) in Y₃(Ga_xAl_5−x)O₁₂ (x = 1, 2, 3, 4, 5) structures.     

Rabi oscillations in two-level semiconductor systems

Rabi oscillations in coherent optical excitations in bulk GaAs and quantum dot two-level systems may be converted into deterministic photocurrents, with the impurities or dots providing the tag for each qubit. Here we perform a theoretical analysis of the damping of Rabi oscillations in two-level semiconductor systems. Present calculations, through optical Bloch equations on excitonic two-level In_xGa_1−xAs quantum-dot systems, are found in good agreement with the corresponding experimental data. Calculated results indicate that the nature underlying the dephasing mechanism associated to the damping of the measured Rabi oscillations, which has previously remained as an open question, may be associated with a field-dependent recombination rate related to the inhomogeneous broadening of the excitonic lines in the In_xGa_1−xAs two-level QD system.     

Effects of electric field and hydrostatic pressure on donor binding energies in a spherical quantum dot

The binding energies of a hydrogenic donor in a GaAs spherical quantum dot in the Ga_1−xAl_xAs matrix are presented assuming parabolic confinement. Effects of hydrostatic pressure and electric field are discussed on the results obtained using a variational method. Effects of the spatial variation of the dielectric screening and the effective mass mismatch are also investigated. Our results show that (i) the ionization energy decreases with dot size, with the screening function giving uniformly larger values for dots which are less than about 25 nm, (ii) the hydrostatic pressure increases the donor ionization energy such that the variation is larger for a smaller dot, and (iii) the ionization energy decreases in an electric field. All the calculations have been carried out with finite barriers and good agreement is obtained with the results available in the literature in limiting cases.     

Exciton bound to an ionized donor impurity in GaAs/Ga1−xAlxAs ellipsoidal finite-potential quantum dots under hydrostatic pressure

In the framework of perturbation theory, a variational method is used to study the ground state of a donor bound exciton in a weakly prolate GaAs/Ga_1−xAl_xAs ellipsoidal finite-potential quantum dot under hydrostatic pressure. The analytic expressions for the Hamiltonian of the system have been obtained and the binding energy of the bound exciton is calculated. The results show that the binding energy decreases as the symmetry of the dot shape reduces. The pressure and Al concentration have a considerable influence on the bound exciton. The binding energy increases monotonically as the pressure or Al concentration increases, and the influence of pressure or Al concentration is more pronounced for small quantum dot size.     

Linear and nonlinear intersubband optical absorption and refractive index changes of asymmetric double semi-parabolic quantum wells

The optical properties of the asymmetric double semi-parabolic quantum wells (DSPQWs) are investigated numerically for typical GaAs/Al_xGa_1−xAs. Optical properties are obtained using the compact density matrix approach. In this work, effects of the structure parameters such as the barrier width and the well widths on the optical properties of the asymmetric DSPQWs are investigated. The results show that the linear and nonlinear optical properties of asymmetric DSPQW are non-monotonic functions of these structure parameters. The behavior of the refractive index changes of asymmetric DSPQW with the variation of the barrier width is different substantially with that of symmetric DSPQW. Results reveal that the resonant peak values of the total absorption coefficient of asymmetric DSPQW is usually greater than that of symmetric DSPQW. Our calculations also show that the total absorption coefficient of asymmetric DSPQW is larger than that of asymmetric double square quantum well.     

A study of nonlinear optical properties of a negative donor quantum dot

In this paper, we studied the nonlinear optical properties of a negative donor center (D⁻) in a disk-like quantum dot (QD) with a Gaussian confining potential. Calculations are carried out by using the method of numerical diagonalization of Hamiltonian matrix within the effective-mass approximation. A detailed investigation of the linear, third-order nonlinear, total optical absorptions and refractive index changes has been carried out for the D⁻ QD and the D⁰ QD. The linear, third-order nonlinear, total optical absorptions and refractive indices have been examined for a double-electron QD with and without impurity. Our results show that the optical absorption coefficients and refractive indices in a disk-like QD are much larger than their values for quantum wells and spherical QDs and the nonlinear optical properties of QDs are strongly affected not only with the confinement barrier height, dot radius, the number of electrons but also the electron-impurity interaction.     

Photoluminescence scanning on InAs/InGaAs quantum dot structures

The photoluminescence spectra of InAs quantum dots (QDs) embedded into four types of In_xGa_1−xAs/GaAs (x = 0.10, 0.15, 0.20 and 0.25) multi quantum well MBE structures have been investigated at 300 K in dependence on the QD position on the wafer. PL mapping was performed with 325 nm HeCd laser (35 mW) focused down to 200 μm (110 W/cm²) as the excitation source. The structures with x = 0.15 In/Ga composition in the In_xGa_1−xAs capping layer exhibited the maximum photoluminescence intensity. Strong inhomogeneity of the PL intensity is observed by mapping samples with the In/Ga composition of x ≥ 0.20-0.25. The reduction of the PL intensity is accompanied by a gradual “blue” shift of the luminescence maximum at 300 K as follows from the quantum dot PL mapping. The mechanism of this effect has been analyzed. PL peak shifts versus capping layer composition are discussed as well.     

Observation of optical properties related to room-temperature ferromagnetism in co-sputtered Zn1−xCoxO thin films

We report on the analysis of optical transmittance spectra and the resulting ferromagnetic characteristics of sputtered Zn_1−xCo_xO films. Zn_1−xCo_xO films were prepared on (0001)-oriented Al₂O₃ substrates by the radio-frequency (rf) magnetron co-sputtering method. The XRD results showed that the crystallinity of films was properly maintained up to x=0.30 and no second phase peaks were detected up to x=0.40. The transmittance spectra showed both the increase of the absorption band intensity and the red shift of the absorption peak as well as the band edge with increasing x. We have proved experimentally that these changes depend on Co concentration. These optical properties suggest that sp-d exchange interactions and typical d-d transitions become activated with increasing x, which leads to the enhancement of ferromagnetic properties in Zn_1−xCo_xO films as shown in the AGM results. Therefore, it is concluded that the ferromagnetism derives from the substitution of Co²⁺ for Zn²⁺ without changing the wurtzite structure.