Laser field effect on the nonlinear optical properties of donor impurities in quantum dots with Gaussian potential

A detailed investigation of optical properties of donor impurities in quantum dots under the influence of laser field with Gaussian potential is performed by using the matrix diagonalization method within the effective mass approximation. Based on the computed energies and wave functions, the dependence of the nonlinear optical properties on the dot size and the potential depth is investigated. The outcome of the calculation suggests that all the factors mentioned above can influence the nonlinear optical properties strongly. We also note that the increase of the laser-dressing parameter leads to important effects on the electronic and optical properties of a quantum dot. This gives a new degree of freedom in various device applications based on the intersubband transition of electrons.     

Intense laser effects on nonlinear optical absorption and optical rectification in single quantum wells under applied electric and magnetic field

In this work the effects of intense laser on the electron-related nonlinear optical absorption and nonlinear optical rectification in GaAs-Ga_1−xAl_xAs quantum wells are studied under, applied electric and magnetic field. The electric field is applied along the growth direction of the quantum well whereas the magnetic field has been considered to be in-plane. The calculations were performed within the density matrix formalism with the use of the effective mass and parabolic band approximations. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Results are presented for the nonlinear optical absorption, the nonlinear optical rectification and the resonant peak of these two optical processes. Several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation have been considered. The outcome of the calculation suggests that the nonlinear optical absorption and optical rectification are non-monotonic functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

Impurity and exciton effects on the nonlinear optical properties of a disc-like quantum dot under a magnetic field

A detailed investigation of the nonlinear optical properties of the (D⁺, X) complex in a disc-like quantum dot (QD) with the parabolic confinement, under applied magnetic field, has been carried by using the perturbation method and the compact density-matrix approach. The linear and nonlinear optical absorption coefficients between the ground (L = 0) and the first excited state (L = 1) have been examined based on the computed energies and wave functions. The competition between the confinement and correlation effects on the one hand, and the magnetic field effects on the other hand, is also discussed. The results show that the confinement strength of QDs and the intensity of the illumination have drastic effects on the nonlinear optical properties. In addition, we note that the absorption coefficients of an exciton in QDs depend strongly on the impurity but weakly on the magnetic field. Furthermore, the light and heavy hole excitons should be taken into account when we study the optical properties of an exciton in a disc-like QD.     

Correlation between electronic states and optical properties in indirect GaAsP/GaP quantum wells with insertion of an ultrathin AlP layer

The optical properties of indirect semiconductor quantum wells (QWs), were studied in relation to their electronic states controlled by an ultrathin AlP layer. The insertion of 1 ML of AlP at the center of a 60 Å GaAsP/GaP QW drastically increased the photoluminescence (PL) intensity and the efficiency of the no-phonon (NP) transition. The NP intensity relative to its TO phonon replica was found to greatly depend on the structural parameters and decreased by decreasing the width of the AlP layer or by increasing the arsenic composition of the GaAsP QW. The comparison with numerical calculation clarified that the efficiency of the NP transition is improved when the X_z electrons rather than X_xy electrons are involved in the radiative recombination. This can be qualitatively understood that the X_z electrons are more strongly localized to the AlP layer, leading to the efficient relaxation of the selection rule. The Arrhenius behavior of the PL intensity was also studied and the quenching of the PL intensity was interpreted as being due to the thermally activated escape of carriers from the well region into the GaP barrier.     

The effect of hydrostatic pressure,temperature and magnetic field on the nonlinear optical properties of asymmetrical Gaussian potential quantum wells

In this study, simultaneous effects of hydrostatic pressure, temperature and magnetic field on the linear and nonlinear intersubband optical absorption coefficients (OACs) and refractive index changes (RICs) in asymmetrical Gaussian potential quantum wells (QWs) are theoretically investigated within the framework of the compact-density-matrix approach and iterative method. The energy eigenvalues and their corresponding eigenfunctions of the system are calculated with the differential method. Our results show that the position and the magnitude of the resonant peaks of the nonlinear OACs and RICs depend strongly on the hydrostatic pressure, temperature and external magnetic field. This gives a new degree of freedom in various device applications based on the intersubband transitions of electrons.     

Nonlinear intersubband optical absorption in semiparabolic quantum wells

The linear and third-order nonlinear optical absorptions in semiparabolic quantum wells are studied in detail. Analytic formulas for the linear and third-order nonlinear optical absorption coefficients are obtained using the compact density matrix approach. Based on this model, numerical results are presented for typical GaAs/AlGaAs semiparabolic quantum wells. The results show that the factors of the incident optical intensity and the semiparabolic confinement frequency have great influences on the total optical absorption coefficients.     

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.     

Polaron effects on linear and nonlinear optical properties of a two-electron quantum dot

The linear and nonlinear optical properties of parabolic quantum dots in which two electrons interact with each other through both coulomb repulsion and longitudinal-optical phonon are studied by using the matrix diagonalization method. With typical semiconducting GaAs-based materials, the linear, third-order nonlinear, total optical absorption coefficients and the optical refractive index have been examined. The effects of different electron-phonon coupling strengths on the linear and nonlinear optical properties are also predicted.     

Effects of hydrostatic pressure and temperature on the nonlinear optical properties of semiparabolic plus semi-inverse squared quantum wells

In this study, the effects of hydrostatic pressure and temperature on nonlinear optical rectification(OR), second-harmonic generation(SHG), third-harmonic generation(THG) and the linear,nonlinear, and total optical absorption coefficients(OACs) of a semiparabolic plus semi-inverse squared quantum well(QW) are theoretically investigated. The results show that hydrostatic pressure and temperature have significant effects on the optical properties of semiparabolic plus semi-inverse squared QWs, and that the energy levels and magnitudes of the resonant peaks of OR, SHG, THG, and the total OACs vary according to the shape of the limiting potential, the hydrostatic pressure, and the temperature. It is easily seen that the peak positions of the resonant peaks of OR, SHG, THG, and the total OACs in the semiparabolic plus semi-inverse squared QW show a red shift with increasing hydrostatic pressure, but a blue shift with increasing temperature. Therefore, the magnitude and position of the resonant peaks of OR, SHG, THG,and the total OACs can be adjusted by changing the hydrostatic pressure and the temperature,which promise a new degree of freedom in the tunability of various electro-optical devices.     

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

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.     

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.     

The nonlinear optical rectification of a confined exciton in a quantum dot

An exciton in a disc-like quantum dot (QD) with the parabolic confinement, under applied electric field, is studied within the framework of the effective-mass approximation. The nonlinear optical rectification between the ground and the first-excited states has been examined through the computed energies and wave functions in details for the excitons. The results show that the optical rectification susceptibility obtained in a disc-like QD reach the magnitude of 10⁻² m/V, which is 3-4 orders of magnitude higher than in one-dimensional QDs. It is found that the second-order nonlinear optical properties of exciton states in a QD are strongly affected by the confinement strength and the electric field.     

Study of nonlinear absorption in GaAs/AlGaAs multiple quantum wells using the reflection Z-scan

The room-temperature third order nonlinearities in GaAs/AlGaAs multiple quantum wells have been studied using reflection Z-scan technique. Band-filling effect is considered to be the major nonlinear mechanism of the nonlinear absorption. A model to calculate the absorption coefficient of quantum wells in the nonlinear regime is presented.      

Nonlinear optical properties of an exciton bound to an ionized donor impurity in quantum dots

In this study, a detailed investigation of the nonlinear optical properties of the (D⁺,X) complex in a disc-like parabolic quantum dot has been carried out by using the matrix diagonalization method and the compact density-matrix approach. First, the numeric calculations and analysis of the oscillator strength of intersubband quantum transition from the ground state into the first excited state at the varying confinement frequency have been performed. Second, the linear, third-order nonlinear, and total absorption coefficients and refractive indices have been investigated. It is observed that the confinement frequency of QDs and the intensity of the illumination have drastic effects on the nonlinear optical properties. In addition, we find that all kinds of absorption coefficients and refractive indices of an exciton in QDs shift to lower energies and their peak values have considerably decreases induced by the impurity.     

Electric field effect on the linear and nonlinear intersubband refractive index changes in asymmetrical semiparabolic and symmetrical parabolic quantum wells

By using the displacement harmonic variant method and the compact density matrix approach, the linear and nonlinear intersubband refractive index changes (RICs) in a semiparabolic quantum well (QW) with applied electric field have been investigated in detail. The simple analytical formulae for the linear and nonlinear RICs in the system were also deduced. The symmetrical parabolic QWs with applied electric fields were taken into account for comparison. Numerical calculations on typical GaAs QWs were performed. The dependence of the linear and nonlinear RICs on the incident optical intensity, the frequencies of the confined potential of the QWs and the strength of the applied electric field were discussed. Results reveal that the RICs in the semiparabolic quantum well system sensitively depend on these factors. The calculation also shows that the semiparabolic QW is a more ideal nonlinear optical system relative to the symmetric parabolic QW systems.     

Linear and nonlinear intersubband optical absorption and refractive index change in asymmetrical semi-exponential quantum wells

Linear and nonlinear intersubband optical absorption and refractive index change in asymmetrical semi-exponential quantum wells are theoretically investigated within the framework of the compact–density–matrix approach and iterative method. The wave functions are obtained by using the effective mass approximation. The energy levels are obtained by numerical method. It is found that the optical absorption coefficients and refractive index changes are strongly affected not only by σ   and _U0$U_0$, but also by the incident optical intensity.     

Trions in quantum wells

An overview is given of our investigation of the energy levels and of the correlation functions of the negatively and positively charged excitons (also called trions) in quantum wells in the presence of a perpendicular magnetic field. A detailed comparison is made with available experimental data in III–V and II–VI semiconductor quantum wells.     

Impact of Al interdiffusion on optical properties in GaSb/AlGaSb multiple quantum wells

We have studied the effect of the Al interdiffusion on the electronic states of AlGaSb–GaSb quantum wells. Measuring the interdiffusion lengths by means of highly depth-resolved secondary ion mass spectrometry we find that the effect of Al diffusion results in the transformation of the ternary–binary system in a ternary–ternary structure. The modified energy levels, calculated on the basis of the measured chemical profile using a Pösch–Teller potential, are consistent with the blue shifts of the PL peaks measured in samples grown with increasing growth temperature.     

Hydrostatic pressure effect on the donor impurity states in asymmetric multiple quantum wells

Based on the effective-mass approximation, hydrostatic pressure effect on the donor binding energy in zinc blende (ZB) InGaN/GaN asymmetric multiple quantum wells (AMQWs) is investigated variationally. Numerical results show that the hydrostatic pressure increases the donor binding energy for any impurity position. Moreover, the hydrostatic pressure effect is more noticeable if the impurity is localized inside the wide well of the AMQWs. For any hydrostatic pressure, the donor binding energy is distributed asymmetrically with respect to the center of the AMQWs. In particular, the donor binding energy of impurity located at the center of the wide well of the AMQWs is insensitive to the increment of the inter-well barrier width if the inter-well barrier width is large.     

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.