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Cumhuriyet Science Journal

Effect of size modulation and donor position on intersubbands refractive index changes of a donor within a spherical core/shell/shell semiconductor quantum dot

E. B. Al

Published Sep 24, 2021• 9 citations
Publisher

Abstract

In the current study, linear, nonlinear and total relative refractive index changes of a single shallow hydrogenic donor atom confined in semiconductor core/shell/shell quantum dot heterostructure are investigated in detail by compact density matrix formalism. For this purpose, the energy eigenvalues and the corresponding wave functions are calculated by diagonalization method in the effective mass approximation. Then, intersubband 1s→1p and 1p→1d donor transition energies are calculated. In the study, the effects of core/shell sizes, donor position and depth of confinement potential are analyzed. The numerical results show that the linear and nonlinear refractive index changes undergo significant changes.

DOI
10.17776/csj.927289

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In the current study, linear, nonlinear and total relative refractive index changes of a single shallow hydrogenic donor atom confined in semiconductor core/shell/shell quantum dot heterostructure are investigated in detail by compact density matrix formalism. For this purpose, the energy eigenvalues and the corresponding wave functions are calculated by diagonalization method in the effective mass approximation. Then, intersubband 1s→1p and 1p→1d donor transition energies are calculated. In the study, the effects of core/shell sizes, donor position and depth of confinement potential are analyzed. The numerical results show that the linear and nonlinear refractive index changes undergo significant changes.
Abstract In this study, linear, nonlinear and total optical absorption coefficients related a single shallow donor atom confined in semiconductor core/shell/shell quantum dot heterostructure are researched in detail within the compact density matrix formalism approximation. For this purpose, firstly, the energies and the wavefunctions are computed by the diagonalization method in the effective mass approach. Moreover, the effects of size modulation, donor position and magnetic field are analyzed. The numerical results indicate that the linear and nonlinear parts of the absorption coefficients related with intersubband 1s-1p and 1p-1d donor transitions undergo significant changes.
In this study, refractive index changes associated with in- tersubband transitions in a spherical quantum dot, GaAs/AlxGa1ixAs, have been theoretically calculated in the presence of impurity. In this regard, the efiect of dot radius, stoichiometric ratio, impurity and in- cident optical intensity on the refractive index changes have been in- vestigated for the transitions between higher energy states, i.e., 1s-1p, 1p-1d and 1d-1f. The results show that these parameters have a great in∞uence on the refractive index changes.
Simultaneous effects of an on-center hydrogenic impurity and band edge non-parabolicity on intersubband optical absorption coefficients and refractive index changes of a typical GaAs/Al x Ga 1−x As spherical quantum dot are theoretically investigated, using the Luttinger—Kohn effective mass equation. So, electronic structure and optical properties of the system are studied by means of the matrix diagonalization technique and compact density matrix approach, respectively. Finally, effects of an impurity, band edge non-parabolicity, incident light intensity and the dot size on the linear, the third-order nonlinear and the total optical absorption coefficients and refractive index changes are investigated. Our results indicate that, the magnitudes of these optical quantities increase and their peaks shift to higher energies as the influences of the impurity and the band edge non-parabolicity are considered. Moreover, incident light intensity and the dot size have considerable effects on the optical absorption coefficients and refractive index changes.
The absorption spectra and the refractive index changes are calculated theoretically for an exciton in a core/shell quantum dot. The advantage of our methodology is that one can investigate the influence of the repulsive core by varying two parameters in the confinement potential. The dimensionality effect of exciton quantum dots on the optical absorptions has been studied. It has been found that in the same regime, the optical absorption intensities of excitons are much smaller for the core/shell quantum dots than for the two-dimensional quantum rings. The linear and the nonlinear optical absorption coefficients and refractive index changes have been examined with the change of the confinement potential. The results show that the optical absorptions and the refractive index changes are strongly affected by the repulsive core of core/shell quantum dots. Moreover, the calculated results also reveal that as the inner radius increases, the peak values of the absorption coefficients and the refractive index changes of an exciton will show the optical Aharonov–Bohm oscillation in core/shell quantum dots.
The effect of geometrical anisotropy is numerically investigated on the linear and nonlinear optical refractive index changes of a GaAs quantum dot which is located at the center of a Ga1-xAlxAs cylindrical nano-wire. The finite difference approximation has been used for obtaining energy eigenvalues and corresponding wave functions. Also, the compact density matrix formalism is applied to investigate linear, third order nonlinear and total optical refractive index (RI) changes. The optical properties are calculated as a function of the incident photon energy for different ellipsoid aspect ratio and dot radius. The results clearly reveal that the dot anisotropy plays an important role in determining the magnitude of nonlinear RI changes which enable us to adjust saturation condition. We also found that the dot anisotropy shifts optical spectrum towards both lower and higher energies which depend on the shape of dot (spherical, prolate or oblate quantum dot) and dot radius.
Intersubband optical absorption coefficient and refractive index changes of a weakly prolate ellipsoidal quantum dot, using the compact-density matrix formalism and iterative method, are investigated. In this regard, the linear and nonlinear intersubband optical absorption coefficient and refractive index changes of a GaAs / Al x Ga 1-x As ellipsoidal quantum dot, as functions of the dot radius, ellipticity constant, stoichiometric ratio and incident light intensity are calculated. The results indicate that absorption coefficient and refractive index changes strongly depend on the light intensity, size and geometry of the dot and structure parameters such as aluminium concentration in GaAs / Al x Ga 1-x As structures.