Laser radiation effect on the optical properties of a spherical quantum dot confined in a cylindrical nanowire

Abstract

An investigation is done in order to calculate the linear, third-order nonlinear and total optical absorption coefficient and refractive index changes of an impure GaAs spherical quantum dot placed at the center of a Ga1−xAlxAs cylindrical nano-wire. In this regard, the finite difference approximation is considered to calculate electronic structure as well as compact density-matrix approach is applied to investigate optical properties. In this study the effects of laser-field radiation along with aluminum (Al) concentration on optical properties are examined for three transitions between four first lower-lying electronic levels, i.e. 1→2, 2→3 and 3→4 transitions. Also the effect of relaxation time on saturation is studied and compared for these transitions. It is found that the magnitude of linear and third order nonlinear absorption coefficient and refractive index changes as well as the optical saturation can be modified by both Al concentration and the amplitude of laser-field radiation. Moreover, for 1→2 transition (3→4 transition) the effect of quantum dot (nano-wire) is dominant whereas for 2→3 transition Al concentration is a major determinant factor. Additionally, for all transitions by increasing the value of laser field a red shift appears in optical spectrum where the extent of this red shift depends on the Al concentration and the energy levels between which the transitions occur. Finally, in contrast to QDs, a red shift appears in absorption spectrum as going up the transitions between higher energy levels.