The linear and nonlinear optical properties of an off-center hydrogenic donor impurity in nanowire superlattices: Comparison between arrays of spherical and cylindrical quantum dots

Abstract

Nanowire superlattices (NWSLs) are objects with a wide range of potential applications in nanoscale electronics and optics. In this paper, we consider an off-center hydrogenic donor impurity in two different NWSL with cylindrical cross-section which involved an array of spherical or cylindrical quantum dots (QDs) along their axis and called Sph and Cyl NWSLs, respectively. The electronic eigenstates and energy eigenvalues of NWSLs are determined by applying finite difference method. Additionally, optical properties such as linear, nonlinear and total intersubband absorption coefficients (ACs) and refractive index (RI) changes are investigated by means of compact density matrix approach. The results show that (I) optical ACs and RI changes of a Sph NWSL are smaller than those of Cyl NWSL. (II) The amplitudes of ACs and RI changes are strongly affected by impurity position and show an oscillatory behavior as the impurity shifts away from the center. (III) Resonance condition in optical spectrum can be adjusted by shape of QDs and impurity position and both blue and red shifts are appeared in ACs as the impurity shifts away from the center of NWSLs. (IV) Total optical absorption saturation intensity can be controlled by the shape of QDs or the impurity position. Moreover, optical incident intensities correspond to the saturation in optical spectrum in Cyl NWSL are smaller than their values for Sph NWSL and have a minimum values as the impurity is located at the center of barriers for both Sph and Cyl NWSLs. (V) By increasing the relaxation time the amplitude of the total AC will increase and for large values of relaxation time a saturation appears in optical spectrum which strongly depends on the Al concentration, impurity position and type of NWSL.