The linear and nonlinear optical properties of GaAs/GaAlAs nanowire superlattices

Abstract

In the present work, we intend to study the effects of aluminum (Al) concentration, hydrogenic donor impurity and shape of quantum dots (QDs) on optical properties of nanowire superlattices (NWSLs). We consider two different NWSL which involved an array of spherical or cylindrical QDs along their axis and called Sph and Cyl NWSLs, respectively. In this regard, the energy eigenvalues and corresponding wave functions are calculated using finite difference approximation within the effective mass framework. Also, the linear, third order nonlinear and total intersubband absorption coefficients (ACs) and refractive index (RI) changes are investigated by means of the compact density matrix approach. The results show that (I) ACs and RI changes of Cyl NWSL are larger than those of Sph NWSL. (II) Presence of impurity decreases ACs and RI changes and shifts optical spectrum towards higher energies (blue shift) for both Sph and Cyl NWSLs. (III) In absence of hydrogenic donor impurity, by increasing the volume of QDs a red shift appears in optical spectrum of Cyl NWSL whereas for Sph NWSL first red shift and then blue shift appear. In the presence of hydrogenic impurity an opposite behavior is observed, i.e., optical spectrum of Cyl NWSL shifts towards higher energies with an augment in volume of QDs while for Sph NWSL first a blue shift and then a red shift appear in optical spectrum. (IV) Through increasing the Al concentration, ACs and RI changes decrease and shift towards higher energies for both Sph and Cyl NWSLs. This blue shift is more enhanced in the case of Sph NWSL. (V) By increasing incident optical intensity the magnitude of total AC decreases and absorption saturation occurs for large values of incident intensity. We find that the critical incident intensity; which corresponds to the saturation, is increased by decreasing aluminum concentration and also is smaller for Cyl NWSL.