Linear and nonlinear optical properties of ZnO/ZnS and ZnS/ZnO core shell quantum dots: Effects of shell thickness, impurity, and dielectric environment

In the present work, we investigated theoretically the linear, nonlinear, and total absorption coefficients and refractive index changes associated with intersubband transitions in ZnO/ZnS core shell quantum dot (CSQD) and ZnS/ZnO inverted CSQD (ICSQD), emphasizing on the influence of the shell thickness, impurity, and dielectric environment. The effect of the polarization charges due to the possible existence of the dielectric mismatch between the system and its surrounding matrix is considered. The electronic structures are numerically calculated by employing the potential morphing method in the framework of effective mass approximation. We find that in both impurity-free CSQD and ICSQD, increasing the shell thickness red shifts significantly the threshold energy and enhances drastically the nonlinear absorption coefficients and all the refractive index changes, independently on the dielectric environments. Similar behaviour has also been observed in most of the cases studied when the impurity is displaced from the core center to the shell center. In contrast, comparing to a dielectrically homogeneous system, dispersing the systems into a matrix with a lower dielectric constant blue shifts all the peak positions of the absorption coefficients and refractive index changes. However, the corresponding magnitudes (in absolute value) are substantially reduced. Finally, we find that the nonlinear properties are more sensitive to the external perturbations, while at a weak radiation intensity, the variation of the total quantities is generally dominated by that of the corresponding linear terms.

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