Simultaneous formation of two types of periodic nanostructure in ZnO single crystal by femtosecond laser direct irradiation

In this paper, we report the simultaneous fabrication of nanogratings and three-dimensional (3D) concentric circles in the same bulk of ZnO single crystal by femtosecond laser irradiation of a single spot. For nanogratings, polarization dependence and selective growth characteristics are two main characteristics; for 3D concentric circles, a noticeable feature is their close relationship with the development of the deep crater ablated by the femtosecond laser. On the basis of the above key points observed from scanning-electron- microscope (SEM) images, possible formation mechanisms for these nanostructures are analyzed.