Effect of laser polarization and crystalline orientation on ZnO surface nanostructuring in the regime of high-density electronic excitation

Abstract

The effect of laser polarization and crystalline orientation on nanoscale morphology of ZnO monocrystal has been studied in the regime of high-density electronic excitation related to the saturation of the exciton fluorescence and appearance of the electron-hole plasma continuum. The irradiation with femtosecond KrF laser (248 nm, 450 fs) was realized in the sub-melting regime of fluences about a half of the ablation threshold. Two types of nanostructures were observed: holes of 10 nm diameter arranged in quasi-periodic zigzag and straight lines, which propagate along the crystalline planes a, c, and m. The nanostructuring sensitively depends on the crystalline plane’s orientation to laser electric field polarization (p¯). In crystalline planes c and m, the zigzag and straight lines propagate along the crystalline cell axes with respectively small (≤30°) and large (90°) inclination to p¯. The nanostructuring in the crystalline plane a is rare and of low intensity. This behavior can be related to a stress field due to an accumulation of space charges in the crystalline structure.