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

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) over bar). In crystalline planes c and m, the zigzag and straight lines propagate along the crystalline cell axes with respectively small (<= 30 degrees) and large (90 degrees) inclination to (p) over bar. 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. (C) 2014 Optical Society of America