Two-dimensional suprawavelength periodic surface structuring of a ZnO single crystal with a UV femtosecond laser

Herein, we report on the one-step formation of a novel microstructure on the surface of crystalline ZnO in ambient air excited by a single femtosecond laser beam (central wavelength 400 nm, pulse duration 35fs), which has photon energy close to the bandgap of ZnO. A twodimensional surface structure with a controlled period of similar to 2-6 mu m is observed, with its orientation independent on the status of laser polarization (linear, circular, or elliptical polarization). We find that the orientation of this two-dimensional structure is defined by the direction of the crystal a and c axes. This structural period of similar to 2-6 micrometers and the independence of its orientation on the laser polarization are in sharp contrast with the traditional laser induced periodic surface structure (LIPSS). In the meantime, surface cracks with a feature size of similar to 30 nm are observed at the bottom of the valley of the two-dimensional structure and theoretical results show there exists strong electric field enhancement on the cracks under 400 nm femtosecond laser irradiation. In view of these unusual features, we attribute the formation of this two-dimensional structure to the mechanical cracking of the ZnO crystal along its (11-20) and (0001) planes induced by the multiple-cyclic heating due to linear absorption of the femtosecond pulses. (C) 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Automated summary

In this study, a novel microstructure was formed on the surface of crystalline ZnO in ambient air by a single femtosecond laser beam. The structure had a controlled period of 2-6 micrometers and was independent of laser polarization. The formation was attributed to mechanical cracking induced by multiple-cyclic heating from linear absorption of the femtosecond pulses.