Deep-subwavelength ripples on the ZnO surface obtained via metal-film-assisted femtosecond laser processing

The femtosecond laser-induced periodic surface structure (LIPSS) in wide-bandgap materials has drawn extensive attention owing to its potential applications in plasmonics, optoelectronics, and biology. However, the weak interaction between femtosecond lasers and wide-bandgap materials leads to a high damage threshold, rendering the modulation of the LIPSS period quite challenging. In this work, 102-nm-period deep-subwavelength ripples (DSRs) were fabricated on the ZnO surface utilizing metal-film-assisted femtosecond laser processing. The linearly polarized femtosecond laser pulses had a pulse duration of 120 fs and a repetition rate of 1 kHz at a wavelength of 800 nm. Compared with the pristine sample, the period of the DSRs on the processed ZnO was reduced by about 58.9%, and the damage threshold was decreased by about 54% through coating with a 50-nm-thick Al film. Besides, Raman spectroscopy of the large-area DSRs generated on the ZnO surface exhibited great crystallinity, and the corresponding results of the water contact-angle measurements indicated an anisotropic wetting behavior. This work provides a new approach to minimize the period of DSRs in wide-bandgap materials, which may result in the discovery of new properties of these materials at the micro and nanoscale.