λ/20 surface nanostructuring of ZnO by mask-less ultrafast laser processing

Fabrication of nanostructures with a feature size much smaller than the laser wavelength is challenging due to the optical diffraction limit. It's well known that the irradiation of polarized ultrafast laser generates periodic nanostructures, so called laser-induced periodic surface structures (LIPSS). Owing to the modulated field, the surface is periodically ablated to form specific patterns, which can be used for some photonic applications including surface-enhanced Raman scattering (SERS). In this paper, we investigate the morphologies of LIPSS on ZnO substrates by mask-less ultrafast laser processing. By adjusting the laser processing parameters, including fluence, pulse number, polarization, and pulse duration, the homogenous nanostrip array and nanopillar array are created. Furthermore, by adjusting the laser fluence, a single nanogroove with a width of ∼20 nm and a single nanocavity with a diameter of ∼24 nm are created. The gold nanoparticles are then coated on the ZnO nanopillar array for SERS application. We found that the concentration of defects in ZnO substrate is increased by the laser irradiation, which is beneficial for SERS performances to achieve an enhancement factor of SERS as high as 2.28 × 10⁷.