Morphology control of nanostructure using microsphere-assisted femtosecond laser double-pulse ablation and chemical etching

In this study, a microsphere-assisted femtosecond (fs) laser temporal shaping fabrication method was proposed. Different from two-dimensional (2D) micro/nanohole pattern, three-dimensional (3D) hexagonal ring-like array can be obtained by microsphere-assisted fs laser double-pulse temporal pulse shaping and chemical etching. The fused silica substrate with polystyrene (PS) microsphere mask was ablated firstly by fs laser and then etched by KOH solution. During laser irradiation, PS microspheres with a diameter of 1 μm were ionized through two-photon absorption and disturbed propagation of fs laser. Concave hexagonal array was ablated because of metal-like mask screening effect of PS microspheres, whose nanowire width can be decreased to 30 nm. The ionization process inside PS microspheres can be controlled using double pulses with different pulse delays. Because of lower absorption and reflection of PS mask under double-pulse irradiation, more laser fluence can transmit into fused silica substrate. Therefore, a uniform hexagonal ring-like array was formed under double pulses after chemical etching, which cannot be obtained through traditional single pulse irradiation. The thickness of hexagonal rings can be controlled by adjusting etching time. The hexagonal ring-like array obtained under double-pulse irradiation can be applied in lubricant reservoir, drug delivery, particle trapping and surface-enhanced Raman scattering (SERS).