Large area laser surface micro/nanopatterning by contact microsphere lens arrays

Laser surface micro/nanopatterning by particle lens arrays is a well-known technique. Enhanced optical fields can be achieved on a substrate when a laser beam passes through a self-assembled monolayer of silica microspheres placed on the substrate. This enhanced optical field is responsible for ablative material removal from the substrate resulting in a patterned surface. Because of the laser ablation, the microspheres are often ejected from the substrate during laser irradiation. This is a major issue impeding this technique to be used for large area texturing. We explored the possibility to retain the spheres on the substrate surface during laser irradiation. A picosecond laser system (wavelength of 515 nm, pulse duration 6.7 ps, repetition rate 400 kHz) was employed to write patterns through the lens array on a silicon substrate. In this experimental study, the pulse energy was found to be a key factor to realize surface patterning and retain the spheres during the process. When the laser pulse energy is set within the process window, the microspheres stay on the substrate during and after laser irradiation. Periodic patterns of nanoholes can be textured on the substrate surface. The spacing between the nanoholes is determined by the diameter of the microspheres. The depth of the nanoholes varies, depending on the number of laser pulses applied and pulse energy. Large area texturing can be made using overlapping pulses obtained through laser beam scanning.