Edge effect at the microsphere colloidal array in near-field particle lithography on polymer surfaces

Abstract

In the near-field colloidal lithography, contact particle-lens arrays provide many field maxima simultaneously under the irradiation by a single laser beam. Hexagonally-symmetric close-packed monolayers of dielectric spheres can be deposited on various surfaces in the course of the self-organization process from commercially-available water suspensions. Here, each of the spheres acts as a focusing microlens providing its own field maximum. However, the spheres located at the edge of the array prove to act differently than those what are inside. We observed this experimentally by collecting statistics of the ablation pits formed on a polymer surface by the arrays of polystyrene microspheres under the irradiation by the femtosecond laser pulse. The performed numerical modeling is in agreement with the obtained experimental results. Namely, the edge spheres provide the greater field enhancement and form the deeper and larger ablation pits. This phenomenon can be qualitatively understood by considering the averaged “immersion medium” formed for the inner spheres by their neighbors. The edge spheres have fewer neighbors, thus, the effect of the “immersion” is less pronounced. The difference in the field enhancements for the differently located spheres can be possibly used to enrich the pattern morphology in the particle-lithography applications.