Effective nanopatterning on metal by optical near-field processing using self-assembled polystyrene monolayer

Contact particle lens array (CPLA) technique has diverse applications in surface engineering such as encrypting laser-induced security features on metal surfaces. In this technique, when a monolayer of microparticles is irradiated with an ultrashort laser pulse, each particle acts as a microlens, concentrating the laser energy beneath it. This localized energy focus generates heat, resulting in material ablation and the formation of nanopits at the particle-substrate interface. This ability of CPLA has been used to generate unique microdimples on stainless steel such that each microdimple was embedded with periodic hexagonally arranged nanopits. For this, a self-assembled monolayer of polystyrene microparticle of sphere diameter 2 μm deposited on a stainless steel (SS) substrate was irradiated with a single shot of 30 ps pulsed laser and the laser-induced nanopits were characterized by scanning electron microscope images. With optimum laser power and microparticle size, various two-dimensional patterns were generated on SS such that each laser-induced microdimple contained ∼3000 numbers of nanopits. The dimension of the nanopits was controlled by varying laser powers. The simulation results obtained from coupled electromagnetic and thermal modelling techniques further verified the light intensity enhancement effect under particle-assisted focusing, providing theoretical support for the experimental results. Precise control over pitting dimension and ability of generating complex two-dimensional patterns demonstrates limitations on easy replication of CPLA-based inscription.