The aim of this article is to introduce a nanowriter system that could lead to a sub-micrometer spot size using a visible light source under ambient conditions. The key component of the system is a focusing optical head, which incorporates a plasmonic-based lens instead of a conventional lens. Based on knowledge of the physical origin of extraordinary transmission and directional beaming, we theorize that the directional beaming phenomenon can be explained simply as a surface plasmon (SP) diffraction along the corrugations as long as the multiple scattering effects are taken into account to modify the dispersion relationship of the surface plasmon. We introduce a Rigorous Coupled Wave Analysis (RCWA) formulation to pursue a precise dispersion relationship needed for the lens design. Comparing the resultant theoretical data between Finite Difference Time Domain (FDTD) simulations and RCWA results, we found good agreement and the many important characteristic parameters needed for an innovative lens design. We also set up a writing-test optomechanical system to examine the photoresist exposure ability of the plasmonic-based lens.