Numerical and experimental studies on sub-wavelength focusing in nano-slit arrays of metallic stripes with variable widths

摘要

We simulated the distributions of light energy transmission through metallic nano-slits for the optimal design of nano-lens structures and observed experimentally the light focusing effect from the nano-lens array. Contrary to reported research where the dielectric slit sizes are controlled, the current design is based on the distributions of metallic claddings of different sizes. The nano-focusing is generated by the surface plasmon polariton (SPP) effect that is known to overcome the diffraction limit and to induce subwavelength focusing. SPPs are collective electron charge oscillations in metallic nano-slits that can be excited by electromagnetic waves, amplifying near-field optical waves at resonance. Here, intensity amplifications in near-fields are maximized for a given light source, mainly based on various distributions of metallic claddings of different sizes in an array. For experimentation gold nano-slit arrays designed from the numerical optimization were fabricated through the lift-off process and confirmed by observations in SEM and confocal microscopes. The SPP effect in nano-slit lenses was explored by light focusing on light-sensitive papers macroscopically and by light transmission microscopically using a dispersive Raman spectrometer in scattering mode.