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

• Numerical simulation for sub-wavelength focusing including surface plasmon polrarition effect.
• Fabrication of experimental metallic nano-slit array through lift-off process.
• Gold nano-slit array of 70 nm in dielectric slit-width and variable widths of metallic stripy claddings.
• Experimental sub-wavelength focusing on light sensitive papers for wavelength of 532 nm.
• Experimental sub-wavelength light transmission using a dispersive Raman spectrometer.

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 sub-wavelength 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.