Metal-slit array fresnel-lens for optical coupling

We proposed, for the first time, to utilize metal-slits array Fresnel lens for the optical coupling form free space into silicon slab waveguide while overcoming near focal length limit of the conventional dielectric Fresnel lens. Physical limits of the conventional Fresnel lens were revealed and it was proposed to employ metal-slit array to overcome these limitations. Comparative study has been carried out with a FDTD simulation between normal Fresnel lens coupling and metal-slit array Fresnel lens coupling.