Optimization of coupling from a sub-wavelength nanoaperture to the fundamental Gaussian mode

We minimize scattering losses and maximize coupling to a given free-space mode from a sub-wavelength metal aperture surrounded by periodic and aperiodic corrugations via optimum choice of the parameters of the structure. We show that introduction of aperiodicity into the corrugations enables coupling to a Gaussian mode [image omitted], which is twice as high as that obtainable from the best periodic-corrugation design. These results indicate the possibility of using such metal nanostructures for quantum- and nonlinear-optical applications.