In this work, we investigate using nanoscale ridge apertures to concentrate light at visible wavelengths into a nanoscale domain. The optical transmission characteristics of ridge apertures in a metal film are first numerically studied using the finite-difference time-domain (FDTD) method. We show that ridge apertures provide optical transmission enhancement compared to conventional square shape nanoscale apertures and confine the transmitted light in the near field to the gap between the ridges due to the fundamental TE10 propagation mode. We fabricated various ridge apertures in metal film coated on quartz substrates by Focused Ion Beam (FIB) milling. These apertures are designed by FDTD calculations to obtain optimal performance.
The ridge apertures are used as a nanometer scale light source for nano-lithography. Holes with sub-100 nm dimension are produced in photoresist with Argon ion laser illumination (wavelength = 457 nm) polarized in the field concentration direction. The performance of the ridge apertures is compared with that of square nanoscale apertures to demonstrate their advantages. Ridge apertures are found to have higher transmission efficiency and better field localization than ordinary-shaped apertures, which is consistent with FDTD simulations. The potentials of using these nanoscale ridge apertures for nano-lithography are demonstrated.
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