Diffraction limit of the focusing waveguide grating coupler for optical probeinformation storage

Abstract

A focusing waveguide grating coupler (FWGC) has been applied to anoptical information storage element. To achieve minimum diffraction,focusing and high-coupling efficiency with high power, several fabrication anddesign difficulties have to be overcome. In this work, we report some newstrategies to improve the fabrication and design, and greatly improvethe pattern quality in defining curved gratings in the nanometre rangeusing single-step electron-beam lithography with a conventional exposuremode. We also present the optimum fabrication conditions for the FWGCand the minimum diffraction limit (1.1 µm)obtained for the structure, which is optically the smallest diffraction spot sizeobtainable from diffraction gratings focusing beams into a spot in air.