Beaming effect and directional emission from photonic-crystal waveguides

Abstract

It is known that free-space focusing of light from sub-wavelength apertures, the so-called <i>beaming effect of light</i>, can be achieved through the excitation of radiative surface modes and their subsequent constructive interference in space surrounding the apertures. This effect, studied extensively in metallic thin films, has recently been shown to exist in photonic-crystal structures. In this paper, we present a comprehensive study of the beaming effect and light directional emission achieved through simple geometric and material engineering of the surface and near-surface structures in two types of photonic-crystal waveguides, classified as increased- and decreased-index structures. We analyze different methods to enhance the directional emission and calculate the resulting efficiencies, highlighting the influence of reflections and matching conditions at the waveguide terminations.