Measurement of losses in planar photonic crystals

Abstract

Waveguiding in planar photonic crystals has been obtained by a number of research groups, using either direct coupling in to the waveguides, or by generating photoluminescence internal to the sample, and then capturing some of this luminescence in a waveguide (Loncar et al, 2000; Tokushima et al, 2000; Fukuya et al, 2000). Although the waveguide structure may differ from one group to another, all samples studied so far have some important design features in common: the photonic crystal matrix consists of a periodic array of holes in a high index ( n>3) material, and a waveguide formed by omitting one or more rows of holes. Light is confined to the waveguide by both total internal reflection ( TIR) and in the plane of the waveguide also by photonic crystal confinement. As a result, all waveguides, even those formed by omission of a single row of holes, are multi-mode. A key parameter of the waveguide performance is the loss for which there are three principal components: coupling loss, transmission loss and loss at the bends or corners in these guides. We discuss four principal methods of measuring loss in a waveguide structure: (a) the cut-back method, where transmission through waveguides of different lengths is compared, (b) measurement of Fabry-Perot resonances, (c) measurement of Q-factors of a resonant cavity in a guide, and (d) measurement of optical and thermal power radiated from a guide.