Light Localization by Defects in Optically Induced Photonic Structures

Abstract

In the past ten years, there has blossomed an interest in the study of collective behavior of wave propagation in periodic waveguide arrays and photonic lattices [1–3]. The unique bandgap structures of these periodic media, coupled with nonlinear effects, give rise to many types of novel soliton structures [1– 26]. On the other hand, it is well known that one of the unique and most interesting features of photonic band-gap structures is a fundamentally different way of waveguiding by defects in otherwise uniformly periodic structures. Such waveguiding has been demonstrated with an “air-hole” in photonic crystal fibers (PCF) for optical waves [27, 28], in an isolated defect in two-dimensional arrays of dielectric cylinders for microwaves [29–31], and recently in all-solid PCF with a lower-index core [32, 33]. In addition, laser emission based on photonic defect modes has been realized in a number of experiments [34–38]. In one-dimensional (1D) fabricated semiconductor waveguide arrays, previous experiments have investigated nonlinearity-induced escape from a defect state [39] and interactions of discrete solitons with structural defects [40] (see also [41]). Despite the above efforts, theoretical understanding on defect guiding was still limited, and experimental demonstrations of defect guiding was still scarce. In addition, when nonlinear effects are significant, how defect guiding is affected by nonlinearity is largely an open issue. Recently, in a series of theoretical and experimental studies, we optically induced 1D, 2D and ringlike photonic lattices with single-site negative defects in photorefractive crystals, and investigated their linear and nonlinear light guiding properties [42–48]. This work will be reviewed in this Chapter. In addition, we present the first experimental demonstration of nonlinear defect modes which undergoes nonlinear propagation through the defects. Our work not only has a direct link to technologically important systems of periodic structures such as PCF, but also brings about the possibility for studying, in an optical setting, many novel phenomena in periodic systems beyond optics such as edge dislocation, defect healing, eigenmode splitting, and nonlinear mode coupling which have been intriguing scientists for decades [49–51].