Observation of surface gap solitons

We predict, in the framework of a nonlinear discrete model, and demonstrate ex-perimentally in defocusing waveguide arrays, that self-localization near the edge of a photonic lattice can result in the formation of surface gap solitons. c©2005 Optical Society of America OCIS codes: 190.4420; 190.5940; Surface modes are a special type of waves localized at an interface between two different media. In periodic systems, staggered modes localized at the surface are known as Tamm states1, first found as localized electronic states at the edge of a truncated periodic potential. Surface states have been studied in different fields of physics, including optics2 where such waves are confined to the interface between periodic and homogeneous dielectric media. Recently it was predicted theoretically and demonstrated experimentally that nonlinear self-trapping of light near the edge of a waveguide array with self-focusing nonlinearity can lead to the formation of discrete surface solitons3,4. However, when light propagates in a defocusing nonlinear medium with periodically modulated refractive index, self-localization can occur in the form of surface modes having a staggered phase structure, thus enabling a full analogy with the localized electronic surface Tamm states. In this work, we study the formation of solitons near the edge of a defocusing nonlinear medium with a semi-infinit