Surface defect gap solitons.

We report on the existence of surface defect gap solitons. Such new type of solitons can be well supported by an interface between the defect of optical lattice and the uniform media with focusing saturable nonlinearity. The surface defect of optical lattice can profoundly affect the properties of solitons. It is shown that for the positive defect, stable solitons exist at the first bandgap and their powers decrease with defect depth; while for negative defect, stable solitons exist at the second bandgap and their powers increase with defect depth. Such solitons with moderate power between lower and higher ones cannot stably existent at the first bandgap.