Saturation effect and forward-dominant second-harmonic generation in single-defect photonic crystals with dual localizations

We study the forward-dominant output of second-harmonic generation (SHG) in single-defect and dual-localized photonic crystals within the saturation limit. We propose that an asymmetric structure can be used to improve the performance of the enhanced SHG. We get two empirical expressions for the total saturation SHG efficiency and the forward factor and provide a reasonable explanation. The theoretical results predict a nearly 100% conversion efficiency and a nearly 100% forward output. Our work is very valuable for designing nonlinear photonic devices. As a test, a practical asymmetrical structure is presented.