Quadratic simultons in linear and nonlinear photonic bandgaps

Energy localization and Bragg solitons can be sustained by the interplay of distributed feedback and quadratic nonlinearity. With reference to second harmonic generation, we review the main features of slowly-moving and stationary two-color solitons or simultons both in singly and doubly resonant structures, and discuss their experimental feasibility and potential applications. Finally, with reference to the case of purely nonlinear feedback through short-period quasi-phase-matching, we show that cavityless parametric oscillations can occur in the presence of gain.