Phase matching in the presence of feedback: Higher order terms and enhancement of second harmonic generation.

We show that inside a multi-layer dielectric stack, consideration of higher-order, fast-oscillating interference terms between counter-propagating waves can dramatically change the dynamics of second harmonic generation, and thus lead to an unusual result: field confinement and overlap can be far better optimized, and conversion efficiencies further enhanced, in the presence of a phase mismatch. One may therefore conclude that phase matching is not always a necessary condition to provide optimized nonlinear frequency conversion efficiency