Modulational instability in periodic quadratic nonlinear materials - art. no. 133901

We investigate the modulational instability of plane waves in quadratic non linear materials with linear and nonlinear quasi-phase-matching gratings. E xact Floquet calculations, confirmed by numerical simulations, show that th e periodicity can drastically alter the gain spectrum but never completely removes the instability. The low-frequency part of the gain spectrum is acc urately predicted by an averaged theory and disappears for certain gratings . The high-frequency part is related to the inherent gain of the homogeneou s non-phase-matched material and is a consistent spectral feature.