We analyze second-harmonic generation (SHG) at a thin effectively quadratic
nonlinear interface between two linear optical media. We predict multistab
ility of SHG for both plane and localized waves, and also describe two-colo
r localized photonic modes composed of a fundamental wave and its second ha
rmonic coupled together by parametric interaction at the interface. [S1063-
651X(99)51307-X].