Propagative cascading at the origin of very large cubic nonlinearities in quadratic polar molecular media

Very large nonlinear shifts ranging from /spl pi//4 to /spl pi//2 corresponding to n/sub 2/ as high as 10/sup -12/ cm/sup 2//W are observed in single crystal cored fibers of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN). Self-phase modulation and direct interferometric measurements have unambiguously ascribed this behaviour to a cascading sequence of second harmonic generation followed by down frequency conversion.