Cascading of second-order processes in quadratic molecular media at the origin of very large cubic effects

We have shown that very large nonlinear shifts ranging from π/4 to π/2 observed at relatively modest input intensities in single-crystal cored fibers of 4-(N,N-dimethylamino)-3-acetamidonitrobenzene (DAN) have their origin in the cascading of two second-order nonlinear processes: second-harmonic generation followed by down-frequency conversion. The nonlinear shifts and corresponding n2 values extracted from independent observationsof self-phase modulation and direct interferometric measurements are as large as 10-12 cm2/W at relatively modest input intensities from 30 to 60 MW/cm2. The cascading origin of large n2 values was fully confirmed by z-scan measurements performed on a single DAN crystal. © 1994.