Sign of chi((3)) in polysilane polymers.

The signs of the third-order optical nonlinear susceptibility chi((3)) of polysilane have been determined to be negative at 532 nm and positive at 1064 nm from single-beam Z-scan measurements. The small and positive chi((3)) at 1064 nm arises from the bound-electronic process. The negative chi((3)) at 532 nm is attributed to two-photon absorption resonance transition.