Solitary Waves and Ring-Formation in Polydiacetylene para-Toluene Sulfonate

Spatial solitary waves propagate in a nonlinear medium by balancing diffraction with a self-focusing nonlinearity. These waves are of fundamental interest, but are also of technological interest if they are stable under propagation. For diffraction in one transverse dimension (ID) a Kerr nonlinearity is sufficient to form stable beams.[1] However, a Kerr nonlinearity does not lead to stable self-trapping in two transverse dimensions (2D).[2] Methods proposed for stabilizing 2D beams have included saturating mechanisms and quintic nonlinearities. [3,4] Until recently, self-trapped beams had only been observed in vapor systems.[5] Current progress in cascaded second-order nonlinear systems has also demonstrated self-trapped beams, which tend to mimic a saturating nonlinearity. [6] However, our recent measurements indicated that polydiacetylene para-toluene sulfonate (PTS) has n2>0 and n3<0 with low linear loss and negligible nonlinear loss at 1600 nm.[7] We now report preliminary experimental demonstrations of 2D spatial solitary waves and ring formation in PTS. The results are supported by numerical simulations of beam propagation in PTS, using measured values for n2 and n3, interpreted in the context of the variational model of nonlinear Gaussian beam propagation. These results confirm, for the first time, the existence of the combination of a third-order and a fifth-order nonlinearity, independent of saturation, in a solid-state material, and the ability of this material to support stable self-trapped beams.