Optical solitons supported by competing nonlinearities.

It is demonstrated that optical solitons can propagate in a dispersive (or diffractive) medium with competing quadratic [i.e., chi((2))] and cubic [i.e., chi((3))] nonlinearities. Strong interplay between the nonlinearities leads to novel effects, in particular the following: (i) stable bright solitons can still exist in a self-defocusing (owing to cubic nonlinearity) medium supported by quadratic parametric interactions and (ii) chi((2)) nonlinearity can lead to instabilities of chi((3)) solitons.