We predict that a photonic crystal fiber whose strands are filled with a defocusing nonlinear medium can support stable bright solitons and also vortex solitons if the strength of the defocusing nonlinearity grows toward the periphery of the fiber. The domains of soliton existence depend on the transverse growth rate of the filling nonlinearity and nonlinearity of the core. Remarkably, solitons exist even when the core material is linear.
Lobanov, V., Borovkova, O., Kartashov, Y.V., Malomed, B., Torner, L. Stable bright and vortex solitons in photonic crystal fibers with inhomogeneous defocusing nonlinearity. "Optics letters", 1 Juny 2012, vol. 37, núm. 11, p. 1799-1801.