Internal oscillations and instability characteristics of (2+1)-dimensional solitons in a saturable nonlinear medium.

Two related problems on scalar (2+1)-dimensional solitons in a saturable nonlinear medium are investigated. The first one is the internal oscillations of fundamental (single-hump) solitons. Internal modes which cause these oscillations, both with and without angular dependence, are discovered. The visual effect of angle-dependent internal modes on the soliton can be a rotation or spatially uneven breathing of the perturbed soliton. These internal oscillations are very robust and persist for a very long distance. The second problem is the instability of double-hump and radially symmetric solitons. Unstable eigenmodes of these solitons are presented. Contrary to intuition, the instability growth rates decrease to zero when the soliton power becomes high. Thus the instability is strongly suppressed at high powers. This phenomenon is corroborated by our direct numerical simulations.