  We show that an optical system involving competing higher-order Kerr
nonlinearities can support the existence of ultrasolitons, namely extremely
localized modes that only appear above a certain threshold for the central
intensity. Such new solitary waves can be produced for powers below the usual
collapse threshold, but they can also coexist with ordinary, lower-intensity
solitons. We derive analytical conditions for the occurrence of multistability
and analyze the dynamics of the different kinds of fundamental eigenmodes that
can be excited in these nonlinear systems. We also discuss the possible
transitions between solitary waves belonging to different nonlinear regimes
through the mechanism of soliton switching.
