Origin of the persistent oscillations of solitary waves in nonlinear quadratic media

We reveal the origin of the persistent oscillations of solitary waves in a quadratically nonlinear medium. It is found that the oscillations are closely correlated to a nontrivial discrete eigenmode of the corresponding linear eigenvalue problem. In addition to this discrete eigenmode a quasibound mode, weakly coupled to the continuous spectrum, is identified. This gives rise to a long-living beating of the solitary wave amplitude.