Embedded Solitons in Lagrangian and Semi-Lagrangian Systems

TL;DR

This paper extends the variational approximation technique to semi-Lagrangian systems and introduces an analytical criterion for identifying embedded solitons within the continuous spectrum, validated on a second-harmonic-generation model.

Contribution

It develops a modified variational approximation for semi-Lagrangian systems and proposes a new orthogonality-based criterion for embedded solitons, with practical validation.

Findings

The VA can be applied to semi-Lagrangian models with modifications.

The criterion accurately predicts embedded solitons with about 1% error.

Exact solutions for embedded solitons are obtained in simplified models.

Abstract

We develop the technique of the variational approximation for solitons in two directions. First, one may have a physical model which does not admit the usual Lagrangian representation, as some terms can be discarded for various reasons. For instance, the second-harmonic-generation (SHG) model considered here, which includes the Kerr nonlinearity, lacks the usual Lagrangian representation if one ignores the Kerr nonlinearity of the second harmonic, as compared to that of the fundamental. However, we show that, with a natural modification, one may still apply the variational approximation (VA) to those seemingly flawed systems as efficiently as it applies to their fully Lagrangian counterparts. We call such models, that do not admit the usual Lagrangian representation, \textit{semi-Lagrangian} systems. Second, we show that, upon adding an infinitesimal tail that does not vanish at infinity, to a usual soliton ansatz, one can obtain an analytical criterion which (within the framework of VA) gives a condition for finding \textit{embedded solitons}, i.e., isolated truly localized solutions existing inside the continuous spectrum of the radiation modes. The criterion takes a form of orthogonality of the radiation mode in the infinite tail to the soliton core. To test the criterion, we have applied it to both the semi-Lagrangian truncated version of the SHG model and to the same model in its full form. In the former case, the criterion (combined with VA for the soliton proper) yields an \emph{exact} solution for the embedded soliton. In the latter case, the criterion selects the embedded soliton with a relative error $\approx 1%$.