Unique properties of quadratic solitons

Quadratic spatial solitons exist in media with second order nonlinearities near the phase-matching condition for frequency mixing processes involving two or three waves of different frequency. Discussed here are a number of p roperties of these special solitons which are different from those of other spatial solitons which rely on optically induced index changes for guiding . First, the self-guiding properties of quadratic solitons are shown to hav e completely different origins than solitons which rely on index changes. S econd, it is shown that there exists a large variety of quadratic solitons which contain two or three distinct spectral components with relative ampli tudes depending on the phase mismatch, dimensionality of the propagation ge ometry, the soliton power and the launching conditions. Third, under approp riate conditions, solitons can be formed even when the group velocity direc tions for the spectral components lead to walk-off under normal circumstanc es. Fourth, for type II phase-matching in bulk crystals, seeded interaction s lead to saturating amplifier characteristics.