Quadratic spatial solitons have proven to be a very interesting soliton system. The guiding mechanisms have to do with photon exchange between multiple waves at two or more frequencies. This results in the locking together of the waves, and mutual self-trapping, even in the presence of group velocity walk-off of the individual waves. The self-trapping mechanism leads to pulse compression and quasi-optical bullets have been The last picture shows a 3D scan of the output three solitons. The collision behavior of quadratic solitons is similar to that found in saturating Kerr media. It is possible, however, to use simple approximate coupled mode theory to predict the nature of the interaction for different relative phase angles between the solitons. The predictions obtained in this way are in good agreement with the experiments. Modulational instability, and instabilities in general, are an interesting and fruitful area of quadratic soliton research. In keeping with expectations linking the existence of solitons to modulational instabilities in the general field of solitons, MI exists in quadratically nonlinear media. MI has been investigated in 1D (waveguides) and 2D (bulk) media. Of these, it proved possible to do a direct comparison between analytical theory and experiment in the 1D case and excellent agreement was obtained. Furthermore, by seeding with an interference pattern, it proved feasible to measure directly the exponential gain coefficients. Another instability that has been investigated is the break up of a bright beam by nesting a vortex inside it. At high enough powers, the broad beam broke up into discrete solitons.
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