The experimental demonstration in 1995 of spatial soliton formation mediated by parametric wave interactions in crystals with quadratic nonlinearities, which had been theoretically predicted in the 1970s, opened a range of exciting new opportunities in the field of soliton science and its applications. For many years, second-order (or quadratic) nonlinearities had been associated only with the frequency conversion of laser light. A crucial step towards richer opportunities was the emergence of cascading, where cross-induced energy- and phase-shifts acquired by multiple light waves that parametrically interact in a material with a quadratic nonlinearity are exploited to perform all-optical operations on signals. To form a soliton, or in other words a self-sustained, localized, non-spreading light packet, such energy- and phase-shifts can also be used to dynamically counteract spreading caused by diffraction and by group-velocity dispersion.
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