A radically new way to think of the problem of multi-dimensional self-trapping has come to light in quadratic nonlinear in teractions. In this article, the authors not only show that the diffraction in spatial walkoff can be defeated, but that a genuine game of billiards can be played with strongly coupled fundamental and harmonic fields
Torruellas, W., Torner, L. A game of billiards with spatial solitary waves in ktp. "Optics and photonics news", Març 1996, vol. 7, núm. 3, p. 34-52.