Nonlinear optical media that are normally dispersive support a new type of localized (nondiffractive and nondispersive) wave packets that are X shaped in space and time and have slower than exponential decay. High-intensity X waves, unlike linear ones, can be formed spontaneously through a trigger mechanism of conical emission, thus playing an important role in experiments.
Nonlinear electromagnetic X waves / Conti, Claudio; S., Trillo; P., Di Trapani; G., Valiulis; A., Piskarskas; O., Jedrkiewicz; J., Trull. - In: PHYSICAL REVIEW LETTERS. - ISSN 0031-9007. - STAMPA. - 90:17(2003), p. 170406. [10.1103/physrevlett.90.170406]