Electromagnetic nonlinear X-waves
  Nonlinear optical media that are normally dispersive, support a new type of localized (nondiffractive and nondispersive) wavepackets 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. 
C. Conti, S. Trillo, P. Di Trapani, G. Valiulis, O. Jedrkiewicz, J.   Trull