X-waves Generated at Second Harmonic
  The process of optical frequency doubling can lead, in the undepleted regime, to the generation of a X-wave envelope with group velocity locked to the pump beam. Its parameters and its angular spectrum, are directly related to the zero- and first-order dispersive features of the nonlinear process. This constitutes a novel mechanism for spatio-temporal localization of light. 
C. Conti and S. Trillo