Vector electromagnetic X waves.

A vector propagation scheme for describing electromagnetic nondiffracting beams (X waves) is introduced. In particular we show that, from the knowledge of the transverse field components on a given transverse plane and at a fixed instant, it is possible to predict the whole electric field everywhere which in particular allows us to investigate the imaging properties of nondiffracting beam. Furthermore, we show that the longitudinal field component crucially depends on the pulse velocity and that it can be neglected only if the velocity is slightly greater than c. The proposed formalism is tested by means of two examples, the vector fundamental and Gaussian X waves which admit analytical treatment. As an application of the propagation scheme, we derive in closed form the expressions for the field propagator showing that its transverse component formally coincides with one of the scalar fundamental X wave.