Gaussian pulsed beams with arbitrary speed.
It is shown that the homogeneous scalar wave equation under a generalized paraxial approximation admits of Gaussian beam solutions that can propagate with an arbitrary speed, either subluminal or superluminal, in free-space. In suitable moving inertial reference frames, such solutions correspond either to standard stationary Gaussian beams or to "temporal" diffracting Gaussian fields.
Longhi
Stefano
S