Zl. Horvath et al., ACCELERATION OF FEMTOSECOND PULSES TO SUPERLUMINAL VELOCITIES BY GOUYPHASE-SHIFT, Applied physics. B, Lasers and optics, 63(5), 1996, pp. 481-484
The phase and the group velocities are calculated in a three-dimension
al neighbourhood of the focus of an aberration-free lens illuminated b
y a spatially Gaussian beam. The Gouy phase shift caused by the diffra
ction results in superluminal pulse propagation on the optical axis wi
thin the Rayleigh range.