Fig. 2: Propagation of variable-group velocity optical wave-packet.
From: Optical wave-packet with nearly-programmable group velocities
a After the ideal thin axicon, in 2D lateral plane the divided two half beams have symmetrical propagation directions (green dash-line arrows) with an angle of 2α. When deformed pulse-front (red−yellow distributions) has an axisymmetric cosine-function-like distribution, longitudinal gap Δz between the intersection of phase-fronts (white solid lines) and optical wave-packet (core of red-yellow distributions) increases and decreases in the first- and second-half propagations, showing subluminal and superluminal group velocities, respectively. b Isosurface plots (10 and 80% of the maximum intensity) show dynamics of optical wave-packet in the entire pulsed beam at different propagating times of −100 ps, −50 ps and 0 in (a). c When deformed pulse-front has an opposite axisymmetric cosine-function-like distribution, longitudinal gap Δz between the intersection of phase-fronts and optical wave-packet decreases and increases in the first- and second-half propagations, showing superluminal and subluminal group velocities, respectively. The (half) conical angle generated by the axicon is α = 0.5°, and then the intersection of phase-fronts travels at a constant velocity of 1.00004c.
