Abstract
The propagation of a Laguerre-Gaussian (LG) beam through a dispersive medium is investigated. The effect of the doughnut-like intensity profile of the probe LG beam on the group velocity is studied. We find an analytical expression for the group velocity out of the optical axis and compare with its projection onto the propagation axis. It is turned out that the group velocity vector is along the optical axis at the waist of the beam and the Rayleigh range. We numerically and analytically investigate the effect of the helical phase front of the coupling LG field on the group velocity of the probe LG field in a four-level double V-type quantum system. Our analysis predicts a strange behavior for the group velocity of the probe LG beam inside a normal dispersive medium in the gain region so that it can exceed the speed of light in free space, leads to the gain-assisted superluminal light propagation in normal dispersion. Such an unusual propagation of the LG beam results from the distortion of its helical phase front via the classical interference of the planar and LG fields. The obtained results may find some potential applications in increasing the velocity of the information transmission in optical communications.
Highlights
A wide variety of theoretical and experimental studies have recently been conducted on the group velocity of structured light fields propagating in free space
It was demonstrated that the addition of orbital angular momentum (OAM) of the structured beam reduces the intrinsic delay of twisted photons with respect to the same beam without OAM [2]
It is noted that the group velocity reduction for the Bessel light beam near the critical frequency has been reported in Ref. [3]
Summary
A wide variety of theoretical and experimental studies have recently been conducted on the group velocity of structured light fields propagating in free space. It was observed that the group velocity of structured single photons is slower in both Bessel and focused Gaussian beams along the propagation axis than the speed of light in free space c [1]. In studying the OAM transfer, we have reported the preliminary results of the group velocity of the probe LG field in a dispersive medium [28]. It is shown that the superluminal (subluminal) group velocity with normal (anomalous) dispersion appears in some regions of the medium Such a strange result is the effect of the classical interference of the coupling planar and LG fields on the helical phase front of the probe LG field, which has not yet been reported before in a dispersive medium
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