Subluminal velocity of OAM-carrying beam

TL;DR

This paper demonstrates that Laguerre-Gauss beams carrying orbital angular momentum travel at subluminal group velocities in free space, with higher OAM values resulting in slower propagation, affecting information transmission and astronomical observations.

Contribution

It provides the first calculation and observation of OAM-dependent group velocity reduction in free space beams, highlighting a new dispersion effect linked to orbital angular momentum.

Findings

Higher OAM leads to slower group velocity in free space.

Group velocity reduction depends on both OAM and beam divergence.

OAM causes temporal separation of light pulses during propagation.

Abstract

We report a consequence of the orbital angular momentum (OAM) of a beam to its group velocity. We calculate the group velocity $v_g$ of Laguerre-Gauss beam ($\emph{LG}$) with $\ell$ and at $p=0$. The $v_g$ reduction of $\emph{LG}$ beam even in free space is observed to have dependence on both orbital or winding number $\ell$ and the beam's divergence $\theta_0$. We found that light possessing higher $\ell$ travels relatively slower than that with lower $\ell$ values. This suggests that light of different OAM separate in the temporal domain along propagation and it is an added effect to the dispersion due to field confinement. Our results are useful for treating information embedded in light with OAM from astronomical sources and/or data transmission in free space.