Subluminal velocity of OAM-carrying beam

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.