The speed of light in a perfect vacuum – a well-known fact, a fundamental constant, a universal truth. This law has been a major principle of modern physics and in no small part does it define our understanding of the universe.

Using a certain type of beam ai??i?? the Laguerre-Gauss (LG) beam, which carries orbital angular momentum (OAM), the propagation of light was shown to be able to be ai???slowedai??? down without passing it through some material. The effect of ai???slowing downai??? the light beam was purely done by changing its structure. This was accomplished by taking advantage of the OAM of the LG beam such that the beam ai???twistsai??? along its propagation axis (imagine the light beam following a path like a corkscrew), therefore effectively making it travel a longer path compared to a beam which travels straight. This shows that even though the LG beam as a whole travels slower than a typical beam of light, the individual photons which make it up still travel at the same fundamental constant ** c **which we are all familiar with.

The orbital angular momentum of light is a property which defines the helical phase pattern of the wavefront. Put simply, OAM describes the amount of ai???twistai??? that the beam has. It describes the degree of rotation of the beam around the axis on which it propagates. This twisting of the light beam allows more information to be encoded on the same number of photons because of the additional degrees of freedom it possesses. This result has possible applications in free space data transmission since a much larger amount of data can be embedded in the beams. A consequence, however, of the twisted LG beam is that though more information can be carried, these information will not arrive at the same time and thus is needed to be further processed with some corrections.

**Abstract:**

That the speed of light in free space *c* is constant has been a pillar of modern physics since the derivation of Maxwell and in Einsteinai??i??s postulate in special relativity. This has been a basic assumption in lightai??i??s various applications. However, a physical beam of light has a finite extent such that even in free space it is by nature dispersive. The field confinement changes its wavevector, hence, altering the lightai??i??s group velocity *v*_{g}. Here, we report the subluminal *v*_{g} and consequently the dispersion in free space of Laguerre-Gauss (*LG*) beam, a beam known to carry orbital angular momentum. The *v*_{g} of *LG* beam, calculated in the paraxial regime, is observed to be inversely proportional to the beamai??i??s divergence *I?*_{0}, the orbital order ai??i?? and the radial order *p*. *LG* beams of higher orders travel relatively slower than that of lower orders. As a consequence, *LG* beams of different orders separate in the temporal domain along propagation. This is an added effect to the dispersion due to field confinement. Our results are useful for treating information embedded in *LG* beams from astronomical sources and/or data transmission in free space.

**Source:**

Bareza, N. D. and Hermosa, N. Subluminal group velocity and dispersion of Laguerre Gauss beams in free space. *Sci. Rep*. 6, 26842; doi: 10.1038/srep26842 (2016).