Subluminal group velocity and dispersion of Laguerre Gauss beams in free space

• Published in: Scientific reports 2016-05, Vol.6 (1), p.26842-26842, Article 26842
• Main Authors: Bareza, Nestor D., Hermosa, Nathaniel
• Format: Article
• Language: English
• Subjects: 639/624/400; 639/766/400; 639/766/400/1102; Humanities and Social Sciences; multidisciplinary; Science
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/srep26842

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 Einstein’s postulate in special relativity. This has been a basic assumption in light’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 light’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 beam’s divergence θ 0 , the orbital order ℓ 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.