Slowing of Bessel light beam group velocity

Peeter Saari

R. R. Alfano, Daniel A. Nolan

B. Macke, B. S�gard

Daniel Giovannini, Jacquiline Romero, Václav Potoček +5 more

That the speed of light in free space is constant is a cornerstone of modern physics. However, light beams have finite transverse size, which leads to a modification of their wave vectors resulting in a change to their phase and group velocities. We study the group velocity of single photons by measuring a change in their arrival time that results from changing the beam's transverse spatial structure. Using time-correlated photon pairs, we show a reduction in the group velocity of photons in both a Bessel beam and photons in a focused Gaussian beam. In both cases, the delay is several micrometers over a propagation distance of ~1 meter. Our work highlights that, even in free space, the invariance of the speed of light only applies to plane waves.

Wang Li-Rong, Yanting Zhao, Jie Ma +3 more

Slowing a light pulse in a degenerate two-level system is observed with a double-frequency sweeping technique. The effects of coupling beam intensity, cell temperature and frequency detunings of the coupling and probe beams in resonance, on the slowing of light propagation in such a system are investigated. It is found that group velocities depend strongly on polarization combinations. A group velocity v g =6760m/s of light pulses in caesium vapour is obtained under the optimal parameters.

Daniela Mugnai