Trapping of Light in Fibre Bragg Gratings

摘要

It is well known that the speed of light, c, is the absolute top speed in the universe at 3 x 10~8 m/s in vacuum. We further know that light can be slowed down to a modest extent in refractive and transparent media, for example water and glass, to velocities typically a factor of 1.5 to 2.0 times slower than c. But there is a limit to how much light can be slowed in normal optical materials, because the larger refractive index associated with slower propagation is inevitably accompanied by increased light absorption. Under special circumstances, however, this limit can be overcome. The first experiment in drastically reducing the speed of light was reported by a group of researchers from Harvard University and Stanford University led by Lene Hau. They load a magneto-optical trap with sodium atoms, and the gas is cooled briefly with a laser to reach temperatures of 50μK. With the lasers switched off, only atoms in the ground state with magnetic dipoles directed opposite to the magnetic field are confined by the magnetic trap. They then evaporatively cool the atoms in this trap to reach temperatures in the region of the Bose-Einstein condensation threshold T_c = 435nK. In these ultra-cold atoms, extremely narrow transparency dips due to quantum interference can be induced using very low powers of the 'coupling' laser beam. Accompanying this low absorption will be a very steep variation in the refractive index according to the Kramer-Kronig's rule. This steep slope and the high sample density in the trapped cloud of atoms lead to ultra-slow light propagation. The light speed they thus obtained is 20 millionth of the free space speed. Several authors wrote articles to explain this phenomenon.