Mode Conversion in Optical Beam in Order to Create Funnel Shaped Optical near Field to Collect Cold Rb Atoms from Mot

Doughnut shaped light beam has been generated from Gaussian mode (TEM00) cw-Ti sapphire laser. After splitting the pump beam into two equal intensity components and introducing unequal convergence and phase delay while they are recombined it results in doughnut mode. Such a beam is tunable and have long propagation length. The evanescent field generated by 360 mW (at 780 nm wavelength) of such a beam creates optical field of 600 nm decay length with a 5.75 neV repulsive dipole potential. Thus cold Rb atoms (at 10μK or less temperature) released from MOT can be reflected by the surface so that the atoms are collected ultimately at the bottom of the prism. By focussing such doughnut beam with 8 cm focal length converging lens, the dark radius reduces to 22μ. We also observe such beam to contain azimuthal phase as well as radial phase distribution. PACS numbers: 42.50.Vk, 42.60.Jf, 32.80.Ys