Blue-detuned evanescent field surface traps for neutral atoms based on mode interference in ultra-thin optical fibres

We present and analyze a novel concept for blue-detuned evanescent field surface traps for cold neutral atoms based on two-mode interference in ultra-thin optical fibres. When two or more transverse modes with the same frequency copropagate in the fibre, their different phase velocities cause a stationary interference pattern to establish. Intensity minima of the evanescent field at any distance from the fibre surface can be created and an array of optical microtraps can thus be obtained in the evanescent field. We discuss three possible combinations of the lowest order modes, yielding traps at one to two hundred nanometres from the fibre surface which, using a few ten milliwatts of trapping laser power, have a depth on the order of 1 mK for caesium atoms and a trapping lifetime exceeding 100 seconds. The resulting trapping geometry is of particular interest because atoms in such microtrap arrays will be coupled to any additional field propagating in the fibre via the evanescent field, thereby realising ensembles of fibre-coupled atoms. PACS numbers: 37.10.Gh, 42.50.-p, 42.25.Hz, 34.35.+a Submitted to: New J. Phys.