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

We present and analyse a novel concept for blue-detuned evanescent field surface traps for cold neutral atoms based on two-mode interference in ultrathin optical fibres. When two or more transverse modes with the same frequency co-propagate 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 surface can be created and an array of optical microtraps can thus be obtained around the fibre. We discuss three possible combinations of the lowest order modes, yielding traps at 100-200 nm from the fibre surface which, using a few tens of milliwatts of trapping laser power, have a depth of the order of 1 mK for caesium atoms and a trapping lifetime exceeding 100 s. 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 realizing ensembles of fibre-coupled atoms.