In recent years, there has been an increasing interest in the fabrication of optical fibres with micrometer and nanometer order diameters. Such fibres can be produced by heating and pulling a single-mode optical fibre to a very small diameter, d, ensuring that the adiabatic criteria for lossless transmission are satisfied. By decreasing d to the submicron range, the taper transition transforms the local fundamental mode from a core mode in the untapered region to a cladding mode in the taper waist, giving rise to a guided mode with a significant evanescent field outside the tapered area of the fibre. Such tapered fibres have been shown to be particularly suited for atom optics experiments, such as modal interferometers, spectral filters and sensors. A novel approach to trapping and guiding neutral atoms has been proposed by Balykin et al. [1]. In contrast to optical waveguides with relatively large diameters, many theoretical and experimental aspects of tapered fibres have not been investigated thoroughly and this is essential in order to achieve goals such as those proposed in [1]. We are currently studying the interaction between cold rubidium atoms in a magneto-optical trap (MOT) and the evanescent field of a sub-micron fibre. The fibre taper is fabricated from 780 nm single mode fibre using a heat-and-pull rig with a CO{sub}2 laser heat source [2]. The fibre is mounted in the UHV chamber using a Teflon feedthrough and the tapered region of the fibre is positioned using a nanometer precision motor (Newscale Technologies UHV squiggle motor) to ensure it passes through the centre of the cloud of atoms. The cloud contains approximately 10{sup}7 atoms at a temperature of 100 μK. A probe beam, modulated ± 15 MHz across the absorption line of 85{sup left}Rb using an AOM in a double-pass configuration, is coupled into the fibre. The transmission through the fibre is monitored by a single photon counting module (SPCM). By recording the number of counts detected by the SPCM with respect to the observation time, any expected changes in the evanescent field due to scattering, absorption and coupling effects can be monitored for different experimental conditions.
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