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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Atomic absorption from the evanescent field of a sub-micron fibre taper

Michael Morrissey, Kieran Deasy, Thejesh Bandi, Brian Shortt, Síle Nic Chormaic

Published Jun 1, 2007• 1 citation
Publisher

Abstract

In this paper, the interaction between cold rubidium atoms in a magneto-optical trap (MOT) and the evanescent field of a sub-micron fibre is studied. The fibre taper is fabricated from 780 nm single mode fibre using a heat-and-pull rig with a CO2laser heat source.
DOI
10.1109/cleoe-iqec.2007.4386696

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This thesis describes the construction and characterisation of a standard six-beam configuration magneto-optical trap (MOT) used to cool and trap neutral 85Rb atoms. This system is capable of producing a cloud of ~108 atoms whose average temperature is in the sub-Doppler range. The fabrication process for the production of subwavelength tapered optical nanofibres (TONFs) is also described, as well as their guiding properties. These have many proposed applications in the field of cold atoms. However, significant technical progress must be made before many of these applications can be realised. The objective of the research work was to combine the TONF with the cold atom setup to develop tapered optical nanofibres as a tool that can be used in cold atom experiments. This thesis presents a novel technique to measure the characteristics of a MOT by monitoring the spontaneous emission from the trapped atoms coupled into the guided mode of the TONF. This has allowed the measurement of cloud profile, loading times and decay times. This serves as a demonstration that TONFs can indeed play an important role in cold atom physics. The work presented here also describes a theoretical model for an atom diffraction grating based on oscillating magnetic fields produced by an array of current carrying wires.
"In this thesis, laser cooling and trapping techniques are implemented to forrn a cold atom cloud of 85Rb atoms in a magneto-optical trap (Mon configuration. The system produces an atom cloud of~ 1.5 mm in diameter, containing ~ 108 atoms and an average temperature in the sub-Doppler range. Tapered optical fibres (TOFs) are also detailed here, including their guiding properties and the process used for their fabrication. Many possible applications for TOFs have been proposed and one such application is investigated, i.e. the use of TOFs as highly sensitive atom probes. The research results contained within this thesis describe two techniques whereby TOFs can be used as probes. By overlapping the TOF and a cold atom cloud spontaneously emitted photons from the trapped atoms are coupled into the guided mode of the TOF. This then allows characteristics of the cold atom cloud, such as the cloud profile, loading time, lifetime and temperature to be determined. The second technique uses light guided by, but external to, the fibre, i.e. within the evanescent field, to probe the surrounding environment, which may be a cold atom cloud or a background vapour. The work presented also describes a theoretical basis for quantum state preparation using multi-level-atom optics, allowing for the creation of spatial superposition states in which one has perfect control over the population distribution and also the phase relation."
Optical nanofibres (ONFs) are very thin optical waveguides with sub-wavelength diameters. ONFs have very high evanescent fields and the guided light is confined strongly in the transverse direction. These fibres can be used to achieve strong light-matter interactions. Atoms around the waist of an ONF can be probed by collecting the atomic fluorescence coupling or by measuring the transmission (or the polarisation) of the probe beam sent through it. This thesis presents experiments using ONFs for probing and manipulating laser-cooled 87Rb atoms. As an initial experiment, a single mode ONF was integrated into a magneto-optical trap (MOT) and used for measuring the characteristics of the MOT, such as the loading time and the average temperature of the atom cloud. The effect of a near-resonant probe beam on the local temperature of the cold atoms has been studied. Next, the ONF was used for manipulating the atoms in the evanescent fields region in order to generate nonlinear optical effects. Four-wave mixing, ac Stark effect (Autler-Townes splitting) and electromagnetically induced transparency have been observed at unprecedented ultralow power levels. In another experiment, a few-mode ONF, supporting only the fundamental mode and the first higher order mode group, has been used for studying cold atoms. A higher pumping rate of the atomic fluorescence into the higher order fibreguided modes and more interactions with the surrounding atoms for higher order mode evanescent light, when compared to signals for the fundamental mode, have been identified. The results obtained in the thesis are particularly for a fundamental understanding of light-atom interactions when atoms are near a dielectric surface and also for the development of fibre-based quantum information technologies. Atoms coupled to ONFs could be used for preparing intrinsically fibre-coupled quantum nodes for quantum computing and the studies presented here are significant for a detailed understanding of such a system.
Optical nanofibres are used to confine light to sub-wavelength regions and are very promising tools for the development of optical fibre-based quantum networks using cold, neutral atoms. To date, experimental studies on atoms near nanofibres have focussed on fundamental fibre mode interactions. In this work, we demonstrate the integration of a few-mode optical nanofibre into a magneto-optical trap for 87 Rb atoms. The nanofibre, with a waist diameter of ∼700 nm, supports both the fundamental and first group of higher order modes (HOMs) and is used for atomic fluorescence and absorption studies. In general, light propagating in higher order fibre modes has a greater evanescent field extension around the waist in comparison with the fundamental mode. By exploiting this behaviour, we demonstrate that the detected signal of fluorescent photons emitted from a cloud of cold atoms centred at the nanofibre waist is larger if HOMs are also included. In particular, the signal from HOMs appears to be about six times larger than that obtained for the fundamental mode. Absorption of on-resonance, HOM probe light by the laser-cooled atoms is also observed. These advances should facilitate the realization of atom trapping schemes based on HOM interference.