Mark Daly

Sub-Doppler temperature measurements of laser-cooled atoms using optical nanofibres

We present a method for measuring the average temperature of a cloud of cold 85Rb atoms in a magneto-optical trap using an optical nanofibre. A periodic spatial variation is applied to the magnetic fields generated by the trapping coils and this causes the trap centre to oscillate, which, in turn, causes the cloud of cold atoms to oscillate. The optical nanofibre is used to collect the fluorescence emitted by the cold atoms, and the frequency response between the motion of the centre of the oscillating trap and the cloud of atoms is determined. This allows us to make measurements of cloud temperature both above and below the Doppler limit, thereby paving the way for nanofibres to be integrated with ultracold atoms for hybrid quantum devices.

Nanostructured optical nanofibres for atom trapping

We propose an optical dipole trap for cold, neutral atoms based on the electric field produced from the evanescent fields in a hollow, rectangular slot cut through an optical nanofibre. In particular, we discuss the trap performance in relation to laser-cooled rubidium atoms and show that a far off-resonance, blue-detuned field combined with the attractive surface-atom interaction potential from the dielectric material forms a stable trapping configuration. With the addition of a red-detuned field, we demonstrate how three dimensional confinement of the atoms at a distance of 140–200 nm from the fibre surface within the slot can be accomplished. This scheme facilitates optical coupling between the atoms and the nanofibre that could be exploited for quantum communication schemes using ensembles of laser-cooled atoms.

Evanescent field trapping of nanoparticles using nanostructured ultrathin optical fibers.

While conventional optical trapping techniques can trap objects with submicron dimensions, the underlying limits imposed by the diffraction of light generally restrict their use to larger or higher refractive index particles. As the index and diameter decrease, the trapping difficulty rapidly increases; hence, the power requirements for stable trapping become so large as to quickly denature the trapped objects in such diffraction-limited systems. Here, we present an evanescent field-based device capable of confining low index nanoscale particles using modest optical powers as low as 1.2 mW, with additional applications in the field of cold atom trapping. Our experiment uses a nanostructured optical micro-nanofiber to trap 200 nm, low index contrast, fluorescent particles within the structured region, thereby overcoming diffraction limitations. We analyze the trapping potential of this device both experimentally and theoretically, and show how strong optical traps are achieved with low input powers.

Multiple particle trapping and self-organization in the evanescent fields of optical micro- and nanofibres

Evanescent fields in optical micro- and nanofibres (MNF) have been proposed as efficient means for multiple microparticle trapping and prolusion. Here, we demonstrate the propulsion of 3 μm polystyrene particle chains in the evanescent fields of the fundamental and first higher order modes in an MNF system. The power at the waist was estimated to be 25mW in both cases Self-assembly and speed variation of particle chains was observed. This observation implies that strong optical interactions between trapped particles within a particle chain occur along the fibre waist region The effect is associated with the long-range, one dimensional optical binding. The numerical finite element and analytical scattering-matrix approaches were used to investigate the field dynamics surrounding the trapped particles. The optical forces and velocities of bound particle chains were calculated using these particle-field dynamics. Both the analytical and numerical analyses show good agreements with the experimental data The observation reveal that higher order modes in a microfibre offer stable multiple particle trapping faster particle propulsion speeds, and allow for the capability to better control each individual trapped object in particle ensembles near the microfibre surface compare to the fundamental mode. In order to trap the particles with diameters down to nanometer sizes, we used a nanostructued optical microfibre to confine the evanescent fields, and hence, trap 200 nm fluorescent polystyrene particles within the nanostructured region. The optical forces and trapping potential of this nanostructured microfibre were theoretically and experimentally analyzed. The observed results show that strong optical nanoparticle traps can be achieved with incident laser powers as low as 1.2mW. This work can be extended for future studies on the trapping and precise control of long-range complex micro- and nanobiological samples in aqueous solutions in the vicinity of the evanescent field of optical micro- or nanofibres.

Submicron particle manipulation using slotted tapered optical fibers

The use of optical micro- and nanofibers has become commonplace in the areas of atom trapping using neutral atoms and, perhaps more relevantly, the optical trapping and propulsion of micro- and nanoscale particles. It has been shown that such fibers can be used to manipulate and trap silica and polystyrene particles in the 1-3 µm range using either the fundamental or higher order modes of the fibers, with the propulsion of smaller particle sizes also possible through the use of metallic and/or high index materials. We previously proposed using a focused ion beam nanostructured tapered optical fiber for improved atom trapping geometries; here, we present the details of how these nanostructured optical fibers can be used as a platform for submicron particle trapping. The optical fibers are tapered to approximately 1.2 µm waist diameters, using a custom-built, heat-and-pull fiber rig prior to processing using a focused ion beam. Slots of approximately 300 nm in width and 10-20 µm in length are milled clean though the waist regions of the tapered optical fibers. High fiber transmissions (> 80%) over a broad range of wavelengths (700-1100 nm) are observed. We present simulation results for the trapping of submicron particles and experimental results on the trapping of 200 nm particles. This work demonstrates even further the functionality of optical micro- and nanofibers as trapping devices across a range of regimes.

Nanostructured tapered optical fibers for particle trapping

Optical micro- and nanofibers have recently gained popularity as tools in quantum engineering using laser-cooled, neutral atoms. In particular, atoms can be trapped around such optical fibers, and photons coupled into the fibers from the surrounding atoms could be used to transfer quantum state information within the system. It has also been demonstrated that such fibers can be used to manipulate and trap silica and polystyrene particles in the 1-3 μm range. We recently proposed using a focused ion beam nanostructured tapered optical fiber for improved atom trapping geometries1. Here, we present details on the design and fabrication of these nanostructured optical fibers and their integration into particle trapping platforms for the demonstration of submicron particle trapping. The optical fibers are tapered to approximately 1-2 μm waist diameters, using a custom-built, heat-and-pull fiber rig, prior to processing using a focused ion beam. Slots of about 300 nm in width and 10-20 μm in length are milled right though the waist regions of the tapered optical fibers. Details on the fabrication steeps necessary to ensure high optical transmission though the fiber post processing are included. Fiber transmissions of over 80% over a broad range of wavelengths, in the 700-11100 nm range, are attainable. We also present simulation results on the impact of varying the slot parameters on the trap depths achievable and milling multiple traps within a single tapered fiber. This work demonstrates even further the functionality of optical micro- and nanofibers as trapping devices across a range of regimes.

Nanofiber optical interfaces for laser-cooled atoms

In this work we present results on the use of optical nanofibers of subwavelength diameter as interfaces for probing and manipulating laser-cooled rubidium atoms. In particular, we concentrate on photon counts emitted through the nanofiber.