Mary Frawley

Spectroscopy, Manipulation and Trapping of Neutral Atoms, Molecules, and Other Particles Using Optical Nanofibers: A Review

The use of tapered optical fibers, i.e., optical nanofibers, for spectroscopy and the detection of small numbers of particles, such as neutral atoms or molecules, has been gaining interest in recent years. In this review, we briefly introduce the optical nanofiber, its fabrication, and optical mode propagation within. We discuss recent progress on the integration of optical nanofibers into laser-cooled atom and vapor systems, paying particular attention to spectroscopy, cold atom cloud characterization, and optical trapping schemes. Next, a natural extension of this work to molecules is introduced. Finally, we consider several alternatives to optical nanofibers that display some advantages for specific applications.

Selective particle trapping and optical binding in the evanescent field of an optical nanofiber.

The evanescent field of an optical nanofiber presents a versatile interface for the manipulation of micron-scale particles in dispersion. Here, we present a detailed study of the optical binding interactions of a pair of 3.13 μm SiO(2) spheres in the nanofiber evanescent field. Preferred equilibrium positions for the spheres as a function of nanofiber diameter and sphere size are discussed. We demonstrated optical propulsion and self-arrangement of chains of one to seven 3.13 μm SiO(2) particles; this effect is associated with optical binding via simulated trends of multiple scattering effects. Incorporating an optical nanofiber into an optical tweezers setup facilitated the individual and collective introduction of selected particles to the nanofiber evanescent field for experiments. Computational simulations provide insight into the dynamics behind the observed behavior.

Interaction of laser-cooled 87Rb atoms with higher order modes of an optical nanofibre

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 87Rb 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.

TAPERED FEW-MODE FIBERS: MODE EVOLUTION DURING FABRICATION AND ADIABATICITY

In this paper, the evolution of the LP11 mode during the fabrication of tapered optical fibers is examined. General trends in the modal coupling are inferred from probe beam transmission plots. Alternative tapered fiber shapes are proposed for the creation of an efficient few-mode fiber for different wavelengths, and core and cladding radii.

The van der Waals interaction of an atom with the convex surface of a nanocylinder

In this work, the van der Waals interaction between an atom and the convex surface of both metal and dielectric nanocylinders is analyzed. We obtain closed analytical equations for the van der Waals interaction potential in the electrostatic approximation and compare this with the case of the atom interaction with a flat surface. We show that the strength of the van der Waals interaction for the nanocylinder differs significantly from that of the flat surface. This is especially visible at distances of the order of a cylinder radius from the surface, where the nanocylinder potential reduces to half that of the flat surface potential. We also note that a weakening of the van der Waals potential for a cylindrical surface as compared with that for a flat surface can be important when considering schemes for trapping atoms around nanofibers.

Optical nanofiber integrated into an optical tweezers for particle manipulation and in-situ fiber probing

Precise control of particle positioning is desirable in many optical propulsion and sorting applications. Here, we develop an integrated platform for particle manipulation consisting of a combined optical nanofiber and optical tweezers system. Individual silica microspheres were introduced to the nanofiber at arbitrary points using the optical tweezers, thereby producing pronounced dips in the fiber transmission. We show that such consistent and reversible transmission modulations depend on both particle and fiber diameter, and may be used as a reference point for in-situ nanofiber or particle size measurement. Therefore we combine SEM size measurements with nanofiber transmission data to provide calibration for particle-based fiber assessment. We also demonstrate how the optical tweezers can be used to create a ‘particle jet’ to feed a supply of microspheres to the nanofiber surface, forming a particle conveyor belt. This integrated optical platform provides a method for selective evanescent field manipulation of micron-sized particles and facilitates studies of optical binding and light-particle interaction dynamics.

Manipulation of self-arranged dielectric particles using optical nanofibers

Propulsion properties of single and chains of particles with well-separated distance between particles in the evanescent field surrounding a nanofiber were studied. Interference of counter-propagating beams is demonstrated, useful for directional propulsion and positional control.

Optical Nanofibers: Microsphere Manipulation and Self-Assembly, and Higher Order Mode Generation

We study the self-assembly phenomenon of a chain of particles in the evanescent field of a nanofiber. We investigate the generation of higher order modes in two-mode fiber using a spatial light modulator.

Higher Order Modes Propagation in Optical Nanofibers

We theoretically study and experimentally demonstrate LP11 mode conservation in nanofibers. We selectively remove the HE21 mode by limiting the minimal waist radius, and optimize tapering to incorporate nanofibers in cold atom and manipulation experiments

Optical Nanofibres: Tools for Particle Propulsion and Manipulation

We study the optimisation of optical nanofiber parameters for particle propulsion, and investigate the null taper coupler as a method of selectively generating higher modes in the nanofiber waist for trapping purposes