Interference of axially-shifted Laguerre–Gaussian beams and their interaction with atoms

Counter-propagating co-axial Laguerre–Gaussian (LG) beams are considered, not in the familiar scenario where the focal planes coincide at z = 0, but when they are separated by a finite axial distance d. The simplest case is where both beams are doughnut beams which have the same linear polarisation. The total fields of this system are shown to display novel amplitude and phase distributions and are shown to give rise to a ring or a finite ring lattice composed of double rings and single central ring. When the beams have slightly different frequencies the ring lattice pattern becomes a finite set of rotating Ferris wheels and the whole pattern also moves axially between the focal planes. We show that the fields of such an axially shifted pair of counter-propagating LG beams generate trapping potentials due to the dipole force which can trap two-level atoms in the components of the ring lattice. We also highlight a unique feature of this system which involves the creation of a new longitudinal optical atom trapping potential due to the scattering force which arises solely when $d\ne 0$. The results are illustrated using realistic parameters which also confirm the importance of the Gouy and curvature effects in determining the ring separation both radially and axially and gives rise to the possibility of atom tunnelling between components of the double rings

Chirality and helicity of linearly-polarised Laguerre-Gaussian beams of small beam waists

The chirality and helicity of a linearly polarised Laguerre-Gaussian (LG) beam are examined. Such a type of light possesses a large longitudinal field amplitude when it is created with a sufficiently small beam waist and so gives rise to substantial magnitudes of chirality and helicity density distributions. In the simplest case of a doughnut beam of winding number ℓ=1 and another identical to it but for which ℓ=−1, we obtain different chirality and helicity distributions in the focal plane z=0. We also show that this chiral behaviour persists and the patterns evolve so that on planes at z0 the beam convergence phase contributes differently to the changes in the chirality and helicity distributions

The zero helicity and chirality of optical vortices

We show that any uniformly linearly-polarised paraxial vortex mode carrying orbital angular momentum (OAM) has zero spin angular momentum (SAM) density, but exhibits non-zero helicity density distributions. Such a mode then possesses chirality as confirmed by experiment and so can engage with chiral matter. We show that confining the treatment for the general paraxial fields only to leading order leads directly to agreement of our theory with the experimental results, provided we ensure that crucially the paraxial fields obey duality. We find that the space integral of the helicity and chirality densities vanish identically for all such optical vortex modes without specifying the kind of mode. These generally applicable properties of optical vortex modes carrying orbital angular momentum thus assert that without optical spin due to elliptical wave polarisation of index σ, an optical vortex alone cannot possess total helicity, even though it always exhibits non-zero helicity density distributions

Chirality-enabled optical dipole potential energy for two-level atoms

We consider the optical dipole potential energy, which arises from the interaction of a two-level atom with a circularly polarized Laguerre-Gaussian laser beam of small waist. The beam is characterized by the existence of a longitudinal electric field component which is responsible for the appearance of a chiral term in the optical dipole potential energy. This term reverses sign if either the winding number or the wave polarization of the beam reverses sign. We propose a scheme of a bi-chromatic vortex interaction with the two-level atom in which the resulting optical dipole potential is fully chiral

An Integrated Routing and Rate Adaptation Framework for Multi-rate Multi-hop Wireless Networks

International audienceIn this paper, we propose a new integrated framework for joint routing and rate adaptation in multi-rate multi-hop wireless networks. Unlike many previous efforts, our framework considers several factors that affect end-to-end performance. Among these factors, the framework takes into account the effect of the relative positions of the links on a path when choosing the rates of operation and the importance of avoiding congested areas. The key element of our framework is a new comprehensive path metric that we call ETM (for expected transmission cost in multi-rate wireless networks). We analytically derive the ETM metric. We show that the ETM metric can be used to determine the best end-to-end path with a greedy routing approach. We also show that the metric can be used to dynamically select the best transmission rate for each link on the path via a dynamic programming approach. We implement the ETM-framework on an indoor wireless mesh network and compare its performance with that of frameworks based on the popular ETT and the recently proposed ETOP metrics. Our experiments demonstrate that the ETM-framework can yield throughput improvements of up to 253 and 368 % as compared with the ETT and ETOP frameworks

The super-chirality of vector twisted light

Vector vortex light of topological order m arises as a superposition of two twisted modes with phase functions e±imϕ (with ϕ the azimuthal variable) and circular polarizations (σ=∓1). We demonstrate that when m is sufficiently large these modes exhibit enhanced helicity densities when compared with the equivalent circularly-polarized Gaussian modes. The enhancement stems from the presence of longitudinal field components which become significant even for moderate beam widths. The super-chirality of light–matter interactions enabled by such modes suggests a high degree of enantioselectivity, surpassing conventional techniques for the chiral selection, so promising useful applications

Could Public Restrooms Be an Environment for Bacterial Resistomes?

PMCID: PMC3547874This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

Atoms in axially shifted tightly focused counter-propagating beams : The role of the Gouy and curvature phases

We consider the interaction of atoms with two tightly focused and axially shifted counter-propagating optical beams. At sub-wavelength focusing, we find that the scattering force potential in the three-dimensional space between the shifted focal planes changes from a feature with a saddle-point to a three-dimensional trapping potential. Further analysis shows that due to the tight focusing, the trapping depends on significant contributions arising from the Gouy and curvature phase gradients of the interfering beams. The physics and its effects are illustrated with reference to the sub-wavelength trapping of sodium atoms