Optical Surface Vortices and Their Use in Nanoscale Manipulation

Following a brief overview of the physics underlying the interaction of twisted light with atoms at near-resonance frequencies, the essential ingredients of the interaction of atoms with surface optical vortices are described. It is shown that surface optical vortices can offer an unprecedented potential for the nanoscale manipulation of absorbed atoms congregating at regions of extremum light intensity on the surface

Optical Dipole Trapping beyond Rotating Wave Approximation: The case of Large Detuning

We show that the inclusion of counter-rotating terms, usually dropped in evaluations of interaction of an electric dipole of a two level atom with the electromagnetic field, leads to significant modifications of trapping potential in the case of large detuning. The results are shown to be in excellent numerical agreement with recent experimental findings, for the case of modes of Laguerre-Gauss spatial profile.Comment: 13 pages, 2 figure

Surface optical vortices

It is shown how the total internal reflection of orbital-angular-momentum-endowed light can lead to the generation of evanescent light possessing rotational properties in which the intensity distribution is firmly localized in the vicinity of the surface. The characteristics of these surface optical vortices depend on the form of the incident light and on the dielectric mismatch of the two media. The interference of surface optical vortices is shown to give rise to interesting phenomena, including pattern rotation akin to a surface optical Ferris wheel. Applications are envisaged to be in atom lithography, optical surface tweezers, and spanners

Graphene-like optical light field and its interaction with two-level atoms

The theoretical basis leading to the creation of a light field with a hexagonal honeycomb structure resembling graphene is considered along with its experimental realization and its interaction with atoms. It is argued that associated with such a light field is an optical dipole potential which leads to the diffraction of the atoms, but the details depend on whether the transverse spread of the atomic wave packet is larger than the transverse dimensions of the optical lattice (resonant Kapitza-Dirac effect) or smaller (optical Stern-Gerlach effect). Another effect in this context involves the creation of gauge fields due to the Berry phase acquired by the atom moving in the light field. The experimental realization of the light field with a honeycomb hexagonal structure is described using holographic methods and we proceed to explore the atom diffraction in the Kapitza-Dirac regime as well as the optical Stern-Gerlach regime, leading to momentum distributions with characteristic but different hexagonal structures. The artificial gauge fields too are shown to have the same hexagonal spatial structure and their magnitude can be significantly large. The effects are discussed with reference to typical parameters for the atoms and the fields

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 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 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

Ultracold atoms in radio-frequency-dressed potentials beyond the rotating wave approximation

We study dressed Bose-Einstein condensates in an atom chip radio-frequency trap. We show that in this system sufficiently strong dressing can be achieved to cause the widely used rotating wave approximation (RWA) to break down. We present a full calculation of the atom - field coupling which shows that the non-RWA contributions quantitatively alter the shape of the emerging dressed adiabatic potentials. The non-RWA contributions furthermore lead to additional allowed transitions between dressed levels. We use RF spectroscopy of Bose-Einstein condensates trapped in the dressed state potentials to directly observe the transition from the RWA to the beyond-RWA regime.Comment: 6 pages, 4 figure