arXiv subjects

Mohamed Babiker

Publications and source records attributed to Mohamed Babiker.

13 recordsLinked to original sources

arXiv2025

Optical forces on atoms subject to higher-order Poincaré vortex modes

The interaction of atoms with higher-order Poincaré optical vortex modes of order $m\geq 0$ is explored for light close to resonance with atomic dipole transitions. It is well-known that atoms subject to optical vortex modes experience both translational and rotational forces acting on the atomic centre of mass, leading to atom dynamics and atom trapping. Here we consider the optical forces on atoms immersed in general paraxial higher-order Poincaré optical vector modes. The coupling to atoms gives rise to wide-ranging scenarios involving such modes in which any specific polarisation is within a spectrum of wave polarisation and all the interactions are treatable within a single formulation. We show that this gives rise to a variety of physical situations, governed by the mode order $m$, the polarisation represented by the angular coordinates of the mode on the surface of the unit Poincaré sphere, the atomic transitions involved, and their selection rules. We present the analytical steps leading to the optical forces on sodium atoms and display their variations in various situations.

quant-ph↗

arXiv2025

Magneto-optical polarisation texturing

Left and right circularly polarized transverse electromagnetic waves propagate at slightly different speeds in a magnetic material leading to a polarization rotation by an amount proportional to the projection of the magnetic field along the direction of the wave propagation. We show how this magneto-optical effect can serve as a vectorial polarization shaper if the input mode is either a radially-polarised or an azimuthally-polarised Laguerre-Gaussian (LG) mode. The specific polarization map of the output field can be achieved by choosing appropriately the magnetic material and/or its geometry. We show further that when the LG beam waist is comparable to the wavelength the fields are no longer purely transverse but acquire an additional longitudinal (axial) component. We demonstrate how this modifies the polarisation texturing.

physics.optics↗

arXiv2024

Intrinsic angular momentum, spin and helicity of higher-order Poincare modes

The availability of coherent sources of higher order Poincare optical beams have opened up new opportunities for applications such as in the optical trapping of atoms and small particles, the manipulation of chirally-sensitive systems and in improved encoding schemes for broad-bandwidth communications. Here we determine the intrinsic properties of Poincare Laguerre-Gaussian (LG) modes which have so far neither been evaluated, nor their significance highlighted. The theoretical framework we adopt here is both novel and essential because it emphasises the crucial role played by the normally ignored axial components of the twisted light fields of these modes. We show that the inclusion of the axial field components enables the intrinsic properties of the Poincare modes, notably their angular momentum, both spin and orbital as well as their helicity and chirality, to be determined. We predict significant enhancements of the intrinsic properties of these modes when compared with those due to the zero order LG modes. In particular, we show that higher order LG Poincare modes exhibit super-chirality and, significantly so, even in the case of the first order

physics.optics↗

arXiv2023

Absorption of hybrid fibre modes by Cs atoms in quadrupole transitions

We evaluate the rate of the absorption of an optical nanofiber mode by a Cs atom in an electric quadrupole transition. With the Cs atom localized near the outer surface of the optical nano-fiber, an interaction occurs between the atomic quadrupole tensor components and the gradients of the vector components of the electric field of a hybrid fiber mode. The absorption rate is evaluated as a function of the radial position of the atom from the fiber axis, assuming a specific value of the laser power and we use experimentally accessible parameters. We find that the absorption of the hybrid modes by the Cs atom decreases as the atom recedes away from the fiber axis and formally vanishes at sufficiently large radial distances. Close to the fiber, however, the absorption rate for the input power chosen can be two orders of magnitude larger than the quadrupole de-excitation rate despite the moderate power used.

physics.optics↗

arXiv2023

Direct Observation of the Faraday Rotation Using Radially-Polarized Twisted Light

A novel experimental technique for the realisation of the optical Faraday effect using Laguerre- Gaussian (LG) light is described. The experiment employs a zero-order vortex half-wave retarder to generate a radially or azimuthally-polarised LG doughnut beam. The light emerging from the retarder then passes through a linear polariser, which gives rise to two intensity lobes, with the orientation of the intensity gap between the two lobes pointing parallel (perpendicular) to the polarization direction of the radially (azimuthally) polarised beam. To complete the Faraday set up, the light traverses a material subject to a magnetic field, before passing through a final linear polariser, which results in a visible rotation of the lobes pattern. This technique exhibits the Faraday effect readily visually, without further elaborate steps to detect changes in the light intensity. The degree of rotation of the plane of polarisation is determined directly by the visibly clear change in the orientation of the intensity gap between the lobes.

physics.optics↗

arXiv2023

Super-chirality of paraxial higher order Poincare modes

We demonstrate that higher order Poincare modes of order m are super-chiral, displaying enhancement factors proportional to $m$ and $m^2$ in their helicity/chirality. With m having arbitrarily large integer values, such modes, in principle, possess unlimited super-chirality. These findings pave the way to applications, including the strong enhancements of optical interactions with chiral matter. The work indicates considerable flexibility in controlling the helicity of any higher order paraxial twisted light mode and it incorporates a very wide range of physical scenarios.

physics.optics↗

arXiv2022

Optical angular momentum in atomic transitions: a paradox

Stated simply the paradox is as follows: it is clear that the orbital angular momentum of a light beam in its direction of propagation is an intrinsic quantity, and therefore has the same value everywhere in the beam. How then can a Gaussian beam, with precisely zero orbital angular momentum, drive a (single-photon) quadrupole transition which requires the transfer of angular momentum 2$\hbar$ to an absorbing atom?

physics.optics↗

arXiv2020

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.

physics.optics↗

arXiv2020

Chirality and helicity of optical vortices of small beam waists

The chirality and helicity of linearly polarised Laguerre-Gaussian (LG) beams are examined. Such a type of light possesses a substantial longitudinal field when its beam waist is sufficiently small and so gives rise to non-zero chirality and helicity. In the simplest case of a doughnut beam of winding number $\ell=1$ and another identical to it but has $\ell= -1$, we obtain different chirality and helicity distributions at the focal plane $z=0$. We also show that this chiral behaviour persists and the patterns evolve so that on planes at $z<0$ and $z>0$ the beam convergence contributes differently to the changes in the chirality and helicity distributions.

physics.optics↗

arXiv2020

Quadrupole absorption rate and orbital angular momentum transfer for atoms in optical vortices

Recent experiments involving the interaction of optical vortices with atoms in quadrupole transitions have been shown to be accompanied by the exchange of orbital angular momentum (OAM) between the electronic states of the atom and the optical vortex field. Earlier work by both theory and experiment had ruled out the transfer of a vortex OAM to the electronic degrees of freedom in an electric dipole atomic transition and it has been confirmed that the lowest multipolar order involving an OAM transfer to the electronic motion is indeed the electric quadrupole. Hitherto, the quadrupole transition involving optical vortices has not been quantified and we have thus set out to evaluate the absorption rate accompanied by an OAM transfer with reference to the $6^2S_{1/2}\rightarrow 5^2D_{5/2}$ in Cs when caesium atoms are subject to the field of a linearly polarized optical vortex. Our results assuming typical experimentally accessible parameters indicate that the absorption rate for moderate light intensities is smaller than the quadrupole spontaneous emission rate, but should still be within the measurement capabilities of modern spectroscopic techniques.

physics.atom-ph↗

arXiv2020

Atom trapping and dynamics in the interaction of optical vortices with quadrupole-active transitions

Recent studies have confirmed the coupling of optical vortices, such as Laguerre-Gaussian and Bessel-Gaussian modes, to quadrupole-active atomic transitions. This interaction has been shown to be enhanced considerably in the case of Laguerre-Gaussian beams due to the gradient coupling, particularly in the case of a relatively large winding number. Here we consider the trapping and the dynamics of atoms in the optical quadrupole potential generated by two co-axial counter-propagating optical vortex beams. We focus on the atomic transition $6^2S_{1/2}\rightarrow 5^2D_{5/2}$ in Cs which is a dipole-forbidden, but a quadrupole-allowed transition. We show how this atomic transition engages with the optical vortex fields at near-resonance, leading to atom trapping in the optical quadrupole potential well accompanied by translational motion. We show how the optical forces generate the motion of the atoms trapped within the quadrupole potential, illustrating the results using typical experimentally accessible parameters.

physics.atom-ph↗

arXiv2012

Quantised orbital angular momentum transfer and magnetic dichroism in the interaction of electron vortices with matter

Following the very recent experimental realisation of electron vortices, we consider their interaction with matter, in particular the transfer of orbital angular momentum in the context of electron energy loss spectroscopy, and the recently observed dichroism in thin film magnetised iron samples. We show here that orbital angular momentum exchange does indeed occur between electron vortices and the internal electronic-type motion, as well as center of mass motion of atoms in the electric dipole approximation. This contrasts with the case of optical vortices where such transfer only occurs in transitions involving multipoles higher than the dipole. The physical basis of the observed dichroism is explained.

quant-ph↗

arXiv2003

Molecular chirality and the orbital angular momentum of light

Optical beams with a new and distinctive type of helicity have become the subject of much recent interest. While circularly polarised light comprises photons with spin angular momentum, these optically engineered 'twisted beams' (optical vortices) are endowed with orbital angular momentum. Here, the wave- front surface of the electromagnetic fields assumes helical form. To date, optical vortices have generally been studied only in their interactions with achiral matter. This study assesses what new features, if any, can be expected when such beams are used to interrogate a chiral system.

physics.optics↗