Transient optical angular momentum effects and atom trapping in multiple twisted beams

Abstract

Theoretical work has already established the existence of a light-induced torque acting on the centre of mass of an atom, ion or molecule immersed in twisted light, where the transition frequency is suitably detuned from that of the twisted light beam. The twisted beam carries l units of orbital angular momentum per photon, and the steady-state saturation form of the torque is also determined by the width of the upper state in the atomic transition. It has been shown that, to leading order, the transfer of orbital angular momentum can only occur between the twisted light and the centre of mass motion. We argue here that, for small linewidth, the full time-dependence of the torque is needed to account correctly for the dynamics of atoms in a twisted light beam. We outline the theoretical framework needed to derive this full timedependence, applying the theory to the motion in a twisted light beam of Eu 3+ ions, which possess a particularly narrow linewidth state. For relatively large linewidth, the steady-state forces and torque are appropriate, but the processes of cooling and trapping require the application of several suitably oriented twisted beams. The description of atomic motion in multiple twisted beams demands the application of special coordinate transformations. We show how to construct the appropriate transformation matrices to represent a twisted light beam propagating in an arbitrary direction, and we proceed to investigate the cooling and trapping of Mg + ions in sets of pairs of counter-propagating twisted beams in two-dimensional and three-dimensional molasses configurations.

Atoms in complex twisted light

“…The effects of structured light on atom dynamics, in both its forms, namely the gross motion and the internal motion have been thoroughly investigated (see references ( [129][130][131][132][133][134][135][136]). These investigations have shown that the interaction of light carrying OAM with atoms introduces new significant features, namely that (i) there is, in addition to translational effects, a light-induced torque which causes a rotational motion of the atoms about the beam axis, and (ii) there are characteristic regions of maximum and minimum intensity in the beam cross-section.…”

Atomic and Molecular Manipulation Using Structured Light

“…Against this background, the theoretical and technological developments that have recently led to the production of structured laser light introduce another tier of possibilities associated with orbital angular momentum (OAM) content [10][11][12][13][14]. The intricate wavefront structures of Laguerre-Gaussian, Bessel, and Mathieu light beams, for example, allow the production of force fields and torques that have no counterpart in conventional optical beams.…”

“…10 Vector field representation of the director orientation of the twisted nematic liquid crystal before application of the twisted light. The orientation at the boundaries z = 0 (bottom) and z = d (top) are fixed as described in text.…”