Spin-orbit interactions and chiroptical effects engaging orbital angular momentum of twisted light in chiral and achiral media

Abstract

There is recurrent interest in the orbital angular momentum (OAM) conveyed by optical vortices, which are structured beams with a helically twisted wavefront. Particular significance is attached to the issue of how, in its interactions with matter, light conveying OAM might prove sensitive to the relative handedness and degree of twist in the associated optical wavefront. As a result of recent experimental and theoretical studies, the supposition that beams with OAM might enable discrimination between oppositely handed forms of matter has become a renewed focus of attention. Some of the tantalizing conclusions that are beginning to emerge from this research have, however, not yet established a definitive basis for a supporting mechanism. To resolve this problem requires the development of theory to support a faithful representation, and a thorough understanding, of the fundamental molecule-photon physics at play in such optical processeseven for processes as basic as absorption. The present analysis establishes mechanisms at play that entail a novel manifestation of optical spin-orbit interactions (SOI), engaging transition electric quadrupole moments. Moreover, powerful symmetry principles prove to render distinctively different criteria governing the exhibition of 2D and 3D chirality. The new results elucidate the operation of such effects, identifying their responsibility for discriminatory optical interactions of various forms in both chiral and achiral media.

Quantum electrodynamics in modern optics and photonics: tutorial

“…Very recently, the QED theory was experimentally vindicated in the observation of modified selection rules for absorption of OAM light in which two units of angular momentum, 2 , were transferred to the valence electron of a 40 Ca + ion through an E2 transition-one quantum of angular momentum from the spin angular momentum, and one from the OAM [221]. Alongside this recent accomplishment, there has also been a proof, again via QED, that the optical OAM of twisted light can produce chiroptical phenomena dependent on the direction of an optical vortex twist in both chiral and achiral matter [37,38,[222][223][224][225]. With these successes-some illustrated in Fig.…”

Quadrupole interaction of non-diffracting beams with two-level atoms

“…Recently, it has shown that the weak optical quadrupole interaction in atoms can be improved considerably while the atom interacts at near resonance with an optical vortex [12,17]. In addition, the LG mode is investigated in the case of the quadrupole approximation and with an appropriate choice of the winding number l of the vortex, the process involving the dipole-forbidden, but quadrupole-allowed, transitions in atoms can be realized [12].…”

“…Then, some appropriate theoretical examinations that are concerned with the exploration of the quadrupole interaction effects have been suggested [10][11][12]. Some such studies are concerned with the exploration of the effects of magnetic-dipole interaction, in the context of the magnetic mirrors applications [13][14][15], while others are dealt with the electric-quadrupole aspects in the framework of the atoms motion in a laser field [7][8][9][10][11][12] and other studies are concerned with the chiroptical interactions using vortex light, which can only occur through the quadrupole transitions [16,17].…”

“…We will illustrate that the inclusion of the quadrupole interactions for some specific cases with forbidden dipole transitions can lead to interesting aspects. In particular, some recent investigations have pointed out the possibility of observing quadrupole interaction and even enhancing its magnitude [12,16,17,28]. Moreover, the realization of optical lattices and vortices of atoms in space, in particular, requires the use of special types of light that are described as possessing non-diffracting beams.…”

Optical spin-orbit interactions in molecular scattering of twisted light

“…Since the interaction involves quadrupole couplings, to calculate the matrix element requires determination of the gradient of the electric field. This has previously been calculated for Laguerre-Gaussian modes 38 and is given as follows:…”