Unveiling Angular Momentum

Karen P. Volke-Sepúlveda, A. Vásquez-Arzola, N. Hernández-Candia +3 more

Optical vortices became a hot topic since almost two decades ago, when it was recognized that Laguerre-Gaussian laser modes carry orbital angular momentum [Allen et al. Phys Rev A 45, 8185 (1992)] related with a screw phase dislocation, and different from the spin angular momentum associated to circular polarization. In 1995, this dynamical quantity was transferred to matter in an optical micromanipulation system for the first time [H. He, et al., Phys. Rev. Lett. 75, 826 (1995)], and since then, a number of studies on angular momentum of light have unveiled different interesting aspects on the subject. However, there are still open questions, which have arisen together with the generation of novel light beams, such as vector vortices, for instance. In contrast with scalar vortices, with usual polarization states (linear, circular, elliptical), the orientation and magnitude of the electric field of vector vortices (solutions of the vector wave equation) is a function of space and time. In this work, we present an experimental study of the local angular momentum density of a Bessel vector vortex of first order by means of an optical trap. For this purpose, we used different probe particles in order to sense the local contribution to the optical angular momentum in each region of the beam. But optical fields are not the only wave fields that may exhibit phase dislocations or singularities. There are close analogies between light and sound fields that can be exploited in order to get a better understanding of common phenomena and study new aspects in both branches of physics. Here we also present the first experimental demonstration and theoretical analysis of acoustical vortices in free field, with similar properties to those of the optical vortices, including the angular momentum that can be transferred to matter. The corresponding analogies and differences with the optical case turn out to be very enlightening for the understanding of the phenomenon of angular momentum in wave fields.

Taeseung Choi, Sam Young Cho

Since the discovery a century ago, spin describing the intrinsic angular momentum of massive elementary particles has exposed its nature and significant roles in wide ranges of (relativistic) quantum phenomena and practical applications for future quantum technology. Emerging inconsistencies have also disclosed its telltale incomplete description. Finding relativistic spins (operators) of massive particles is a long-standing fundamental problem from the beginning of relativistic quantum mechanics. Here we present the rigorous derivation and the representation of spin operators from the spacetime symmetry. The covariant parity operation, defined by the spin operators, naturally leads to a fundamental equation equivalent to the covariant Dirac equation, which manifests existent relativistic spins. Proper understanding position operator in the Dirac theory on account of the spin operator through total angular momentum predicts no Zitterbewegung as well as conserving orbital and spin currents. The spin operators can be applicable for unraveling the inconsistencies and for exploring unveiled physics of massive particles.

James D. Louck

Jeff Greensite

Christian Tomás Schmiegelow, Nicolás A. Nuñez Barreto

We present a series of four experiments unveiling intriguing characteristics of structured beams when interacting with individual, well-localized atoms. The initial investigation explores the placement of a single atom within the dark center of a vortex/doughnut beam, revealing unexpected excitations. Furthermore, the study demonstrates the transfer of the beam’s chirality to the internal and external motion of the atom, as evidenced by alterations in allowed atomic transitions. The research showcases the ability of both intrinsic (polarization) and extrinsic (orbital/structural) angular momentum of the beam to influence the atom, resulting in the transfer of two units of angular momentum. Notably, structured beams induce motion transversal to their propagation direction. The paper concludes with a position-resolved measurement of the azimuthal Doppler shift of a structured beam, uncovering its characteristic divergence at the center and its scale-invariant nature. These findings offer valuable insights into the understanding of structured beam-atom interactions, potentially constituting the first indirect observation of the elusive super-kicks predicted by Barnett and Berry in 2013.