Abstract
Abstract Spin angular momentum associated with circular polarization is a fundamental and important aspect of photons both in classical and quantum optics. The interaction of this optical spin with matter and structures results in many intriguing optical effects and state-of-the-art applications covered under the emerging subject of spin optics. Distinct from longitudinal optical spin along the mean wavevector, transverse spin, the corresponding vector of which is perpendicular to the mean wavevector, prevails and plays a significant role in confined electromagnetic waves such as focused beams, guided waves, and evanescent waves. In the optical near-field, these transverse spins are generated owing to the spatial variation of the kinetic momentum of confined electromagnetic waves, where the spin and orbital angular momenta are strongly coupled, leading to many interesting topological spin structures and properties. Several reviews on optical transverse spins have been published in recent years in which their concepts and the various configurations producing them were introduced systematically. Here, we introduce in this review the underlying physics and dynamics of transverse spin and the resultant topological structures and properties such as the photonic skyrmions and merons. We term this sub-area ‘spin photonics’, its scope being to cover the design and research of spin structures in strongly confined electromagnetic fields with unique properties and applications. The concepts and framework reviewed have importance in optics, topological photonics, metrology, and quantum technologies and may be used to extend spin-dynamics concepts to fluidic, acoustic, and gravitational waves.
Highlights
In the classical physics, light is an electromagnetic (EM) wave, which has multiple degrees of freedom including frequency, amplitude, phase, and polarization [1, 2]
The angular momentum (AM) of an optical beam is classed as: (i) intrinsic orbital AM (i-orbital angular momentum (OAM)) Lint [4,5,6,7,8,9,10,11,12,13,14,15] related to the optical phase singularity, which is characterized by the vortex topological charge; (ii) extrinsic orbital AM (e-OAM) Lext [16, 17] associated with the beam trajectory, which depends on the transverse coordinates of the beam centroid; and (iii) spin AM (SAM) [18,19,20,21,22,23,24] associated with the rotation of electric and magnetic polarizations
We reviewed the recent progress in spin photonics associated with transverse spins, with an emphasis on introducing their underlying physical dynamics, their topological properties, and resultant structures
Summary
Light is an electromagnetic (EM) wave, which has multiple degrees of freedom including frequency, amplitude, phase, and polarization [1, 2]. The circulation of the canonical momentum immediately yields the orbital angular momentum (OAM) density of the EM field, expressly, L = r × po This density has intrinsic and extrinsic parts associated with the optical vortex and photon trajectory, respectively [4,5,6,7,8,9,10,11,12,13,14,15,16,17]. The SAM density is an intrinsic quantity, but its direction with respect to the wave momentum (whether the kinetic momentum or the canonical momentum) is not specified in the generic case This suggests that the photonic spin vector associated with the three-dimensional rotating of electric and magnetic polarizations can be oriented in an arbitrary direction and contains components both perpendicular and parallel to the mean wavevector. The spin and orbital AMs both manifest in very different manners in local light–matter interactions, and they should be considered as independent physical properties, corresponding to different degrees of freedom
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