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Register a new userAngular Momenta and Spin-Orbit Interaction of Nonparaxial Light in Free Space
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We give an exact self-consistent operator description of the spin and orbital angular momenta, position, and spin-orbit interactions of nonparaxial light in free space. Both quantum-operator formalism and classical energy-flow approach are presented. We apply the general theory to symmetric and asymmetric Bessel beams exhibiting spin- and orbital-dependent intensity profiles. The exact wave solutions are clearly interpreted in terms of the Berry phases, quantization of caustics, and Hall effects of light, which can be readily observed experimentally.
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Recently, spatiotemporal optical vortex pulses carrying a purely transverse intrinsic orbital angular momentum were generated experimentally [{it Optica} {bf 6}, 1547 (2019); {it Nat. Photon.} {bf 14}, 350 (2020)]. However, an accurate theoretical analysis of such states and their angular-momentum properties remains elusive. Here we provide such analysis, including scalar and vector spatiotemporal Bessel-type solutions as well as descrption of their propagational, polarization, and angular-momentum properties. Most importantly, we calculate both local densities and integral values of the spin and orbital angular momenta, and predict observable spin-orbit interaction phenomena related to the coupling between the trasnverse spin and orbital angular momentum. Our analysis is readily extended to spatiotemporal vortex pulses of other natures (e.g., acoustic).
We develop a novel theoretical framework describing polariton-enhanced spin-orbit interaction of light on the surface of two-dimensional media. Starting from the integral formulation of electromagnetic scattering, we exploit the reduced dimensionality of the system to introduce a quantum-like formalism particularly suitable to fully take advantage of rotational invariance. Our description is closely related to that of a fictitious spin one quantum particle living in the atomically thin medium, whose orbital, spin and total angular momenta play a key role in the scattering process. Conservation of total angular momentum upon scattering enables to physically unveil the interaction between radiation and the two-dimensional material along with the detailed exchange processes among orbital and spin components. In addition, we specialize our model to doped extended graphene, finding such spin-orbit interaction to be dramatically enhanced by the excitation of surface plasmon polaritons propagating radially along the graphene sheet. We provide several examples of the enormous possibilities offered by plasmon-enhanced spin-orbit interaction of light including vortex generation, mixing, and engineering of tunable deep subwavelength arrays of optical traps in the near field. Our results hold great potential for the development of nano-scaled quantum active elements and logic gates for the manipulation of hyper-entangled photon states as well as for the design of artificial media imprinted by engineered photonic lattices tweezing cold atoms into the desired patterns.
The edge diffraction of a homogeneously polarized light beam is studied theoretically based on the paraxial optics and Fresnel-Kirchhoff approximation, and the dependence of the diffracted beam pattern of the incident beam polarization is predicted. If the incident beam is circularly polarized, the trajectory of the diffracted beam centre of gravity experiences a small angular deviation from the geometrically expected direction. The deviation is parallel to the screen edge and reverses the sign with the polarization handedness; it is explicitly calculated for the case of a Gaussian incident beam with plane wavefront. This effect is a manifestation of the spin-orbit interaction of light and can be interpreted as a revelation of the internal spin energy flow immanent in circularly polarized beams. It also exposes the vortex character of the weak longitudinal field component associated with the circularly polarized incident beam.
This paper analyzes the algebraic and physical properties of the spin and orbital angular momenta of light in the quantum mechanical framework. The consequences of the fact that these are not angular momenta in the quantum mechanical sense are worked out in mathematical detail. It turns out that the spin part of the angular momentum has continuous eigenvalues. Particular attention is given to the paraxial limit, and to the definition of Laguerre--Gaussian modes for photons as well as classical light fields taking full account of the polarization degree of freedom.
Light beam with optical vortices can propagate in free space only with integer orbital angular momentum. Here, we invert this scientific consensus theoretically and experimentally by proposing light beams carrying natural non-integer orbital angular momentum. These peculiar light beams are actually special solutions of wave function, which possess optical vortices with the topological charge l+0.5, where l is an integer. Owing to the interaction of phase and polarization singularity, these vortex beams with fractional topological charge can maintain their amplitude and vortex phase even when they propagate to an infinite distance. This work demonstrates another state of optical vortices in free space, which will fundamentally inject new vigor into optics, and other relate scientific fields.
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