From Circular Polarization to Photonic Wheels – Different Types of Angular Momenta

Evanescent fields, waveguide modes, propagating surface plasmons and focused light beams share an intriguing feature. As a direct consequence of the spatial confinement they are resulting from, components of the field oscillating along the propagation direction are formed, which are usually dephased by ±π/2 with respect to their transverse counterparts. Hence, the electric or magnetic field (or both) are elliptically polarized with the polarization ellipse coinciding with a plane of propagation. Richards and Wolf first recognized this subtle detail for tightly focused light beams and Huard and Vigoureux for evanescent waves. In the notation of angular momenta of light, such confined fields therefore exhibit components of the spin, which are orthogonal to the propagation direction in contrast to the conventional case of circularly polarized light. This research area of transverse angular momenta was initiated in 2009 and, since then, received considerable attention. In some cases, the angular momentum (spin) density or the net angular momentum might be even purely transverse. In this presentation, we discuss the concept of photonic wheels, i.e., light carrying an angular momentum perpendicular to the propagation direction, as a versatile and very flexible tool for nanophotonics. Recent studies involving transverse angular momenta are reviewed..