The photonic wheel - demonstration of a state of light with purely transverse angular momentum

Abstract

In classical mechanics, a system may possess angular momentum which can be either transverse (e.g. in a spinning wheel) or longitudinal (e.g. for a spiraling seed falling from a tree) with respect to the direction of motion. However, for light, a typical massless wave system, the situation is less versatile. Photons are well-known to exhibit intrinsic angular momentum which is longitudinal only: the spin angular momentum defining the polarization and the orbital angular momentum associated with a spiraling phase front. Here we show that it is possible to generate a novel state of the light field that contains purely transverse angular momentum, the analogue of a spinning mechanical wheel. We realize this state by tight focusing of a polarization tailored light beam and measure it using an optical nano-probing technique. Such a novel state of the light field can find applications in optical tweezers and spanners where it allows for additional rotational degree of freedom not achievable in single-beam configurations so far

Spin–orbit coupling in surface plasmon scattering by nanostructures

“…Discussion The specific transverse spin topology, with the spin vector normal to the wavevector, is required to achieve opposite handedness in the opposite, mirror-symmetric scattering directions. Such transverse spin can be realized with an SPP wave, or other guided modes, or indeed in free space in special cases of light focusing 12 . The effect does not exist (for an isotropic particle) if the illuminating field does not possess such a spin structure.…”

“…However, in any direction contained within the x-z plane (orthogonal to the incident light spin), the radiation is always linearly polarized because the projection of the particle polarization in a plane perpendicular to any direction in the x-z plane is always linearly polarized. An SPP-like field having longitudinal (y direction) and transverse (z direction) components while propagating in free space, with its spin transverse to the propagation (y) direction can be obtained in specifically focused beams 12 . In such a field, the particle will be polarized elliptically in the y-z plane (Fig.…”

“…1c; Supplementary Note 1). This form of transverse angular momentum can also be observed in free space by engineering the optical field near a tight focal spot 12 . SPP fields, being transversemagnetic in nature, have a mirror symmetry in the k-z plane.…”

Linear and angular momenta in tightly focused vortex segmented beams of light (Invited Paper)

“…In this work, we investigate the occurrence of a negative longitudinal component of p o , a phenomenon sometimes referred to as 'backflow' [21,[23][24][25] , in tightly focused composite beams consisting of two spatially separated vortices with opposite charge and circular polarization of opposite handedness. Similar to a spin segmented beam [26,27] , focusing of such vortex segmented beams (VSBs) yields transverse AM, which depends on the charge of the input vortices [28] . The transverse AM manifests itself in the occurrence of vortices oriented perpendicular to the propagation direction [17][18][19][20][21][22]29,30] .…”

Tighter spots of light with superposed orbital-angular-momentum beams

“…In the limit of tight focusing of light beams, the electromagnetic field distribution can become highly complex in the focal plane [5][6][7][8]29]. In this context, the occurrence of longitudinally oscillating field components or, more generally, of three-dimensional field distributions gives rise to a variety of interesting effects and phenomena, including spin-to-orbit coupling [30], transverse angular momentum [31][32][33][34], the creation of complex polarization topologies [35] at the nanoscale, and also the possibility of focusing light more tightly, as observed for instance for radially polarized light [4,7].…”