Vortex Beam Generation by Spin-Orbit Interaction with Bloch Surface Waves

Abstract

Axis-symmetric grooves milled in metallic slabs have been demonstrated to promote the transfer of Orbital Angular Momentum (OAM) from far- to near-field and vice versa, thanks to spin-orbit coupling effects involving Surface Plasmons (SP). However, the high absorption losses and the polarization constraints, which are intrinsic in plasmonic structures, limit their effectiveness for applications in the visible spectrum, particularly if emitters located in close proximity to the metallic surface are concerned. Here, an alternative mechanism for vortex beam generation is presented, wherein a free-space radiation possessing OAM is obtained by diffraction of Bloch Surface Waves (BSWs) on a dielectric multilayer. A circularly polarized laser beam is tightly focused on the multilayer surface by means of an immersion optics, such that TE-polarized BSWs are launched radially from the focused spot. While propagating on the multilayer surface, BSWs exhibit a spiral-like wavefront due to the Spin-Orbit Interaction (SOI). A spiral grating surrounding the illumination area provides for the BSW diffraction out-of-plane and imparts an additional azimuthal geometric phase distribution defined by the topological charge of the spiral structure. At infinity, the constructive interference results into free-space beams with defined combinations of polarization and OAM satisfying the conservation of the Total Angular Momentum, based on the incident polarization handedness and the spiral grating topological charge. As an extension of this concept, chiral diffractive structures for BSWs can be used in combination with surface cavities hosting light sources therein.

Probing and Imaging Photonic Spin‐Orbit Interactions in Nanostructures

“…[ 58 ] Recently, Stella et al demonstrated an alternative mechanism for far‐field OV generation based on the diffraction of Bloch surface waves, as depicted in Figure 5d. [ 66 ] A spiral grating on top of the dielectric multilayers induced designated phase distributions and diffracted the Bloch surface waves into the far‐field.…”

“…Reproduced with permission. [ 66 ] Copyright 2020, American Chemical Society. e) Tunable unidirectional coupling by coupling SAM into curved nanoscale plasmonic waveguides, where a circular decoupler is fabricated at the end of each route to visualize the optical flow.…”

“…[ 24,25 ] Hence, different from the traditional interfaces and bulk materials, nanostructures have become versatile platforms for enhancing PSOIs and demonstrating many novel optical phenomena, such as the photonic spin‐Hall effect (PSHE), [ 26–40 ] the quantum spin‐Hall effect (QSHE) of light, [ 41–50 ] and the spin‐to‐vortex conversion (SVC). [ 31,51–74 ] These phenomena have attracted broad interest as they can provide new and robust ways to manipulate light at nanoscale, including the directionality (wavevector), the spin state (CP state), and IOAM (phase distribution). So far, PSOI effects have found many important applications.…”

Arbitrary-Order and Multichannel Optical Vortices with Simultaneous Amplitude and Phase Modulation on Plasmonic Metasurfaces

“…Through careful design of each element structure, the metal metasurfaces can provide an additional phase gradient or surface momentum [22,23]. Metal metasurfaces based on the Pancharatnam-Berry (PB) phase [24] or geometric phase can produce OVs through the spin-orbit interaction (SOI) [25], which are robust and nondispersive [26]. Although these surfaces have multiple functions, they still face some obvious challenges and shortcomings.…”

Surface States on Photonic Crystals As Hybrid Dielectric Metasurface Bound States of the Termination Layer

“…Similar to surface plasmon polaritons, it was examined whether or not electromagnetic modes can be sustained bound at the interface between air and a finite dielectric photonic crystal. − The possible appearance of air/photonic crystal localized states have attracted great attention due to the fact that, with the addition of a supplementary defect, they may couple to external radiation in a controlled way, being transformed into a leaky mode. This enables fascinating applications like beam wavefront steering and beam shaping, − further facilitating the communication of the components in a photonic crystal circuit. It was found that, in the typical two-dimensional rectangular dielectric photonic crystal, we need a corrugation, that is, a layer of different size, material, or lattice, to sustain the bound mode .…”