Algebra of optical dislocations with plasmonic nanostructures.

Plasmonic structures with physical and Berry-type dislocations are shown to produce vortices with phase singularity according to the system and illumination parameters. We demonstrate that, by combining the two types of dislocations in one structure, the manipulation with the topological charge of the emerging vortex beams can be controlled in an intriguing way. As a result, the plasmonic field distribution can be conveniently modified and selectively excited.

Topologically protected plasmonic phases in randomized aperture gratings.

We experimentally show the excitation of surface plasmons by topologically protected diffraction from gratings with randomized periodicity. The structures are designed such that the plasmonic excitation is conditioned by the proper combination of the geometric and the dynamic phases. Accordingly, it is possible to obtain a precise interaction of the incident light signal and a specific plasmonic directional mode in a polarization dependent manner.

Directional Plasmonic Excitation by Helical Nanotips.

The phenomenon of coupling between light and surface plasmon polaritons requires specific momentum matching conditions. In the case of a single scattering object on a metallic surface, such as a nanoparticle or a nanohole, the coupling between a broadband effect, i.e., scattering, and a discrete one, such as surface plasmon excitation, leads to Fano-like resonance lineshapes. The necessary phase matching requirements can be used to engineer the light-plasmon coupling and to achieve a directional plasmonic excitation. Here, we investigate this effect by using a chiral nanotip to excite surface plasmons with a strong spin-dependent azimuthal variation. This effect can be described by a Fano-like interference with a complex coupling factor that can be modified thanks to a symmetry breaking of the nanostructure.

Particle trapping and beaming using a 3D nanotip excited with a plasmonic vortex.

Recent advances in nanotechnology have prompted the need for tools to accurately and noninvasively manipulate individual nano-objects. Among the possible strategies, optical forces have been widely used to enable nano-optical tweezers capable of trapping or moving a specimen with unprecedented accuracy. Here, we propose an architecture consisting of a nanotip excited with a plasmonic vortex enabling effective dynamic control of nanoparticles in three dimensions. The structure illuminated by a beam with angular momentum can generate an optical field that can be used to manipulate single dielectric nanoparticles. We demonstrate that it is possible to stably trap or push the particle from specific points, thus enabling a new, to the best of our knowledge, platform for nanoparticle manipulation.

Spin-locking metasurface for surface plasmon routing.

Nanophotonic circuitry requires an ability to externally control and analyze optical signals tightly confined in subwavelength volumes. Various schemes of surface plasmon (SP) routing have been presented using active and passive metasurfaces. One of the most appealing approaches is the use of plasmonic spin-orbit interaction where the incident light spin state is efficiently coupled to an orbital degree of freedom of the surface wave. Recently, a major attention has been drawn to an additional plasmonic degree of freedom - the transverse spin and some application for near-field plasmonic manipulations have been presented. Here we propose a spin-locking metasurface incorporating a transverse spin of the SP wave to selectively route the near-field beams. Owing to the combination of the oblique incidence of circularly polarized light with the accurately designed momentum matching of the grating we achieve a precise directional control over the plasmonic distributions. The experimental verification of the directional launching is performed by a time-resolved leakage radiation measurements allowing one to visualize the shape and the dynamics of the excited beam.

Unusual polarizing effect of cylindrical plasmonic holes.

We observe an unusual polarization state conversion in the light that passes through a cylindrical hole in a thick metal film. This phenomenon is related to the helicity locking of the guided mode due to the plasmonic transverse spin-an intrinsic angular momentum of the surface waves. We show how this effect is linked to the generation of the plasmonic vortex inside the hole and can be altered by varying the hole diameter. In addition, the total light transmission through the hole is shown to be partially contributed from the direct transmission, which can further modify the resulting light polarization state.

Helicity locking of chiral light emitted from a plasmonic nanotaper.

Surface plasmon waves carry an intrinsic transverse spin, which is locked to its propagation direction. Apparently, when a singular plasmonic mode is guided on a conic surface this spin-locking may lead to a strong circular polarization of the far-field emission. Specifically, a plasmonic vortex excited on a flat metal surface propagates on an adiabatically tapered gold nanocone where the mode accelerates and finally beams out from the tip apex. The helicity of this beam is shown to be single-handed and stems solely from the transverse spin-locking of the helical plasmonic wave-front. We present a simple geometric model that fully predicts the emerging light spin in our system. Finally, we experimentally demonstrate the helicity-locking phenomenon by using accurately fabricated nanostructures and confirm the results with the model and numerical data.

Beaming of Helical Light from Plasmonic Vortices via Adiabatically Tapered Nanotip.

We demonstrate the generation of far-field propagating optical beams with a desired orbital angular momentum by using a smooth optical-mode transformation between a plasmonic vortex and free-space Laguerre-Gaussian modes. This is obtained by means of an adiabatically tapered gold tip surrounded by a spiral slit. The proposed physical model, backed up by the numerical study, brings about an optimized structure that is fabricated by using a highly reproducible secondary electron lithography technique. Optical measurements of the structure excellently agree with the theoretically predicted far-field distributions. This architecture provides a unique platform for a localized excitation of plasmonic vortices followed by its beaming.

Ultrathin plasmonic chiral phase plate.

A thin free-standing gold membrane with complex plasmonic structures engraved on both sides is shown to perform as an ultrathin phase plate. Specifically, we demonstrate the generation of a far-field vortex beam propagating at a desired angle. The angular momentum of the beam is generated by the groove helicity, together with the geometric phase arising from a plasmonic spin-orbit interaction. The radial chirp of the back-side structure is used to modify the emission angle via a specific momentum matching condition.

Optical vortex beam generator at nanoscale level.

Optical beams carrying orbital angular momentum (OAM) can find tremendous applications in several fields. In order to apply these particular beams in photonic integrated devices innovative optical elements have been proposed. Here we are interested in the generation of OAM-carrying beams at the nanoscale level. We design and experimentally demonstrate a plasmonic optical vortex emitter, based on a metal-insulator-metal holey plasmonic vortex lens. Our plasmonic element is shown to convert impinging circularly polarized light to an orbital angular momentum state capable of propagating to the far-field. Moreover, the emerging OAM can be externally adjusted by switching the handedness of the incident light polarization. The device has a radius of few micrometers and the OAM beam is generated from subwavelength aperture. The fabrication of integrated arrays of PVLs and the possible simultaneous emission of multiple optical vortices provide an easy way to the large-scale integration of optical vortex emitters for wide-ranging applications.

Generating far-field orbital angular momenta from near-field optical chirality.

We demonstrate orbital angular momentum (OAM) transfer by chiral plasmonic nanostructures designed on both sides of a thin suspended metallic membrane. We show how far-field vortex beams with tunable OAM indices can be tailored through nanostructure designs. We reveal the crucial role played by the central aperture that connects the two sides of the membrane from which OAM selection rules are derived in perfect agreement with experimental data.

Optical spin Hall effects in plasmonic chains.

Observation of optical spin Hall effects (OSHEs) manifested by a spin-dependent momentum redirection is presented. The effect occurring solely as a result of the curvature of the coupled localized plasmonic chain is regarded as the locally isotropic OSHE, while the locally anisotropic OSHE arises from the interaction between the optical spin and the local anisotropy of the plasmonic mode rotating along the chain. A wavefront phase dislocation was observed in a circular curvature, in which the dislocation strength was enhanced by the locally anisotropic effect.

Geometric doppler effect: spin-split dispersion of thermal radiation.

A geometric Doppler effect manifested by a spin-split dispersion relation of thermal radiation is observed. A spin-dependent dispersion splitting was obtained in a structure consisting of a coupled thermal antenna array. The effect is due to a spin-orbit interaction resulting from the dynamics of the surface waves propagating along the structure whose local anisotropy axis is rotated in space. The observation of the spin-symmetry breaking in thermal radiation may be utilized for manipulation of spontaneous or stimulated emission.

Observation of optical spin symmetry breaking in nanoapertures.

Observation of a spin symmetry breaking effect in plasmonic nanoscale structures due to spin-orbit interaction is presented. We demonstrate a nanoplasmonic structure which exhibits a crucial role of an angular momentum (AM) selection rule in a light-surface plasmon scattering process. In our experiment, the intrinsic AM (spin) of the incident radiation is coupled to the extrinsic momentum (orbital AM) of the surface plasmons via spin-orbit interaction. Due to this effect, we achieved a spin-controlled enhanced transmission through a coaxial nanoaperture.

Topological spin-orbit interaction of light in anisotropic inhomogeneous subwavelength structures.

Spin-orbit interaction resulting from spatial polarization state manipulation is demonstrated. Polarization-state manipulation is achieved by utilizing the effective birefringent nature of subwavelength structures acting as an anisotropic inhomogeneous medium. Experimental verification is obtained by measuring the effect of the unavoidable spin-dependent Pancharatnam-Berry phase modulation on the far-field diffraction pattern of the beam. Unlike the usual dynamic spin-orbit interaction that splits spin states in the temporal frequency (energy) domain, this topological spin-orbit interaction results in the splitting of spin states degenerated by their spatial frequencies (momentum).

Coriolis effect in optics: unified geometric phase and spin-Hall effect.

We examine the spin-orbit coupling effects that appear when a wave carrying intrinsic angular momentum interacts with a medium. The Berry phase is shown to be a manifestation of the Coriolis effect in a noninertial reference frame attached to the wave. In the most general case, when both the direction of propagation and the state of the wave are varied, the phase is given by a simple expression that unifies the spin redirection Berry phase and the Pancharatnam-Berry phase. The theory is supported by the experiment demonstrating the spin-orbit coupling of electromagnetic waves via a surface plasmon nanostructure. The measurements verify the unified geometric phase, demonstrated by the observed polarization-dependent shift (spin-Hall effect) of the waves.

Manipulation of polarization-dependent multivortices with quasi-periodic subwavelength structures.

Multiple vortices with different topological charges are formed by the use of two sequential geometric phase elements. These elements are realized by quasi-periodic subwavelength gratings. The first element is a spiral phase element and the second element is a spherical phase element. We provide a theoretical analysis and an experimental demonstration of the formation of the multiple vortices that comprise scalar vortices and a vectorial vortex.

Space-variant polarization manipulation for far-field polarimetry by use of subwavelength dielectric gratings.

A method for polarimetric measurement that uses a discrete space-variant subwavelength dielectric grating is presented. One retrieves the polarization state by measuring the far-field intensity of a beam emerging from the grating followed by a polarizer. The analysis for a partially polarized, quasi-monochromatic beam is performed by use of the beam coherence polarization matrix along with an extended van Cittert-Zernike theorem. We experimentally demonstrate polarization measurements of both fully and partially polarized light.