Asymmetric transport is an uneven physical response of counter-propagating signals that has significantly contributed to fundamental science and revolutionized advanced technology via a variety of significant devices including diodes and isolators in electronics, optics, acoustics, and heat transfer. Photonic metasurfaces are two-dimensional ultrathin arrays of engineered subwavelength meta-atoms, acting as local phase shifters, which unprecedentedly mold wavefronts at will with a virtually flat optical element. While such an architecture can be potentially harnessed to achieve two-way asymmetric response of free-space light at an optically thin flatland, asymmetric light transport cannot be fundamentally achieved by any linear system including linear metasurfaces. Here, we report asymmetric transport of free-space light at nonlinear metasurfaces, with harmonic generation, upon transmission and reflection. We also derive the nonlinear generalized Snell’s laws of reflection and refraction which were experimentally verified by angle-resolved anomalous refraction and reflection of the nonlinear light. The asymmetric transport at optically thin nonlinear interfaces is revealed by comparing the original path of light through the metasurface with its corresponding reversed propagation path. Such a two-way asymmetric response at metasurfaces opens a new paradigm for free-space ultrathin lightweight optical devices with one-way operation including unrivaled optical valves and diodes.
Spinoptics provides a route to control light, whereby the photon helicity (spin angular momentum) degeneracy is removed due to a geometric gradient onto a metasurface. The alliance of spinoptics and metamaterials offers the dispersion engineering of a structured matter in a polarization helicity dependent manner. We show that polarization-controlled optical modes of metamaterials arise where the spatial inversion symmetry is violated. The emerged spin-split dispersion of spontaneous emission originates from the spin-orbit interaction of light, generating a selection rule based on symmetry restrictions in a spinoptical metamaterial. The inversion asymmetric metasurface is obtained via anisotropic optical antenna patterns. This type of metamaterial provides a route for spin-controlled nanophotonic applications based on the design of the metasurface symmetry properties.
KEYWORDS: Plasmonics, Antennas, Polarization, Dispersion, Nanostructures, Near field optics, Scattering, Near field scanning optical microscopy, Scanning electron microscopy, Optical components
Spin-symmetry breaking in nanoscale structures caused by spin-orbit interaction, leading to a new branch in optics -
spinoptics is presented. The spin-based effects offer an unprecedented ability to control light and its polarization state in
nanometer-scale optical devices, thereby facilitating a variety of applications related to nano-photonics. The direct
observation of optical spin-Hall effect that appears when a wave carrying spin angular momentum (AM) interacts with
plasmonic nanostructures is introduced. A plasmonic nanostructure exhibits a crucial role of an AM selection rule in a
light-surface plasmon scattering process. A spin-dependent dispersion splitting was obtained in a structure consisting of a
coupled thermal antenna array. The observed effects inspire one to investigate other spin-based plasmonic effects and to
propose a new generation of optical elements for nano-photonic applications.
KEYWORDS: Near field optics, Plasmonics, Antennas, Polarization, Electrons, Anisotropy, Scattering, Spiral phase plates, Near field scanning optical microscopy, Scanning electron microscopy
Spin-Hall effect is a basic phenomenon arising from the spin-orbit coupling of electrons. In particular, the spatial
trajectory of the moving electrons is affected by their intrinsic angular momentum. The optical spin-Hall effect (OSHE)
- beam deflection due to the optical spin (polarization helicity) - was recently presented. The effect was attributed to the
optical spin-orbit interaction occurring when the light passes through an anisotropic and inhomogeneous medium. Here,
we present and experimentally observe the OSHE in coupled localized plasmonic chains. The OSHE is due to the
interaction between the optical spin and the path of the plasmonic chain with an isotropic plasmonic mode. In addition,
OSHE was observed due to the interaction between the optical spin and the local anisotropy plasmonic mode, which is
independent on the chain path. A spin-dependent orbital angular momentum was observed in a circular path. Moreover,
a wavefront phase dislocation due to the scattering of surface plasmons from a topological defect is directly measured in
the near-field by means of interference. The dislocation strength is shown to be equal to the incident optical spin and
with analogy to the magnetic flux parameter in the Aharonov-Bohm effect. OSHE in spontaneous emission was also
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 OSHE in the nanoscale provides an additional degree of freedom in spin-based optics.
The spin-Hall effect - the influence of the intrinsic spin on the electron trajectory, which produces transverse
deflection of the electrons, is a central tenet in the field of spintronics. Apparently, the handedness of the light's
polarization (optical spin up/down) may provide an additional degree of freedom in nanoscale photonics. The direct
observation of optical spin-Hall effect that appears when a wave carrying spin angular momentum interacts with
plasmonic nanostructures is presented. The measurements verify the unified geometric phase, demonstrated by the
observed spin-dependent deflection of the surface waves as well as spin-dependent enhanced transmission through
coaxial nanoapertures even in rotationally symmetric structures. Moreover, spin-orbit interaction is demonstrated by
use of inhomogeneous and anisotropic subwavelength dielectric structures. The observed effects inspire one to
investigate other spin-based plasmonic effects and to propose a new generation of optical elements for nanophotonic
applications.
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