Igor Yulevich

Spin-optical metamaterial route to spin-controlled photonics.

Making Metamaterials Controlling the propagation of electromagnetic waves is a key requirement in communication technologies. The components tend to be bulky, however, which can make it difficult to integrate with microelectronics circuits. Using arrays of metallic nanoantennae patterned on a substrate surface, Shitrit et al. (p. 724) fabricated a novel class of metamaterials: anisotropic materials without inversion symmetry. The materials may pave the way to polarization-dependent nanophotonics. Designed arrays of metallic nanoantennas provide a route for the polarization-dependent propagation of light. Spin optics 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 spin optics 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 spin-optical 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.

Photonic spin-controlled multifunctional shared-aperture antenna array

Multifunction planar optics Specially designed two-dimensional (2D) arrays of nanometer-scale metallic antennas, or metasurfaces, may allow bulky optical components to be shrunk down to a planar device structure. Khorasaninejad et al. show that arrays of nanoscale fins of TiO can function as high-end optical lenses. At just a fraction of the size of optical objectives, such planar devices could turn your phone camera or your contact lens into a compound microscope. Maguid et al. interleaved sparse 2D arrays of metal antennas to get multifunctional behavior from the one planar device structure (see the Perspective by Litchinitser). The enhanced functionality of such designed metasurfaces could be used in sensing applications or to increase the communication capacity of nanophotonic networks. Science, this issue pp. 1190 and 1202; see also p. 1177 A 2D nanophotonic system can be designed with multifunctional optical capability. The shared-aperture phased antenna array developed in the field of radar applications is a promising approach for increased functionality in photonics. The alliance between the shared-aperture concepts and the geometric phase phenomenon arising from spin-orbit interaction provides a route to implement photonic spin-control multifunctional metasurfaces. We adopted a thinning technique within the shared-aperture synthesis and investigated interleaved sparse nanoantenna matrices and the spin-enabled asymmetric harmonic response to achieve helicity-controlled multiple structured wavefronts such as vortex beams carrying orbital angular momentum. We used multiplexed geometric phase profiles to simultaneously measure spectrum characteristics and the polarization state of light, enabling integrated on-chip spectropolarimetric analysis. The shared-aperture metasurface platform opens a pathway to novel types of nanophotonic functionality.

Multifunctional interleaved geometric-phase dielectric metasurfaces

Shared-aperture technology for multifunctional planar systems, performing several simultaneous tasks, was first introduced in the field of radar antennas. In photonics, effective control of the electromagnetic response can be achieved by a geometric-phase mechanism implemented within a metasurface, enabling spin-controlled phase modulation. The synthesis of the shared-aperture and geometric-phase concepts facilitates the generation of multifunctional metasurfaces. Here shared-aperture geometric-phase metasurfaces were realized via the interleaving of sparse antenna sub-arrays, forming Si-based devices consisting of multiplexed geometric-phase profiles. We study the performance limitations of interleaved nanoantenna arrays by means of a Wigner phase-space distribution to establish the ultimate information capacity of a metasurface-based photonic system. Within these limitations, we present multifunctional spin-dependent dielectric metasurfaces, and demonstrate multiple-beam technology for optical rotation sensing. We also demonstrate the possibility of achieving complete real-time control and measurement of the fundamental, intrinsic properties of light, including frequency, polarization and orbital angular momentum.

Rashba-like spin degeneracy breaking in coupled thermal antenna lattices

Observation of a spin degeneracy breaking in thermal radiation emitted from an inhomogeneous anisotropic lattice composed of coupled antennas supporting surface waves is presented. The spin degeneracy removal is manifested by a spin-dependent momentum splitting of the radiative mode which resembles the Rashba effect. The spin split dispersion arises from the inversion asymmetry of the lattice. Our experiment confirms that the spatial rate of the inhomogeneity determines the degree of the spin- dependent momentum redirection. The influence of the inversion asymmetry on the dispersion was studied by comparing the results to those produced by homogeneous lattices and characterizing the behavior of the isolated thermal antennas.

Spin-controlled plasmonics via optical Rashba effect

Observation of the optical Rashba effect in plasmonics is reported. Polarization helicity degeneracy removal, associated with the inversion symmetry violation, is attributed to the surface symmetry design via anisotropic nanoantennas with space-variant orientations. By utilizing the Rashba-induced momentum in a nanoscale kagome metastructure, we demonstrated a spin-based surface plasmon multidirectional excitation under a normal-incidence illumination. The spin-controlled plasmonics via spinoptical metasurfaces provides a route for spin-based surface-integrated photonic nanodevices and light-matter interaction control, extending the light manipulation capabilities.

Optical Mode Control by Geometric Phase in Quasicrystal Metasurface.

We report on the observation of optical spin-controlled modes from a quasicrystalline metasurface as a result of an aperiodic geometric phase induced by anisotropic subwavelength structure. When geometric phase defects are introduced in the aperiodic structured surface, the modes exhibit polarization helicity dependence resulting in the optical spin-Hall effect. The radiative thermal dispersion bands from a quasicrystal structure are studied where the observed bands arise from the optical spin-orbit interaction induced by the aperiodic space-variant orientations of anisotropic antennas. The optical spin-flip behavior of the revealed modes that arise from the geometric phase pickup is experimentally observed within the visible spectrum by measuring the spin-projected diffraction patterns. The introduced ability to manipulate the light-matter interaction of quasicrystals in a spin-dependent manner provides the route for molding light via spin-optical aperiodic artificial planar surfaces.

Disorder-induced optical transition from spin Hall to random Rashba effect

Phase transition of scattered light Disordered structures can give rise to intriguing scattering phenomena owing to the unpredictable nature of their interaction with light. Using subwavelength-scale disordered metasurfaces, Maguid et al. observed a phase transition in how the light is scattered as a function of disorder. Weak disorder induced a photonic spin Hall effect, whereas strong disorder led to spin-split modes in momentum space, a random optical-Rashba effect. Thus, designed photonic structure could offer a versatile platform to study similar phenomena in complex solid-state systems. Science, this issue p. 1411 An optical phase transition is observed as a function of disorder of the scattering structure. Disordered structures give rise to intriguing phenomena owing to the complex nature of their interaction with light. We report on photonic spin-symmetry breaking and unexpected spin-optical transport phenomena arising from subwavelength-scale disordered geometric phase structure. Weak disorder induces a photonic spin Hall effect, observed via quantum weak measurements, whereas strong disorder leads to spin-split modes in momentum space, a random optical Rashba effect. Study of the momentum space entropy reveals an optical transition upon reaching a critical point where the structure’s anisotropy axis vanishes. Incorporation of singular topology into the disordered structure demonstrates repulsive vortex interaction depending on the disorder strength. The photonic disordered geometric phase can serve as a platform for the study of different phenomena emerging from complex media involving spin-orbit coupling.

Spinoptical Metamaterials: A Novel Class of Metasurfaces

Photonic metasurfaces are metamaterials with reduced dimensionality composed of engineered subwavelength-scale meta-atoms enabling a custom-tailored electromagnetic response of the medium. These 2-D metastructures are also at the forefront of the physical enigma: What is the effect of surface symmetry properties on light-matter interactions?

Spin-controlled multifunctional metasurfaces

Multitasking shared-aperture systems have initially emerged as radar phased array antennas. Recently, the shared-aperture concept has been suggested as a platform for multifunctional optical phased array antennas, accomplished by a reflective metasurface [1]. Metasurfaces consist of metallic or dielectric subwavelength nanoantennas, capable of manipulating light by controlling the local amplitude and phase of an incident electromagnetic wave [2-6]. An effective control of the electromagnetic response can be achieved by a geometric phase mechanism implemented within a metasurface, enabling spin-controlled phase modulation. Shared-aperture geometric phase metasurface (GPM) paves the way for multifunctional nano-optical device. Shared-aperture interleaved phased arrays are formed by the random interspersing of sub-arrays, thus resulting in a device with high flexibility in multifunctional wavefront generation and the angular resolution of the shared aperture. Each sub-array is associated with a specific phase function, sparsely sampled at randomly chosen lattice points. We presented multifunctional spin-dependent dielectric metasurfaces, and demonstrated multiple-beam technology for complete real-time control and measurement of the fundamental intrinsic properties of light, including frequency, polarization, and orbital angular momentum (OAM) [7].

Shared-aperture geometric phase metasurfaces

Shared-aperture multifunctional planar systems (in which a number of tasks are performed concurrently) have recently been introduced in the field of phased array antennas, for radar applications.1 Indeed, these shared-aperture phased antenna arrays—see Figure 1(a)—are a promising way to increase functionality in photonics. Recent achievements in the fast-growing field of metasurfaces (i.e., metamaterials of reduced dimensionality) are particularly relevant because they provide a route to developing virtually flat optics. Such metasurfaces consist of a dense arrangement of small-scale resonant optical antennas. For example, in a phased antenna array, phase accumulators are arranged so that they form a wavefront (because of phase differences across the array). Light–matter interactions of individual nanoantennas therefore allow local light scattering properties to be controlled with these arrays. The local phase pickup in a phased antenna array can be manipulated by changing the antenna material, size, shape, and environment (known as antenna resonance shaping), or via the geometric phase concept. The latter concept is fundamental to geometric phase metasurfaces (GPMs), in which the phase pickup originates from space-variant orientations of the anisotropic nanoantennas that compose the metasurface. In addition, the geometric phase concept is an efficient way to achieve spin-controlled phase modulation, whereas the photon spin is associated with the intrinsic angular momentum of light.2–8 GPMs can also be used to transform incident circularly polarized light into a beam of opposite helicity (imprinted with a geometric phase). In our work,9 we have used a new technique to synthesize shared-aperture phased antenna arrays. We combine these arrays with the geometric phase concept to realize multifunctional GPM photonic arrays. In addition, we have incorporated Figure 1. Schematic illustrations of (a) the shared-aperture concept and (b) structured wavefronts emerging from an interleaved geometric phase metasurface. In (a) each wavefront is represented by a different color. The wavefronts are illustrated with solid lines and the dashed lines indicate the propagation directions.