Francisco J. Rodríguez-Fortuño

Spin–orbit interactions of light

This Review article provides an overview of the fundamental origins and important applications of the main spin–orbit interaction phenomena in modern optics that play a crucial role at subwavelength scales. Light carries both spin and orbital angular momentum. These dynamical properties are determined by the polarization and spatial degrees of freedom of light. Nano-optics, photonics and plasmonics tend to explore subwavelength scales and additional degrees of freedom of structured — that is, spatially inhomogeneous — optical fields. In such fields, spin and orbital properties become strongly coupled with each other. In this Review we cover the fundamental origins and important applications of the main spin–orbit interaction phenomena in optics. These include: spin-Hall effects in inhomogeneous media and at optical interfaces, spin-dependent effects in nonparaxial (focused or scattered) fields, spin-controlled shaping of light using anisotropic structured interfaces (metasurfaces) and robust spin-directional coupling via evanescent near fields. We show that spin–orbit interactions are inherent in all basic optical processes, and that they play a crucial role in modern optics.

Near-Field Interference for the Unidirectional Excitation of Electromagnetic Guided Modes

Controlling Light Propagation Surface plasmons are light-induced collective electronic excitations in a metal that offer the possibility of manufacturing optoelectronic devices at nanometer scale. Before such shrinking can be achieved, the propagation direction and lifetime of the plasmonic excitations have to be controlled (see the Perspective by Miroshnichenko and Kivshar). Rodríguez-Fortuño et al. (p. 328) show how this is done using polarized light. Alternatively, using an array of metallic nanoantennae (in this case, slits) patterned into a thin gold film, Lin et al. (p. 331) present a further improvement on current plasmonic coupling schemes that has the potential to encode information contained in both the intensity and polarization of light. Near-field interference can be used to control the directional propagation of electromagnetic excitations. [Also see Perspective by Miroshnichenko and Kivshar] Wave interference is a fundamental manifestation of the superposition principle with numerous applications. Although in conventional optics, interference occurs between waves undergoing different phase advances during propagation, we show that the vectorial structure of the near field of an emitter is essential for controlling its radiation as it interferes with itself on interaction with a mediating object. We demonstrate that the near-field interference of a circularly polarized dipole results in the unidirectional excitation of guided electromagnetic modes in the near field, with no preferred far-field radiation direction. By mimicking the dipole with a single illuminated slit in a gold film, we measured unidirectional surface-plasmon excitation in a spatially symmetric structure. The surface wave direction is switchable with the polarization.

Photonic spin Hall effect in hyperbolic metamaterials for polarization-controlled routing of subwavelength modes

The routing of light in a deep subwavelength regime enables a variety of important applications in photonics, quantum information technologies, imaging and biosensing. Here we describe and experimentally demonstrate the selective excitation of spatially confined, subwavelength electromagnetic modes in anisotropic metamaterials with hyperbolic dispersion. A localized, circularly polarized emitter placed at the boundary of a hyperbolic metamaterial is shown to excite extraordinary waves propagating in a prescribed direction controlled by the polarization handedness. Thus, a metamaterial slab acts as an extremely broadband, nearly ideal polarization beam splitter for circularly polarized light. We perform a proof of concept experiment with a uniaxial hyperbolic metamaterial at radio-frequencies revealing the directional routing effect and strong subwavelength λ/300 confinement. The proposed concept of metamaterial-based subwavelength interconnection and polarization-controlled signal routing is based on the photonic spin Hall effect and may serve as an ultimate platform for either conventional or quantum electromagnetic signal processing.

Spin–orbit coupling in surface plasmon scattering by nanostructures

The spin Hall effect leads to the separation of electrons with opposite spins in different directions perpendicular to the electric current flow because of interaction between spin and orbital angular momenta. Similarly, photons with opposite spins (different handedness of circular light polarization) may take different trajectories when interacting with metasurfaces that break spatial inversion symmetry or when the inversion symmetry is broken by the radiation of a dipole near an interface. Here we demonstrate a reciprocal effect of spin-orbit coupling when the direction of propagation of a surface plasmon wave, which intrinsically has unusual transverse spin, determines a scattering direction of spin-carrying photons. This spin-orbit coupling effect is an optical analogue of the spin injection in solid-state spintronic devices (inverse spin Hall effect) and may be important for optical information processing, quantum optical technology and topological surface metrology.

Double-negative polarization-independent fishnet metamaterial in the visible spectrum

We show that a second-order magnetic resonance present in the fishnet metamaterial can be enhanced so as to achieve simultaneous negative permittivity and permeability in the visible range. The double-negative behavior leads to reduced losses in this particular fishnet metamaterial. We also study the stacking of several functional layers, verifying the convergence of the refractive index.

Squeezing and expanding light without reflections via transformation optics.

We study the reflection properties of squeezing devices based on transformation optics. An analytical expression for the angle-dependent reflection coefficient of a generic three-dimensional squeezer is derived. In contrast with previous studies, we find that there exist several conditions that guarantee no reflections so it is possible to build transformation-optics-based reflectionless squeezers. Moreover, it is shown that the design of antireflective coatings for the non-reflectionless case can be reduced to matching the impedance between two dielectrics. We illustrate the potential of these devices by proposing two applications in which a reflectionless squeezer is the key element: an ultra-short perfect coupler for high-index nanophotonic waveguides and a completely flat reflectionless hyperlens. We also apply our theory to the coupling of two metallic waveguides with different cross-section. Finally, we show how the studied devices can be implemented with non-magnetic isotropic materials by using a quasi-conformal mapping technique.

Highly-sensitive chemical detection in the infrared regime using plasmonic gold nanocrosses

In this work, we report the design, fabrication, and characterization of gold nanocrosses for chemosensing purposes. The nanocrosses are designed to exhibit a localized surface plasmon resonance which is very sensitive to refractive index changes in the surrounding medium, resulting in sensitivity values of around 500–700 nm per refractive index unit at wavelengths around 1.4 μm. We experimentally demonstrate the functionalization of the gold nanocrosses and the successful sensing of chemical monolayers.

Sorting linearly polarized photons with a single scatterer

Intuitively, light impinging on a spatially mirror-symmetric object will be scattered equally into mirror-symmetric directions. This intuition can fail at the nanoscale if the polarization of the incoming light is properly tailored, as long as mirror symmetry is broken in the axes perpendicular to both the incident wave vector and the remaining mirror-symmetric direction. The unidirectional excitation of plasmonic modes using circularly polarized light has been recently demonstrated. Here, we generalize this concept and show that linearly polarized photons impinging on a single spatially symmetric scatterer created in a silicon waveguide are guided into a certain direction of the waveguide depending exclusively on their polarization angle and the structure asymmetry. Our work broadens the scope of polarization-induced directionality beyond plasmonics, with applications in polarization (de)multiplexing, unidirectional coupling, directional switching, radiation polarization control, and polarization-encoded quantum information processing in photonic integrated circuits.

Negative refractive index metamaterials aided by extraordinary optical transmission

We study under which conditions extraordinary optical transmission (EOT) structures can be used to build negative refractive index media. As a result, we present a metamaterial with superimposed EOT and negative index at visible wavelengths. The tailoring process starting from a simple hole array until achieving the negative index is detailed. We also discuss the so-called fishnet metamaterial (previously linked to EOT) under the same prism. Using the ideas put forward in this work, other structures with negative index could be engineered in the optical or visible spectrum.

Universal method for the synthesis of arbitrary polarization states radiated by a nanoantenna

Optical nanoantennas efficiently convert confined optical energy into free-space radiation. The polarization of the emitted radiation depends mainly on nanoantenna shape, so it becomes extremely difficult to manipulate it unless the nanostructure is physically altered. Here, a simple way is demonstrated to synthetize the polarization of the radiation emitted by a single nanoantenna so that every point on the Poincare sphere becomes attainable. The nanoantenna consists of a single scatterer created on a dielectric waveguide and fed from its both sides so that the polarization of the emitted optical radiation is controlled by the amplitude and phase of the feeding signals. The nanoantenna is created on a silicon chip using standard top-down nanofabrication tools, but the method is universal and can be applied to other materials, wavelengths and technologies. This work will open the way towards the synthesis and control of arbitrary polarization states in nano-optics.