12 May 2013

Technion scientists developed a new class of photonic materials, named Spin-Optical Metamaterial, which is based on nano-antennas controlling radiative modes assisted through the spin of light. The discovery will enable the realization of new optical surface components on the nano-scale (metasurfaces), which are based on the fundamental elements of the optical antenna’s material (optical meta-atoms), and symmetry properties – the breaking of symmetry of the photonic structure. By designing the metasurface symmetry properties with space-variant oriented anisotropic nano-antennas, it is possible to control the light-matter interaction and to design the radiative and surface optical modes.

The scientists have recently published their findings on the experimental “Optical Rashba Effect,” which enables addition or removal of light momentum by controlling the spin of light (the intrinsic spin degree of photon - circular polarization) and the symmetry properties (breaking of symmetry) of the structure. This breakthrough research was done by scientists from Professor Erez Hasman’s research group – comprising of Dr. Vladimir Kleiner, and his research students Nir Shitrit, Igor Yulevich, Elhanan Maguid, Dror Ozeri and Dekel Veksler. According to Professor Hasman from Technion’s Faculty of Mechanical Engineering: “In the experiment we created a structure in the shape of a “kagome” (similar to the structure of the Star of David) and we demonstrated an ability to control the behavior of thermal radiation emission with the support of an optical spin-orbit interaction as a result of the breaking of symmetry of the system.” The researchers named this scientific phenomenon the “Optical Rashba Effect” as an analogy for the Rashba effect of electrons in condensed matter. The electronic Rashba effect allows to control electrons with the help of the spin under broken inversion symmetry of the structure. This effect is used today in the new field of “Spintronics” that allows to control nanoelectronic components by the spin of electrons.


(from Science 10 May 2013 online: www.sciencemag.org):
Light-matter interaction control by spin-optical metamaterials
Thermal radiation modes emitted by spin-optical metamaterial based on inversion asymmetric kagome lattice. The illuminated color spirals symbolize the photon helicity-split due to optical Rashba effect. False-colored electric field distribution near the metasurface obtained by numerical simulation. The design of photonic metasurface symmetries via geometric gradient of the optical antenna patterns provides a route to control light-matter interaction at the nanoscale.

The inspiration for the study was derived from solid-state research: many studies have been done on natural anti-ferromagnetic crystalline structures in the form of kagome. In these materials, there is a physical effect termed “Frustration”: the magnetic moments can be arranged in different orientations (different phases) but the energy continues to be degenerated. Technion scientists noticed that there are significant differences in the symmetry of the phases: the reorder of the local magnetic moments transforms the lattice from an inversion symmetric to an asymmetric structure. Hence, Hasman's group selected the kagome as a platform for investigating the symmetry influence on spin-based manipulation of photonic materials. It is well known that the physical laws of conservation are determined by the symmetry of the system. Noether's theorem states that for every symmetry there is a corresponding conservation law; particularly, invariance with respect to spatial translation or rotation correspond to conservation of linear and angular momenta, respectively. Moreover, when the spatial inversion symmetry is violated, one can expect to observe spin-split dispersion (dispersion is the dependent of mode's frequency on the momentum). Instead of different spatial arrangements of magnetic moments, Hasman's group used anisotropic antennas geometrically arranged in such a way that their orientations are aligned with the original spin direction in the magnetic kagome phases.

Optical nano-antennas can create local electromagnetic modes dependant on the geometry of the antenna. The researchers used meta-material based on the alignment of antennas upon SiC, and measured the thermal emission from the different structures. The scientists showed that polarization-controlled optical modes of metamaterials arise where the spatial inversion symmetry is violated. They observed spin-split dispersion of the thermal emission originates from the spin-orbit interaction of light, generating a selection rule based on symmetry restrictions in the spin-optical metamaterials. The design of metamaterial symmetries via geometric gradient of the antennas' configuration provides the route for spin-controlled nanophotonic applications.  

Technion scientists believe that their discovery will facilitate the development of new type of optical components and nanophotonic devices that will make possible to implement optical logic gates controlled by photon-spin, novel optical computing components, new light sources dependant on spin, ultra thin surface lenses, laser beam shaping using nanoscale surface-optics, and control over thermal radiation. Using the novel photonic materials, it will be possible to design the properties of the emission, absorption, transmission and scattering of light, as well as manipulating the surface waves for photonic chips. As an outcome of the study, Professor Hasman intends to expand his research group and has plans to recruit additional excellent research students from different areas, in order continue to develop the field of Spinoptics studied by his group over the last thirteen years. For additional information visit: www.technion.ac.il/optics