Optical Manifestations of Planar Chirality

Abstract

We report that planar chiral structures affect the polarization state of light in a manner similar to three-dimensional chiral (optical active) media. In experiments with artificial metal-on-silicon chiral planar gratings of 442 wallpaper group symmetry, containing millions of chiral elements per square centimeter, we observed rotation of the polarization azimuth in excess of 30 of light diffracted from it. The rotation was found to change its sign for two enantiomeric forms of the media and to have components associated with both the structural arrangement and the chirality of individual structural elements. DOI: 10.1103/PhysRevLett.90.107404 PACS numbers: 78.67.-n, 78.20.Ek The ability of left-right asymmetrical (chiral) threedimensional molecules to rotate the polarization state of light known as optical activity is one of the most remarkable effects in optics that has been extensively studied since its discovery at the beginning of the 19th century. An optical active medium (for example, a medium consisting of randomly oriented helixlike molecules) will show opposite signs of polarization azimuth rotation for the two mirror-symmetric (enantiomeric) forms of the constituting molecule. The general concept of chirality also exists in two dimensions [1][2][3][4], where a planar object is said to be chiral if it cannot be brought into congruence with its mirror image unless it is lifted from the plane. One could therefore envisage a planar chiral medium that consists of ''flat'' chiral elements possessing no line of symmetry in the plane. So far, there have been only a few theoretical publications on the optical manifestations of such planar chirality. Hecht and Barron predicted incoherent circular differential Rayleigh and Raman light scattering from an ensemble of planar chiral molecules [5]. They showed that genuine strong chiral scattering phenomena could be generated through pure electric dipole interactions (in comparison with the much weaker processes involving magneto-dipole interaction in threedimensional chirality), while Arnaut and Davis calculated the scattered fields from the two-dimensional chiral structure of a metallic wire gammadion and found rotation of the polarization azimuth of the scattered field [6]. However, there have as yet been no reports of experimental observations of any optical manifestations of planar chirality, apart from the observation of a random chiral component in a highly localized near-field polarization effect in metallic fractal aggregates [7]. Consequently, it has yet to be shown whether planar chiral media could affect the far-field polarization state of light scattered on it in a manner similar to three-dimensional chiral media when the polarization effect is sensitive to the handedness of the structure. Here we report that we have manufactured left-and right-handed forms of a regular artificial medium consisting of microscopic chiral metallic objects distributed regularly in a plane, with a density of several millions per square centimeter. In this artificial medi...

Metamaterials: a new frontier of science and technology

“…In 2003, Zheludev's group reported the first optical manifestation of chirality in a two-dimensional (2D) planar system, which was composed of a single layer of metallic rosette structures arranged in a square lattice. 55 A large rotation of polarization more than 301 was observed experimentally. The rotation was believed to be associated with both the chirality of the structure itself and the arrangement of the structures.…”

A new model of geometric chirality for two-dimensional continuous media and planar meta-materials

“…Finally, we will outline the utility of our new model by demonstrating its ease of use and its consequences. We will also compare its theoretical results with those derived using other models, as well as with the results of recent optical experiments [2,3].…”

“…If we define K i jk as the chirality index of a triangular set of points {r i , r j and r k }, then [2,3], the mass can be thought of as the integral of the 2D mass density, ρ 2D (r), over a given area. The 2D mass density is then just the product of the material density of the film, ρ (=19.3 g cm −3 for gold), and the film thickness at that point, in which case the mass term will have units of kilograms.…”

“…Recent experiments investigating the optical properties of planar chiral media [2,3] have now shown that two-dimensional (2D) chiral objects with characteristic sizes ∼2 µm are also capable of manipulating the polarization state of coherent light. In particular, it was found that linearly polarized light in the visible and infrared regions of the spectrum experiences azimuth rotation and elliptization when diffracted from arrays of gammadion-shaped chiral holes that are etched into a thin metallic film and supported on a silicon substrate.…”

“…In particular, it was found that linearly polarized light in the visible and infrared regions of the spectrum experiences azimuth rotation and elliptization when diffracted from arrays of gammadion-shaped chiral holes that are etched into a thin metallic film and supported on a silicon substrate. These 3 Author to whom any correspondence should be addressed. polarization changes were strongly correlated to the degree of chirality of the gammadions and were found to reverse sign when the handedness of the gammadions was reversed.…”