Metamaterials: Optical Activity without Chirality

Abstract

We report that the classical phenomenon of optical activity, which is traditionally associated with chirality (helicity) of organic molecules, proteins and inorganic structures, can be observed in artificial media which exhibit neither volume nor planar chirality (helicity). We have designed a metamaterial film periodically structured on the subwavelength scale which behaves indistinguishably from chiral three-dimensional media yielding strong circular dichroism and birefringence in transmission for waves incident at oblique angles.The phenomenon of optical activity that is the ability to rotate the polarization state of light is a fundamental effect of electrodynamics which is traditionally associated with mirror asymmetry (chirality) of organic molecules. The effect has enormous importance for analytical chemistry, crystallography, molecular biology, and the food industry and is also a signature effect used to detect life forms in space missions. The recognition of chirality as a source negative refraction of light [1-6] needed for the creation of a perfect lens [7] inspired intense work in developing microwave and optical artificial chiral metamaterials [8][9][10][11][12] and yielded a demonstration of negative index due to chirality [8,13]. In this paper we present a somewhat surprising result that very strong optical activity may be seen in a metamaterial system consisting of meta-molecules that itself are not chiral. Here chirality is drawn from the mutual orientation of the wave propagation direction and the two-dimensional metamaterial. We demonstrate the optical activity effect using an artificially created non-chiral planar metamaterial structure and show that it behaves indistinguishably from chiral three-dimensional molecular systems manifesting resonant birefringence and dichroism for circularly polarized electromagnetic waves.Through the efforts of several generations of researchers optical activity was linked to the threedimensional property of molecules known as chirality: a molecular structure such as a helix for which mirror images are not congruent possesses a sufficient asymmetry to manifest polarization rotation (optical activity). The effect of optical activity is linked to the phenomenon of circular dichroism, i.e. differential absorbtion for left and right circular polarizations. The recent effort in creating artificial optically active metamaterials that aimed to achieve strong optical activity was focused on different types of arrays of 3D-chiral meta-molecules [8][9][10][11][12][14][15][16][17]. It is significantly less acknowledged that optical activity can also be seen when oriented non-chiral molecules make a chiral triad with the wave vector of light. This mechanism of optical activity was first described by Bunn [18] and detected in liquid crystals [19]. Here we show that this is a highly significant mechanism of optical activity in metamaterials that can be seen as essentially planar structures that possess neither 2D chirality [20,21] nor FIG. 1: (Color online) Planar slit m...

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Evidence of Optical Circular Dichroism in GaAs‐Based Nanowires Partially Covered with Gold

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