Optical Properties of Planar Chiral Meta-Materials
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Optical Properties of Planar Chiral Meta-Materials A. Potts, A. Papakostas1, N.I. Zheludev, H.J. Coles, R. Greef 2 and D.M. Bagnall1 Department of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK 1 Department of Electronics and Computer Science, University of Southampton, 2 Department of Chemistry, University of Southampton,
ABSTRACT The polarization state of visible light is found to be altered upon reflection from artificial two-dimensional chiral media. Arrays of metallic planar chiral structures were fabricated by electron beam lithography and ion beam milling. The characteristic dimensions on the chiral elements correspond to wavelengths in the near-IR. Our chiral media are found to induce strong polarization effects, with the handedness of individual elements having a direct effect on the sense and magnitude of rotation of the diffracted light. INTRODUCTION The ability of left- and right-handed asymmetrical (chiral) three-dimensional helical molecules to rotate the polarization state of light (known as optical activity) is one of the cornerstones of modern optics. However, this concept of chirality also exists in two-dimensions [1], 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. The question as to whether planar medium could affect the polarization state of light, in a manner similar to three-dimensional chiral molecules, has so far remained unanswered. Recently, there has been considerable theoretical interest in this subject, with many authors [2-4] proposing planar structures with chirality that could exhibit selective electromagnetic properties. Now, using electron beam lithographic techniques, we have manufactured left and right-handed forms of an artificial medium consisting of microscopic chiral metallic objects distributed regularly in a plane. In this artificial medium we have observed optical manifestations of planar chirality in the form of handedness-sensitive rotations of the polarization state and elliptization of visible light diffracted from the chiral structures. DEVICE DESIGN AND FABRICATION PROCESS The artificial two-dimensional media that we have created consist of microscopic chiral metallic objects (gammadions) that are arranged in regular two-dimensional square gratings to produce a wallpaper structure of 442 group symmetry (see figure 1).
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Figure 1. A SEM image of an array of right-handed gammadions etched in a Ti/Au/Ti film on a silicon substrate.
Figure 2. A high magnification SEM image of an array of left-handed gammadions with the internal angle (θ) and characteristic length (ξ) marked.
Arrays of both clockwise and anticlockwise gammadions, but of otherwise identical design specifications, were fabricated to pr
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