Poster MM
MM 23 Anisotropic response from a textured fourfold symmetric substrate: when Fresnel
meets Bragg
H. Wormeester1, T. W. H. Oates2, M. Jin1, E. Carlen1, H. Zandvliet1 1MESA Institute for Nanotechnology, University of Twente, The Netherlands
2Leibniz-Institut für Analytische Wissenschaften - ISAS-Berlin, Germany
Email: h.wormeester@utwente.nl
Periodic metal - dielectric structures with a characteristic length scale similar to the wavelength of light have attracted large attention for future photonic applications. New effective dielectric properties may arise from these artificially structured materials. We analyzed the optical response of a nanotextured silicon surface containing fourfold symmetric pyramids with a separation of 224 nm. This surface was covered with a 70 nm Au layer. Mueller matrix ellipsometry was used for a thorough experimental characterisation of the optical properties of such periodic structures. We found that despite the fourfold symmetry of the surface, the optical response changes substantially at oblique incidence when the sample is rotated around its normal. When the light beam is incident along the high symmetry direction, the anisotropic Mueller elements are negligible whereas a 30◦
rotation to the left or right leads to significant anisotropic Mueller elements of opposite sign, see figure below. This observation shows that the optical response of this nanotextured surface does not comply with the morphological symmetry. A similar response is also found for the bare Si pyramids, i.e. without the presence of the gold layer. A first explanation of these observations is provided by considering diffraction. With diffraction, not only the intensity, but also the polar-ization of the specular light beam can be strongly altered. This leads to an optical response of a surface that at a first glance mimics chiral behaviour.
Figure 1: Azimuthal dependence of the M13 Mueller matrix component of a fourfold symmetric nanostruc-ture.
