arXiv:1001.4339v1 [physics.optics] 25 Jan 2010 Twisted split-ring-resonator photonic metamaterial with huge optical activity M. Decker1, R. Zhao2,3, C. M. Soukoulis3,4, S. Linden1, and M. Wegener1 1Institut f��ur Angewandte Physik and DFG-Center for Functional Nanostructures (CFN), Karlsruhe Institute of Technology (KIT), D-76128 Karlsruhe, Germany, and Institut f��ur Nanotechnologie, Karlsruhe Institute of Technology (KIT), D-76021 Karlsruhe, Germany 2Applied Optics Beijing Area Major Laboratory, Department of Physics, Beijing Normal University, Beijing 100875, China 3Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, U.S.A. 4IESL-FORTH and Dept. of Materials Science and Technology, University of Crete, 71110, Heraklion, Greece Coupled split-ring-resonator metamaterials have previously been shown to exhibit large coupling effects, which are a prerequisite for obtaining large effective optical activity. By a suitable lateral arrangement of these building blocks, we completely eliminate linear birefringence and obtain pure optical activity and connected circular optical dichroism. Experiments at around 100-THz frequency and corresponding modeling are in good agreement. Rotation angles of about 30 degrees for 205 nm sample thickness are derived. 1

Optical activity in effective media refers to a difference ���n = nRCP ��� nLCP in the real parts of the refractive indices for left- and right handed circularly polarized incident light. The Kramers-Kronig relations connect these differences to the imaginary parts of the refrac- tive indices, i.e., to circular dichroism. Optical activity requires a magnetic-dipole response mediated by the electric field of the electromagnetic light wave [1���3]. In natural substances like solutions of chiral molecules, these effects are quite small, i.e., |���n| ��� 1. Strong effective magnetic dipoles can arise from the coupling of Mie-like electric-dipole resonances. For ex- ample, coupled gold crosses have recently been discussed in this context [4,5]. Yet stronger coupling effects have been reported for twisted split-ring resonators (SRR) [6]. However, the latter structures do not only exhibit strong optical activity but also strong linear birefrin- gence. A clean separation is desirable for both understanding the physics and for applications such as ���poor-man���s��� optical isolators [7]. In this Letter, we discuss a novel lateral arrangement of such twisted SRR that, by sym- metry, exhibits vanishing linear optical birefringence. This structure has been mentioned theoretically previously [8]. The circular dichroism measured by us translates to values of |���n| ��� 2, which outperform our previous best results on twisted crosses [4] by about a factor of six. The twisted SRR pair in Ref. [6] exhibits an obvious asymmetry between the x and the y-direction. To avoid the resulting linear birefringence, we design a square unit cell which is formed by four of these pairs, whereby the pairs are rotated by 0, 90, 180, and 270 degrees with respect to the stacking axis (see Fig.1(a)). The resulting overall crystal structure has four-fold rotational symmetry, no center of inversion, and no mirror planes. Hence, it is truly Fig. 1. (a) Illustration of our metamaterial���s chiral unit cell composed of gold SRR. The lateral dimensions of the SRR are indicated on the left-hand side. (b) Electron micrographs of a typical fabricated structure. The normal-incidence image illustrates the high alignment accuracy of the two stacked layers. The inset shows an oblique view onto the sample. The scalebars are 400nm. 2