Optical activity in extrinsically chiral metamaterial

Abstract

We demonstrate optical activity in an intrinsically non-chiral anisotropic planar metamaterial. The phenomenon is due to extrinsic chirality resulting from the mutual orientation of the metamaterial structure and the incident electromagnetic wave. The polarization effect, which has a resonant nature, features a spectral band where linear birefringence is practically absent and can be easily tuned by tilting the plane of the metamaterial relative to the incident beam. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3021082͔Recent predictions that strong optical activity could result in a negative index of refraction 1,2 inspired intense work on developing artificial three-dimensionally ͑3D͒ chiral metamaterials, i.e., arrays of identical meta-molecules, where the meta-molecules themselves are different from their mirror image and therefore intrinsically 3D-chiral. [3][4][5][6][7][8][9] Although not widely known, optical activity in the form of circular birefringence and dichroism can also be observed in extrinsically chiral systems. In such a system a non-chiral structure together with the incident wave forms a geometrical arrangement that cannot be superimposed with its mirror image and thus the whole arrangement is chiral. This mechanism of polarization rotation was first identified by Bunn 10 and has been observed in liquid crystals. 11 In this letter we show that extrinsic chirality is a highly significant source of optical activity in metamaterials. We demonstrate strong polarization rotary power and circular dichroism at oblique incidence in a planar metamaterial, which is neither two-dimensionally ͑2D͒ chiral 12,13 nor 3D-chiral. The effect is inherently tunable: its sign and magnitude are controlled by the tilt of the metamaterial plane relative to the incident beam. Importantly, this type of tunable optical activity occurs in simple planar metamaterial designs that are ideally suited for well-established planar manufacturing technologies.We studied optical activity in a planar metamaterial structure consisting of a regular 2D array of metal split rings supported by a 1.6 mm thick dielectric substrate ͑see Fig. 1͒. The rings were split asymmetrically into pairs of arcs of different lengths separated by equal gaps. Each split ring had a line of mirror symmetry but had no axis of twofold rotation. The planar metamaterial sample was approximately 220ϫ 220 mm 2 large and had a square unit cell of 15ϫ 15 mm 2 , which ensured no diffraction at normal or oblique incidence for frequencies below 10 GHz. Our measurements were performed in an anechoic chamber using microwave broadband horn antennas ͑Schwarzbeck BBHA 9120D͒ equipped with lens concentrators and a vector network analyzer ͑Agilent E8364B͒. We measured losses and phase delays for circularly polarized waves transmitted by the metamaterial at various angles of incidence in the range from −30°to +30°achieved by tilting the sample around its symmetry axis. In practical terms we measured the complex transmission matrix E i out = t ij E j in relating the...

Silicon nanofin grating as a miniature chirality-distinguishing beam-splitter

“…Interestingly, our approach is conceptually different from previous methods for distinguishing LCP from RCP that made use of chiral structures, for example, see refs [15][16][17][18][19][20][21][22][23]. By contrast, our device is achiral and possesses neither intrinsic [15][16][17][18][19][20][21][22][23] nor extrinsic 27,28 chirality (see Supplementary Fig. 1 and Note 1).…”

Specular optical activity of achiral metasurfaces

“…1(c)] if the interface does not feature a line of mirror symmetry in the plane of incidence. Extrinsic 3D chirality is known to lead to optical activity in transmission [13][14][15], but its effect on reflected waves had not been studied.…”

Design of a Terahertz Polarization Rotator Based on a Periodic Sequence of Chiral-Metamaterial and Dielectric Slabs

“…Helicity (handedness) is one example of chirality. From left to right, chiral medium based on the random orientation of helical inclusions [17]; uniaxial chiral medium based on oriented wire helices [17]; enantiomeric helicoidal bilayered twisted rosettes [25,27]; bilayered twisted cut-wire chiral MTM [30,35,41]; uniaxial chiral metamaterial constructed by double-layered four "U" split ring resonators [32,42].…”