Circular dichroism of planar chiral magnetic metamaterials

Abstract

We propose, fabricate, and study a double-layer chiral planar metamaterial that exhibits pronounced circular dichroism at near-infrared wavelengths. The antisymmetric oscillation modes of the two coupled layers allow local magnetic-dipole moments and enhanced polarization effects compared with similar single-layer systems where only electric-dipole moments occur. Experiment and rigorous theoretical calculations are in good agreement. © 2007 Optical Society of America OCIS codes: 160.4760, 260.5430. Planar chiral metallic structures have recently been introduced. [1][2][3][4] These structures are composed of periodically arranged metallic building blocks that are chiral, i.e., that cannot be brought into congruence with their mirror image (the so-called enantiomer) unless lifted off the substrate. Thus, the optical response of these structures is different for righthanded circularly polarized (rcp) and left-handed circularly polarized (lcp) incident light, respectively. Recent work 3,4 has shown that the polarization behavior neither violates reciprocity nor time-reversal symmetry, in contrast to earlier claims.2 This means that the polarization behavior is roughly similar to optical activity (though clearly not isotropic) but is distinct from the Faraday effect. Intuitively, this behavior is directly connected to the planar nature of these structures: while a three-dimensional spiral or helix keeps its handedness when looked at from the other side, a planar chiral structure obviously changes its handedness.The optics of these planar chiral structures could be understood in terms of plasmon modes leading to local electric-dipole moments oscillating in a particular manner. Here, we discuss a novel structure where two metal layers are separated by a dielectric spacer [ Fig. 1(a)]. As a result, strong magnetic-dipole moments (or, equivalently, electric quadrupoles) can occur as well. We show that the resulting polarization effects are yet stronger than for single-layer structures. Our spectroscopic experiments are in good agreement with rigorous theoretical calculations.In our samples, the chiral building blocks or "photonic atoms" are arranged in a square lattice with lattice constant a = 340 nm. We emphasize that the operation wavelength is large with respect to the lattice constant such that our structures can be viewed as effective materials in the same spirit as, e.g., magnetic 5,6 or negative-index photonic metamaterials.7-9 Precisely, we have / a Ͼ 2 ( / a =2 is the fundamental Bragg condition), whereas previous work had / a Ϸ 0.16 (Refs. 1 and 2) and / a Ϸ 1 (Ref. We have also fabricated gammadion structures with angles other than 90°between the long and the short arms (e.g., 45°). These structures show smaller polarization effects and are not discussed here. Indeed, from symmetry it is obvious that structures with 0°and 180°a ngles would not exhibit any circular dichroism (CD) at all.For the optical characterization we perform broadband linear-optical transmittance spectroscopy with circularly polarized...

Metamaterials: a new frontier of science and technology

“…Pronounced chiral behaviours such as optical activity and circular dichroism were observed in the optical region based on nanoscaled bilayered chiral structures. [58][59][60] Very recently, light propagation through three-dimensional gold helices has been investigated in the mid-infrared region. 61 The structure can block the circular polarization with the same handedness as the helices, while it transmits the other with very high efficiency over a broad wavelength band (Fig.…”

Twisted optical metamaterials for planarized ultrathin broadband circular polarizers

“…In part inspired by biological species that are well equipped with broadband, conformal circular polarization sensors 12 , optical activity and circular dichroism have been shown in either planar [13][14][15][16][17][18] or three-dimensional metamaterials 3,19,20 . Planar structures generally exhibit much weaker circular dichroism compared with 3D geometries, because an infinitesimally thin device is inherently achiral, and excitation at oblique incidence or some form of nonreciprocal response is required to distinguish between LCP and RCP transmissivity 21 .…”

“…Planar structures generally exhibit much weaker circular dichroism compared with 3D geometries, because an infinitesimally thin device is inherently achiral, and excitation at oblique incidence or some form of nonreciprocal response is required to distinguish between LCP and RCP transmissivity 21 . In this context, the concept of extrinsic chirality has been recently put forward, showing that, by pairing two closely spaced planarized inclusions with some form of 3D chirality, may produce a narrow-band chiral response near their resonance [13][14][15][16][17][18] .…”

Ultra-Compact Multi-Band Chiral Metamaterial Circular Polarizer Based on Triple Twisted Split-Ring Resonator

“…Chiral metamaterials (CMMs) is lack of any mirror symmetry and also neither violates reciprocity nor time-reversal symmetry, which has been paid attention increasingly due to their exotic electromagnetic (EM) properties [1][2][3][4][5][6][7][8][9]. From microwave to optical ranges, negative refraction [1], giant gyrotropy [2], optical activity [3][4][5][6], circular dichroism (CD) [7][8][9], asymmetric transmission (AT) [10][11][12][13], and linear or/to circular polarization conversion [14][15][16] can be explored and realized by special CMM design.…”

“…From microwave to optical ranges, negative refraction [1], giant gyrotropy [2], optical activity [3][4][5][6], circular dichroism (CD) [7][8][9], asymmetric transmission (AT) [10][11][12][13], and linear or/to circular polarization conversion [14][15][16] can be explored and realized by special CMM design. Essentially, these unique EM properties mainly originate from cross-coupling of the electric and magnetic fields among these chiral structures [17][18][19][20][21][22][23].…”