Chiral Metafoils for Terahertz Broadband High-Contrast Flexible Circular Polarizers Jianfeng Wu, 1,2 Binghao Ng, 3 Haidong Liang, 2,4 Mark B. H. Breese, 2,4 Minghui Hong, 1 Stefan A. Maier, 3 Herbert O. Moser, 5 and Ortwin Hess 3,* 1 Department of Electrical and Computer Engineering, National University of Singapore, 4 Engineering Drive 3, 117576 Singapore, Singapore 2 Department of Physics, Center for Ion Beam Applications (CIBA), National University of Singapore, 2 Science Drive 3, 117542 Singapore, Singapore 3 The Blackett Laboratory, Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom 4 Singapore Synchrotron Light Source (SSLS), National University of Singapore, 4 Engineering Drive 3, 117576 Singapore, Singapore 5 Karlsruhe Institute of Technology (KIT), Institute of Microstructure Technology (IMT), Postfach 3640, D-76021 Karlsruhe, Germany (Received 8 May 2014; published 18 July 2014) Metamaterial concepts have opened the door to high-optical anisotropy beyond levels of naturally occurring materials but usually with limited spectral bandwidth. Here, we report on the design, realization, and experimental and numerical characterization of broadband chiral metafoils exhibiting at the same time exceptionally high-contrast circular polarization in the THz range. We demonstrate that the observed chirality is achieved by a simple modification of the structure of well-established metafoils, namely, through a shift of interconnecting lines by half a unit cell length between each row. The bandwidth of the observed circular-polarization-selective response is demonstrated to be up to about one octave at a center frequency of 2.4 THz and the ratio of the transmission of oppositely circularly polarized radiation is shown to reach about 700%, with the maximum transmittance being approximately 70%. Although implemented here at THz frequencies, broadband chiral metafoils may be extended to higher infrared and optical frequencies by hot embossing or nanoimprinting. They may be used as circular polarizers and beam stops in THz optical and infrared systems. DOI: 10.1103/PhysRevApplied.2.014005 I. INTRODUCTION Circular optical dichroism, the differential absorption of right- and left-handed circularly polarized light, is a characteristic attribute of light and an important means of using light to study the structure and properties of chiral molecules [1]. While generally, 2D or 3D objects are called chiral if their mirror image at a line or a plane, respectively, cannot be transformed into the original by translations and/ or rotations, many biologically relevant molecules such as naturally existing amino acids, enzymes, and sugars are chiral and chiral molecules are also of great importance in chemistry, biology, and pharmaceuticals. However, in naturally chiral molecules the chirality is relatively low. Not surprisingly, artificial chiral “molecules, ” such as helical coils, are thus studied with an aim to achieving augmented chiral effects [2,3]. Combining chirality with metamaterials, chiral metamaterials have recently attracted considerable attention due to their exotic properties, such as giant optical activity as well as their potential to realize materials with negative refraction [4–17]. In chiral metamaterials, strong magnetoelectric coupling gives rise to an appreciably different optical response of left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light. On the basis of this concept, chiral metamaterials have opened a new route to building circular polarizers with octave-wide frequency ranges of operation. For conventional circular polarizers, two methods have commonly been used to generate circularly polarized light: (1) Bragg reflection using a cholesteric liquid-crystal film and (2) a linear polarizer laminated with a quarter- wave film [18]. However, their narrow-band response at an a priori designed frequency seriously limits their integration into many devices and systems. Recently, a gold-helix photonic metamaterial [19,20] was introduced and investigated as a compact and broad- band circular polarizer. For light propagating along the helix axis, the structure was demonstrated to block circular polarization with the same handedness as the helices but to transmit the other, for a frequency range exceeding one octave. At the same time, three-dimensional bichiral plasmonic crystals [21,22], tapered helices [23], and multi- helical structures [24,25] have also been conceived and shown to further improve the performance of circular polarizers. However, the complex structure design and * o.hess@imperial.ac.uk PHYSICAL REVIEW APPLIED 2, 014005 (2014) 2331-7019=14=2(1)=014005(8) 014005-1 © 2014 American Physical Society