PHYSICAL REVIEW B 97, 085105 (2018) Observation of asymmetric electromagnetic field profiles in chiral metamaterials Nobuyuki Hisamoto,1 Tetsuya Ueda,1,* Kei Sawada,2 and Satoshi Tomita3 1Department of Electrical Engineering and Electronics, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan 2RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan 3Graduate School of Materials Science, Nara Institute of Science and Technology, Ikoma, Nara 630-0192, Japan (Received 29 June 2017; published 2 February 2018) We experimentally observe asymmetric electromagnetic field profiles along two-dimensional chiral metamate- rials. The asymmetric field profiles depending on the chirality and the operation frequency have been reproduced well by the numerical simulation. Around a chiral meta-atom, distribution of a Poynting vector is found to be shifted asymmetrically. These results are explained in terms of an analogy with the side-jump mechanism in the electronic anomalous Hall systems. DOI: 10.1103/PhysRevB.97.085105 I. INTRODUCTION chiral metamaterials, determine the macroscopic constitutive relations of the effective media including not only permittivity The electromagnetic phenomena such as unidirectional and permeability but also chirality for bi-anisotropic media, by electromagnetic wave propagation have been of much in- appropriately designing meta-atoms [26–34]. So far, various terest. Unidirectional backscattering immune propagation of applications of the chiral metamaterials have been proposed surface waves, referred to as electromagnetic edge modes, and demonstrated [35–39]. However, little is known about the was observed in electromagnetic band gap structures such as edgelike mode propagation in the chiral metamaterials at the photonic crystals [1–3] with topological properties [4,5]. The operation frequencies far below the photonic band gaps due to unidirectional surface wave propagations that appear at the the Bragg scattering. boundary between two different areas with and without broken In this paper we demonstrate electromagnetic edgelike time-reversal symmetry using ferromagnetic medium [6,7] mode propagation in chiral metamaterials at microwave fre- or broken space-inversion symmetry due to chirality [8–13] quencies. We consider two-dimensional (2D) microwave chiral are topologically protected and robust against scattering by metamaterials of triangular lattice that are constructed us- disorder. ing helical meta-atoms [40,41]. Experimental results show Even in nontopological systems without periodicity, asym- asymmetric magnetic field profiles, which are relevant to metric propagation of surface plasmon polaritons due to the edgelike mode propagation in the metamaterials. The field unidirectional excitation [14–17] has been demonstrated. In concentration side is switched by inverting the chirality of these systems, unidirectional wave propagation was observed the meta-atom and by changing the operation frequencies. in various structures with space-inversion symmetry by ap- The asymmetric field profiles are reproduced by numerical propriately setting up incident wave conditions, such as by simulations based on the finite element method. Furthermore, controlling source positions [14], incident angles [15,16], and the numerical simulations visualize asymmetric energy and the polarization [17]. However, these approaches focused on Poynting vector distribution in the metamaterials, which en- wave incidence techniques rather than a design of wave- ables us to understand the behaviors of asymmetric field guiding structures. profiles. At frequencies far below the electromagnetic band gaps, where the wavelength of propagating waves becomes much longer than the lattice constant, the artificial periodic structures II. EXPERIMENTAL AND NUMERICAL SETUPS can be regarded as effective media based on the concept of metamaterials [18]. Metamaterials are artificial electromag- Figure 1 shows schematic illustrations of 2D chiral metama- netic media which are composed of small elements compared terials and the calculated dispersion diagram. A right-handed to the wavelength in order to manipulate electromagnetic wave (RH) chiral meta-atom was composed of a metallic-wire RH propagation and to discover new electromagnetic phenomena. helix. The chiral axis is along the z direction. As illustrated in On the other hand, chirality with broken space-inversion Fig. 1(a), design parameters for the meta-atom are as follows: = = symmetry, such as helices [19,20] or gammadions [21], gives wire diameter dw 0.4 mm, numbers of turns in the helix t = = rise to optical activity [22–25], which can rotate a plane of 3.5, pitch of turns p 1 mm, and helix diameter dh 2.4 mm. the polarization or may convert linear to circular polarizations The resonance frequency of the lowest mode for the and vice versa. Metamaterials with the chirality, referred to as isolated chiral meta-atom is calculated to be 7.0 GHz, which is determined by the total length of metal wire corresponding to a half wavelength of microwaves. Array of the meta-atoms comprises the 2D chiral metamaterial with thickness of *[email protected] a single meta-atom. Figure 1(c) illustrates the dispersion 2469-9950/2018/97(8)/085105(6) 085105-1 ©2018 American Physical Society HISAMOTO, UEDA, SAWADA, AND TOMITA PHYSICAL REVIEW B 97, 085105 (2018) FIG. 1. (a) Side view of a RH chiral meta-atom. (b) Top view of a finite rhombus-shaped 2D RH chiral metamaterial with triangular FIG. 2. Measured [(a)–(c)] and simulated [(d)–(f)] profiles for the lattice. Red arrow at the bottom indicates the magnetic-dipole excita- y component of the magnetic fields. (a) and (d) at 6.85 GHz for the tion position. (c) Dispersion diagram of a 2D RH chiral metamaterial LH chiral metamaterial, (b) and (e) at 6.85 GHz for the RH chiral with infinite triangular lattice. (d) The enlarged dispersion diagram in metamaterial. (c) and (f) also for the RH chiral meta-atoms but at 7.5 the vicinity of the lowest resonant mode in (c). and 8.4 GHz, respectively. will be a future issue. Here we focus our attention on the lowest diagram calculated for the 2D RH chiral metamaterials band at approximately 7.0 GHz where such topological effects consisting of the RH chiral meta-atoms in infinite triangular are negligibly small. lattice under the periodic boundary condition. In the numerical We experimentally and numerically investigate finite simulation setup for the eigenmode analysis, a perfect magnetic rhombus-shaped 2D chiral metamaterials as shown in Fig. 1(b). conductor (PMC) has been set on top and bottom boundaries The meta-atoms with the chiral axis parallel to the z axis were of the calculation region, because we are focusing on the TE placed on each grid of the 2D triangular lattice on the x-y plane mode propagation near the lowest-order magnetic dipolelike with 7 × 7 cells. The meta-atom design is the same as shown resonance of chiral meta-atoms. The lattice spacing among in Fig. 1(a). In microwave experiments, RH and LH chiral the meta-atoms were set to 8 mm. meta-atoms were made of stainless-steel wires. Forty-nine For the eigenmode calculation, a commercially available RH(LH) chiral meta-atoms embedded in a polystyrene foam electromagnetic field solver based on the finite element method slab comprise the 2D RH(LH) chiral metamaterials so that the ANSYS HFSS ver. 16 was utilized. Although Fig. 1(c) shows wire top edges are aligned to the −x direction. the dispersion diagram of the 2D RH chiral metamaterial, the In the measurement of microwave field profiles, two diagram is independent of the sign of chirality and identical identical loop antennas with diameter of 2.4 mm were used; to that of 2D left-handed (LH) chiral metamaterials consisting one for a magnetic dipole excitation source and another for a of the LH chiral meta-atoms. Figure 1(d) shows the enlarged magnetic field detection probe. Both antennas are connected to dispersion diagram in the vicinity of the lowest resonant mode two ports of a vector network analyzer (Agilent PNA N5224A) in Fig. 1(c).InFig.1(d), mutual coupling among neighboring through coaxial cables. While the meta-atom at the bottom meta-atoms under the magnetic-dipolelike lowest-mode reso- vertex of the rhombus designated by a red arrow in Fig. 1(b) nance at approximately 7.0 GHz gives rise to a backward-wave was excited by the excitation source antenna, magnetic field mode propagation. In the present case, the passband width of distribution at the slab top surface was measured by scanning the mode is extremely narrow due to the weak coupling. The the magnetic field detection probe in the x-y plane at a distance coupling between the chiral backward-wave mode and TEM of 1 mm on the opposite side of the excitation. In order to mode in free space results in a narrow gap at approximately reproduce experimental results by the numerical simulation, a 7.2 GHz between the and K points as well as between and time-varying magnetic dipole moment normal to the lattice is M points. placed as an incident wave source just below the meta-atom Additionally, Fig. 1(c) shows a band gap due to the Bragg at the bottom vertex of a rhombus at a distance of 3 mm. scattering at the K and M points around 20 GHz. We have numerically confirmed that this gap is caused by the meta- atom’s chirality. When the meta-atom structure is achiral such III. RESULTS AND DISCUSSION as with finite-length straight wire or ring shape, this gap will disappear. It would be interesting to control the dispersion A. Measured and calculated magnetic field profiles by changing the meta-atom’s chirality among RH, LH, and Figure 2 shows the measured and calculated magnitude achiral, giving topological states to the electromagnetic waves. of the magnetic field distribution for the y component However, such a study is out of the scope of this paper, and around 7.0 GHz corresponding to the magnetic-dipolelike 085105-2 OBSERVATION OF ASYMMETRIC ELECTROMAGNETIC … PHYSICAL REVIEW B 97, 085105 (2018) lowest-mode resonance frequency of the meta-atoms.