A Transparent Metasurface Supporting Pseudoanapole by Mirroring Split-Ring Resonators

This letter explores a novel possibility to create a transparent metasurface without any size modulation scheme, simply by mirroring split-ring resonators (SRRs). With this technique, a pseudoanapole state can be formed, where all major multipoles can be nullified, forming a transparency window with full transmission. In the mirrored SRR, magnetic dipoles are induced at the center of each SRR with antiphase components, thus forming the so-called toroidal dipole. The toroidal dipole constitutes a separate higher order moment whose destructive interference with the fundamental electric dipole moment forms the so-called anapole mode. However, under the $H_{||}$ excitation scenario, a pseudoanapole state can be excited, demonstrated by using the multipole scattering expansion to find the first resonant dipoles of the metasurface (electric dipole, toroidal dipole, magnetic dipole, and quadrupole). Theoretical results are experimentally validated using microwave measurements in an anechoic chamber facility.

I. Introduction

The scattering of electromagnetic waves from small particles [1], [2] can be manipulated for intriguing phenomena, ranging from super scattering [3], [4] to cloaking devices [5], [6], [7] and more recently transparent metasurfaces [8], [9]. When an electromagnetic wave interacts with subwavelength particles, electric and magnetic dipole moments are induced in these composites: the first is due to the oscillations of electric charges, the second to circulating electric current loops. The electric and magnetic dipole resonances could be coalesced to occupy the same spectral range by modifying the geometry of the particle. When these moments oscillate in phase, forward scattering along the propagation direction can be achieved with zero backward scattering, a phenomenon known as the first Kerker's condition [10], [11]. On the other hand, when electric and magnetic moments possess out-of-phase oscillations, they create nonradiating conditions, in which the object becomes invisible due to destructive dipole interference [12], [13] forming radiationless sources [14]. Recently, another moment has gained considerable research interest due to its similarity with the scattering characteristics of the electric dipole resonance: the toroidal moment [15], [16], [17]. Toroidal moments are excited due to the poloidal electric current flowing over a torus [see Fig. 1(a)], first observed in atomic systems by Zeldovich [15], but, in conventional structures, its scattering contribution could be neglected in comparison with the fundamental moments. However, recently, a variety of structures have been proposed that exhibit strong toroidal moments that exceed scattering from the fundamental multipoles [18], where the coupled magnetic dipole moments sustain electric toroidal moments, as shown in Fig. 1(b). To manipulate them with elementary dipoles, an in-phase oscillation of electric and toroidal moments in a metasurface usually gives coherent forward scattering, whereas an out-of-phase oscillation gives destructive interference, resulting in the formation of the so-called anapole state [19], [20], [21], [22].