Ultrahigh Sensitive Toroidal Mode in All-Dielectric Metasurfaces

Hui-Hsin Hsiao^1*, Ai-Yin Liu¹

¹Institute of Electro-Optical Engineering, National Taiwan Normal University, Taipei, Taiwan

* Presenter:Hui-Hsin Hsiao, email:hhhsiao@ntnu.edu.tw

Toroidal moment (TD), known as the vortex distribution of magnetic dipoles (MDs), originates from a current flowing on the surface of a torus along its meridian [1]. Due to its unique charge-current configuration, the toroidal metamaterials usually require sophisticated structural designs or specific excitation schemes to enhance the toroidal response and suppress the completing electric and magnetic multipoles simultaneously. Recently, the low-loss and high-index dielectric nanostructures have attracted a vast amount of attention and provide an alternative platform to support the multipolar resonances. The toroidal response and the associated anapole mode have been demonstrated either in a coupling system consisting of dielectric cylindrical quadrumers [2] or in individual specialized nanostructures, such as nanodisks [3] and nanoparticles [4]. In this paper, we numerically and experimentally demonstrate the excitation of a transverse TD response and the associated anapole-like state in periodic amorphous silicon (α-Si) nano-pillars embedded in a spin-on glass (SOG) layer. Upon normal incidence, a dominant TD mode can be observed in the optical regime with a bandwidth of 80 nm. The strong toroidal response stems from a complete head-to-tail configuration of the MD moments inside each nanopillar and exhibits ultra-high sensitivity to the variation of external environments. The measured peak wavelength variation as a function of refractive index reaches 370 nm/RIU, showing good correspondence to our numerical prediction of 480 nm/RIU. In addition, a mode transition between the non-radiating TD configuration and strongly radiating electric dipole mode was performed through the selective inclusion of appropriate liquids on the surface of one single metamaterial device. Our findings establish a robust basis for building up strong and broadband dielectric toroidal metamaterials and paves the way towards innovative active metadevices and biosensors on demand.
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Keywords: Toroidal dipoles, magnetic dipoles, dielectric metasurfaces, refractive-index sensors