All-dielectric metasurfaces based on toroidal and quasi-bound state in the continuum resonances in sensing applications

Hui-Hsin Hsiao^1*

¹Department of Engineering Science and Ocean Engineering, National Taiwan University, Taipei, Taiwan

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

Toroidal dipole (TD), known as the vortex distribution of magnetic dipoles (MDs), originates from a current flowing on the surface of a torus along its meridian. 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 [1]. In this work, 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 [2]. Upon normal incidence, a dominant TD mode can be observed in the optical regime, stemming from a complete head-to-tail configuration of the MD moments inside each nanopillar. Such TD mode sustains a large structural tolerance and can be spectrally tuned by elongating the cylindrical axis perpendicular to the light polarization, corresponding to a cross-sectional variation from circular to elliptical shapes. The excited TD mode is found to exhibit ultrahigh refractive index sensitivity compared to other multipoles, reaching a sensitivity of 459 nm (470 nm) per external refractive index change in our experiment (calculation). In addition, to further narrow down the spectral linewidth of the resonant mode for sensing applications with lower detection limit, we employed the design of silicon-based asymmetric nanobar pairs to excite a quasi-bound states in the continuum (quasi-BIC) with a dominant TD and electric quadrupole (EQ) resonant feature in the near-infrared [3]. The TD-EQ quasi-BIC mode is found to exhibit strong and tightly confined optical fields at the surface of tilted nanobar pairs, and its refractive-index sensitivity can be dramatically increased for nanopillars with larger aspect-ratio. The measured (simulated) sensitivity and figure of merit for nanobar pairs with a height of 450 nm reach 608 nm/RIU and 46 (612 nm/RIU and 85), respectively. These ultrahigh sensitive all-dielectric metadevices are promising for on-chip integration and sensor miniaturization to a wide range of diagnostic applications.
REFERENCES
[1] N. Papasimakis, V. A. Fedotov, V. Savinov, T. A. Raybould, and N. I. Zheludev, “Electromagnetic toroidal excitations in matter and free space,” Nat. Mater. 15, 263 (2016).
[2] H.-H. Hsiao and A.-Y. Liu, “Ultrasensitive and Broadband Optical Toroidal Modes in all-Dielectric Nanostructures,” Laser & Photonics Reviews 16(3), 2100404 (2022).
[3] H.-H. Hsiao, Y.-C. Hsu, A.-Y. Liu, J.-C. Hsieh, and Y.-H. Lin, “Ultrasenstive refractive index sensing based on the quasi-bound state in the continuum of all-dielectric metasurfaces,” Advanced Optical Materials 10(19), 2200812 (2022).

Keywords: Toroidal dipole, quasi-bound states in the continuum, all-dielectric metasurfaces, refractive-index sensor