Ultrasensitive and Broadband Optical Toroidal Modes in all-Dielectric Nanostructures

Dynamic toroidal dipole (TD) with its peculiar characteristic of broken space-inversion and time-reversal symmetries plays an important role in the fundamental physics of light–matter interaction. Here, TD metamaterials comprised of amorphous silicon nanopillar arrays embedded in spin-on-glass layer are experimentally demonstrated. Upon normal incidence of plane wave, the transverse toroidal moment and the associated anapole-like state are excited in optical regime. The strong TD response stems from a complete head-to-tail configuration of the magnetic dipole moments within each individual nanopillar. Both the experimental and simulation results show that 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, resulting in a sensitivity of 459 nm (470 nm) per external refractive index change in the experiment (calculation). This approach provides a simple and straightforward path in realizing toroidal metamaterials and establishes a new flat-optics platform for realizing active metadevices, sensors, and nonlinear nanophotonics.