We experimentally demonstrated an unprecedented simple way to excite strong and broadband toroidal dipole (TD) response in the optical regime using dielectric metasurfaces upon plane-wave excitation. A comprehensive study of the geometric dependence for the TD mode was performed. In addition, with the change of local refractive index surrounding dielectric nanopillars, the TD mode was found to reach a sensitivity of 459 nm per external refractive index (RIU) change, showing good correspondence to our numerical prediction of 470 nm/RIU. In addition, arrays of tilted angle of nanorod pairs were designed to sustain quasi-bound states in the continuum (quasi-BIC) with TD and electric quadrupole (EQ) resonant property, showing strongly optical fields at the surface of tilted nanobar pairs and resulting in a measured (simulated) sensitivity and figure of merit of 608 nm/RIU and 46 (612 nm/RIU and 85), respectively.
Toroidal moment, induced by currents flowing on the surface of a torus, plays an important role in the fundamental physics of light-matter interaction. However, natural toroidal response was usually overlooked due to weak interaction with other electromagnetic resonances. Recently, toroidal moments can be effectively excited in artificial metamaterial by utilizing complex nanostructures or special light sources. Here, we have proposed periodic amorphous silicon cylinders embedded in spin-on-glass layer that can successfully generate transverse toroidal dipole (TD) in optical regime under normal incident illumination. Both experimental and simulation results indicate that such TD mode sustains a large structural tolerance and can be spectrally tuned by stretching the cylindrical axis perpendicular to the light polarization. In addition, the excited TD mode also displays ultrahigh refractive index sensitivity. This approach provides a simple and straightforward way to implement TD metamaterials and serves a powerful platform for high-sensitivity biosensors and nonlinear optics.
Light-matter strong coupling provides a powerful way to modify the nonlinear optical properties of materials. The coupling strength of the state-of-the-art strongly coupled systems are restricted by weak field confinement of the cavity, which limits the enhancement of the optical nonlinearity. Here, we investigate a new type of strong-coupling system, where the Mie resonant modes of the dielectric nanocavities are strongly coupled to an epsilon-near-zero mode, which results in an Rabi splitting of 220 meV. Static and dynamic nonlinear optical measurements reveal a very large enhancement in the intensity-dependent refractive index change at the coupled resonance. In addition, we observe a transient response of hundreds of femtoseconds for the coupled system. The ultrafast and large optical nonlinear coefficients in this work offer a new route towards strong coupling-assisted high-speed photonics.
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