Wide angle anapole excitation in stacked resonators.

In the search for resonances with high localized field strengths in all-dielectric nanophotonics, novel states such as anapoles, hybrid anapoles and bound states in the continuum have been realized. Of these, the anapoles are the most readily achievable. Interaction between vertically stacked disks supporting anapole resonances increases the field localization further. When fabricated from materials with high non-linear coefficients, such stacked disk pillars can be used as non-linear antennas. The excitation of such 3D pillars often includes off normal incidence when using focusing optics. Therefore, it is important to evaluate the angular and polarization response of such pillars. In the paper we fabricate pillars with three AlGaAs disks in a stack separated by stems of GaAs. The angular and polarization responses are evaluated experimentally with integrating sphere measurements and numerically through simulation, multipole decomposition and quasi-normal modes. We find that the stacked geometry shows hybridized anapole excitation for a broad span of incidence angles, with tunability of the individual multipolar response up to octupoles, including an electric octupole anapole, and we show how the average enhanced confined energy varies under angled excitation. The results show that the vertical stacked geometry can be used with highly focusing optics for efficient in-coupling to the hybridized anapole.

Strong optical coupling in metallo-dielectric hybrid metasurfaces.

Metasurfaces consisting of hybrid metal/dielectric nanostructures carry advantages of both material platforms. The hybrid structures can not only confine electromagnetic fields in subwavelength regions, but they may also lower the absorption losses. Such optical characteristics are difficult to realize in metamaterials with only metal or dielectric structures. Hybrid designs also expand the scope of material choices and the types of optical modes that can be excited in a metasurface, thereby allowing novel light matter interactions. Here, we present a metallo-dielectric hybrid metasurface design consisting of a high-index dielectric (silicon) nanodisk array on top of a metal layer (aluminum) separated by a buffer oxide (silica) layer. The dimensions of Si nanodisks are tuned to support anapole states and the period of the nanodisk array is designed to excite surface plasmon polariton (SPP) at the metal-buffer oxide interface. The physical dimensions of the Si nanodisk and the array periods are optimized to excite the anapole and the SPP at normal incidence of light in the visible-NIR (400-900 nm) wavelength range. Finite difference time domain (FDTD) simulations show that, when the nanodisk grating is placed at a specific height (∼200 nm) from the metal surface, the two modes strongly couple at zero detuning of the resonances. The strong coupling is evident from the avoided crossing of the modes observed in the reflectance spectra and in the spectral profile of light absorption inside the Si nanodisk. A vacuum Rabi splitting of up to ∼ 129 meV is achievable by optimizing the diameters of Si nanodisk and the nanodisk array grating period. The proposed metasurface design is promising to realize open cavity strongly coupled optical systems operating at room temperatures.