Strong near-field couplings of anapole modes and the formation of higher-order electromagnetic modes in stacked all-dielectric nanodisks

We theoretically study the near-field couplings of two stacked all-dielectric nanodisks, where each disk has an electric anapole mode consisting of an electric dipole mode and an electric toroidal dipole (ETD) mode. Strong bonding and anti-bonding hybridizations of the ETD modes of the two disks can occur. The bonding-hybridized ETD can interfere with the dimer's electric dipole mode and induce a new electric anapole mode. The anti-bonding hybridization of the ETD modes can induce a magnetic toroidal dipole (MTD) response in the disk dimer. The MTD and magnetic dipole resonances of the dimer form a magnetic anapole mode. Thus, two dips associated with the hybridized modes appear on the scattering spectrum of the dimer. Furthermore, the MTD mode is also accompanied by an electric toroidal quadrupole mode. The hybridizations of the ETD and the induced higher-order modes can be adjusted by varying the geometries of the disks. The strong anapole mode couplings and the corresponding rich higher-order mode responses in simple all-dielectric nanostructures can provide new opportunities for nanoscale optical manipulations.

Microwave realization of multiresonant metasurfaces for achromatic pulse delay

We propose a microwave realization of a metasurface that can delay broadband pulses without distortion in reflection. In order to obtain large and broadband pulse delay, we harness the synergetic phase delay of five sharply-resonant meta-atoms. More specifically, three electric-LC and two split ring resonators, supporting electric and magnetic dipole resonances, respectively, are combined in a subwavelength unit cell. The resonances are spectrally interleaved and specifically designed to provide a spectrally-constant reflection amplitude and group delay according to the prescription in [ACS Photonics 5, 1101, 2018]. The designed metasurface is electrically ultrathin (λ0/19), since it relies on resonant phase delay exclusively, instead of phase accumulation via propagation. We show delay of 700-MHz Gaussian pulses centred at 11 GHz by 1.9 ns, corresponding to approximately 21 carrier cycles. Our results highlight the practical potential of metasurfaces for broadband dispersion control applications.

Significantly enhanced second-harmonic generations with all-dielectric antenna array working in the quasi-bound states in the continuum and excited by linearly polarized plane waves

The recently emerging all-dielectric optical nanoantennas based on high-index semiconductors have proven to be an effective and low-loss alternative to metal-based plasmonic structures for light control and manipulations of light–matter interactions. Nonlinear optical effects have been widely investigated to employ the enhanced interactions between incident light and the dielectrics at the Mie-type resonances, and in particular magnetic dipole resonances, which are supported by the semiconductor. In this paper, we explore the novel phenomenon of bound states in the continuum supported by high-index semiconductor nanostructures. By carefully designing an array of nanodisk structures with an inner air slot as the defect, we show that a novel high quality-factor resonance achieved based on the concept of bound state in the continuum can be easily excited by the simplest linearly polarized plane wave at normal incidence. This resonance further enhances the interactions between light and semiconductors and boosts the nonlinear effects. Using AlGaAs as the nonlinear material, we demonstrate a significant increase in the second-harmonic generation efficiency, up to six orders of magnitude higher than that achieved by magnetic dipole resonances. In particular, a second-harmonic generation efficiency around 10% can be numerically achieved at a moderate incident pump intensity of 5 MW/cm2.