Machine learning techniques using artificial neural networks (ANN) have proven to be extremely ef-fective in designing nanophotonic systems. This presentation focuses on two applications where ANNs are utilized for designing nanophotonic scatterers.
In the first scenario, ANNs act as surrogate solvers for Maxwell's equations, allowing the design of scatterers tailored to specific fabrication technologies like laser nanoprinting. Designing low-index material scatterers is complex, so solving the inverse problem multiple times from different starting points is crucial. A Fourier neural operator ANN serves as a surrogate Maxwell solver, simplifying this process.
The second scenario integrates ANNs into a holistic metasurface design framework. Individual meta-atoms are efficiently described by their scattering responses, typically expressed as polarizability or T-matrix that provide metasurfaces with functionality on demand. Then, suitably trained ANNs are used to identify feasible physical objects that offer the desired T-matrices.
A distinguishing feature of high-index dielectric nanoparticles is their ability to support strong Mie resonances, thereby enhancing the interaction of light with matter and minimizing Ohmic losses, leading to unprecedented efficiency. An important advancement in this field is the investigation of the "transverse Kerker" effect, in which both forward and backward scattering are significantly reduced while lateral scattering is enhanced. We uncover that the realization of a perfect transverse Kerker effect is possible even in passive structures, by exploiting the physics of bound states in the continuum—electromagnetic states remaining localized in photonic structures, coexisting with outgoing waves. Such 'transverse Kerker BICs' are polarization independent, and in momentum space are pinned at the center of polarization vortices with high order topological charges.
Here we develop a theory of bound states in the continuum (BICs) in multipolar lattices – periodic arrays of resonant multipoles. We show that off-Γ BIC can be pinned in the k-space in this multipole approximation. The developed approach set a direct relation between the topological charge of BIC and the asymptotic behavior of Q-factor of the radiative modes in its vicinity.
The most common way to enhance interaction of electromagnetic waves with matter at the nanoscale is to use microresonators and resonant optical nanoantennas. In virtue of small size their optical properties are well described in terms of multipole decomposition, namely, by first several terms in the multipole expansion. The specific multipole content of the mode is completely determined by its symmetry and shape of the resonator. Here, we classify eigenmodes in resonators of the simplest shapes depending on their symmetry group. For each type of mode, we found its multipole content. As an illustrative example, we apply the developed formalism to the analysis of dielectric triangular prism and demonstrate the formation of high-Q resonances originated due to suppression of the scattering through the main multipole channel.
The study of resonant dielectric nanostructures with a high refractive index is a new research direction in the nanoscale optics and metamaterial-inspired nanophotonics. Because of the unique optically induced electric and magnetic Mie resonances, high-index nanoscale structures are expected to complement or even replace different plasmonic components in a range of potential applications. We study a strong coupling between modes of a single subwavelength high-index dielectric resonator and analyze the mode transformation and Fano resonances when the resonator’s aspect ratio varies. We demonstrate that strong mode coupling results in resonances with high-quality factors, which are related to the physics of bound states in the continuum when the radiative losses are almost suppressed due to the Friedrich–Wintgen scenario of destructive interference. We explain the physics of these states in terms of multipole decomposition, and show that their appearance is accompanied by a drastic change in the far-field radiation pattern. We reveal a fundamental link between the formation of the high-quality resonances and peculiarities of the Fano parameter in the scattering cross-section spectra. Our theoretical findings are confirmed by microwave experiments for the scattering of high-index cylindrical resonators with a tunable aspect ratio. The proposed mechanism of the strong mode coupling in single subwavelength high-index resonators accompanied by resonances with high-quality factors helps to extend substantially functionalities of all-dielectric nanophotonics, which opens horizons for active and passive nanoscale metadevices.
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