Switching from visibility to invisibility via Fano resonances: theory and experiment

Subwavelength structures demonstrate many unusual optical properties which can be employed for engineering functional metadevices, as well as scattering of light and invisibility cloaking. Here we demonstrate that the suppression of light scattering for any direction of observation can be achieved for an uniform dielectric object with high refractive index, in a sharp contrast to the cloaking with multilayered plasmonic structures suggested previously. Our finding is based on the novel physics of cascades of Fano resonances observed in the Mie scattering from a homogeneous dielectric rod. We observe this effect experimentally at microwaves by employing high temperature-dependent dielectric permittivity of a glass cylinder with heated water. Our results open a new avenue in analyzing the optical response of hight-index dielectric nanoparticles and the physics of cloaking.Comment: 8 pages, 4 figure

Quadruplets of exceptional points and bound states in the continuum in dielectric rings

In photonics, most systems are non-Hermitian due to radiation into open space and material losses. At the same time, non-Hermitianity defines a new physics, in particular, it gives rise to a new class of degenerations called exceptional points, where two or more resonances coalesce in both eigenvalues and eigenfunctions. The point of coalescence is a square root singularity of the energy spectrum as a function of interaction parameter. We investigated analytically and numerically the photonic properties of a narrow dielectric resonator with a rectangular cross section. It is shown that the exceptional points in such a resonator exist in pairs, and each of the points is adjacent in the parametric space to a bound state in the continuum, as a result of which quadruples of singular photonic states are formed. We also showed that the field distribution in the cross section of the ring is a characteristic fingerprint of both the bound state in the continuum and the exceptional point.Comment: 12 pages, 5 figure

Phase diagram for the transition from photonic crystals to dielectric metamaterials

Photonic crystals and metamaterials represent two seemingly different classes of artificial electromagnetic media but often they are composed of similar structural elements arranged in periodic lattices. The important question is how to distinguish these two types of periodic photonic structures when their parameters, such as dielectric permittivity and lattice spacing, vary continuously. Here, we discuss transitions between photonic crystals and all-dielectric metamaterials and introduce the concept of a phase diagram and an order parameter for such structured materials, based on the physics of Mie and Bragg resonances. We show that a periodic photonic structure transforms into a metamaterial when the Mie gap opens up below the lowest Bragg bandgap where the homogenization approach can be justified and the effective permeability becomes negative. Our theoretical approach is confirmed by detailed microwave experiments for a metacrystal composed of a square lattice of glass tubes filled with heated water. This analysis yields deep insight into the properties of periodic photonic structures, and it also provides a useful tool for designing different classes of electromagnetic materials in a broad range of parameters.Comment: 7 pages, 6 figure

Fano resonances in antennas: General control over radiation patterns

The concepts of many optical devices are based on fundamental physical phenomena such as resonances. One of the commonly used devices is an electromagnetic antenna that converts localized energy into freely propagating radiation and vise versa, offering

Switchable invisibility of dielectric resonators

The study of invisibility of an infinite dielectric rod with high refractive index is based on the two-dimensional Mie scattering problem, and it suggests strong suppression of scattering due to the Fano interference between spectrally broad nonresonant waves and narrow Mie-resonant modes. However, when the dielectric rod has a finite extension, it becomes a resonator supporting the Fabry-Perot modes which introduce additional scattering and eventually destroy the invisibility. Here we reveal that for shorter rods with modest values of the aspect ratio r/L (where r and L are the radius and length of the rod, respectively), the lowest spectral window of the scattering suppression recovers completely, so that even a finite-size resonator may become invisible. We present a direct experimental verification of the concept of switchable invisibility at microwaves using a cylindrical finite-size resonator with high refractive index.This work was supported by the Russian Foundation for Basic Research (Grant No. 16-02-00461), and the Australian Research Counci

Bound states in the continuum and Fano resonances in the strong mode coupling regime

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.The numerical calculations were performed with support from the Ministry of Education and Science of the Russian Federation (Project 3.1500.2017/4.6) and the Australian Research Council. The experimental study of the cylinder SCS in the microwave frequency range was supported by the Russian Science Foundation (17-79-20379). The analytical calculations with resonant-state expansion method were performed with support from the Russian Science Foundation (17-12-01581). A. A. B., K. L. K. and Z. F. S. acknowledge support from the Foundation for the Advancement of Theoretical Physics and Mathematics “BASIS” (Russia)