109 research outputs found
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
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
Optical "fingerprints" of dielectric resonators
The complete picture of the optical properties of resonant structures, along
with the frequency, quality factor, and line shape in the scattering spectra,
is determined by the electromagnetic field distribution patterns, which are a
kind of "fingerprint" of each resonant eigenmode. In this paper, we
simultaneously analyze the changes in the spectra and the transformation of the
field pattern during the topological transitions from a thin disk to a ring
with a gradually increasing thickness and further to a split ring. In addition,
we demonstrate characteristic optical fingerprints for well-known interference
effects such as bound states in the continuum and Fano resonances.Comment: 11 pages, 5 figure
Influence of the pseudogap on the superconductivity-induced phonon renormalization in high-T superconductors
We investigate the influence of a d-density wave (DDW) gap on the
superconductivity-induced renormalization of phonon frequency and linewidth.
The results are discussed with respect to Raman and inelastic neutron
scattering experiments. It turns out that the DDW gap can enhance the range of
frequencies for phonon softening depending on the underlying band
structure. Moreover we show that an anisotropic 'd-wave' pseudogap can also
contribute to the q-dependent linewidth broadening of the 340cm phonon
in YBaCuO.Comment: 4 page
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
Fano Resonance Between Mie and Bragg Scattering in Photonic Crystals
We report the observation of a Fano resonance between continuum Mie
scattering and a narrow Bragg band in synthetic opal photonic crystals. The
resonance leads to a transmission spectrum exhibiting a Bragg dip with an
asymmetric profile, which can be tunably reversed to a Bragg rise. The Fano
asymmetry parameter is linked with the dielectric contrast between the
permittivity of the filler and the specific value determined by the opal
matrix. The existence of the Fano resonance is directly related to disorder due
to non-uniformity of a-SiO2 opal spheres. Proposed theoretical "quasi-3D" model
produces results in excellent agreement with the experimental data
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)
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