189 research outputs found
Magnetic dipole radiation tailored by substrates: numerical investigation
Nanoparticles of high refractive index materials can possess strong magnetic
polarizabilities and give rise to artificial magnetism in the optical spectral
range. While the response of individual dielectric or metal spherical particles
can be described analytically via multipole decomposition in the Mie series,
the influence of substrates, in many cases present in experimental
observations, requires different approaches. Here, the comprehensive numerical
studies of the influence of a substrate on the spectral response of high- index
dielectric nanoparticles were performed. In particular, glass, perfect electric
conductor, gold, and hyperbolic metamaterial substrates were investigated.
Optical properties of nanoparticles were characterized via scattering
cross-section spectra, electric field profiles, and induced electric and
magnetic moments. The presence of substrates was shown to introduce significant
impact on particle's magnetic resonances and resonant scattering
cross-sections. Variation of substrate material provides an additional degree
of freedom in tailoring properties of emission of magnetic multipoles,
important in many applications.Comment: 10 page, 28 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
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