Biomedical Engineering Reference
In-Depth Information
generation of optical vortices with the characteristic core size well
beyond the diffraction limit. Such effects are found for weakly
dissipative metallic nanoparticles within the Mie theory. Important
peculiarities of the far-field scattering and near-field Poynting flux
are manifested in the so-called “nano-Fano resonances” introduced
and discussed here. Control of the orbital momentum of photons
with the help of nanostructures is a novel research direction, which
is very attractive for many applications in quantum optics and
information technologies, and it opens an unprecedented way for
manipulating optical vortices at the nanoscale.
9.1 Introduction
The fascinating physics of light scattering offers two classes
of important interference phenomena: optical vortices and Fano
resonances. The optical vortices are observed in the structure of
the wave phase that carries topological singularities, and therefore,
they are associated with the so-called topological optics (also
called singular optics and dislocations of the wave front). The Fano
resonances are associated with a sharp asymmetric variation of the
wave transmission. In general, these two phenomena are seemingly
unrelated, and Fano resonances are observed without topological
effects as well as singular optics is not necessary accompanied by
Fano resonance effect. However, the situations change dramatically
as soon as we reduce the size of the scattering structure and
approach the nanoscale.
For a majority of plasmonic nanostructures with weak dissipa-
tion, the pronounced Fano resonance appears when the typical size
parameter becomes of the order of unity,
q
=
2
π
R
/λ
≈
1, where
R
is the characteristic scale of the structure and
λ
is the radiation
wavelength. The term “pronounced” used here means that the
corresponding Fano resonance is distinguished in comparison with
thecharacteristicscaleofthedipoleresonance(Rayleighscattering).
This condition implies that a majority of plasmonic nanostructures
with Fano resonances suffer from scaling.
Meanwhile, it is possible to generate Fano resonances within
the nanostructures with a very small size parameter
q
1
