Biomedical Engineering Reference
In-Depth Information
implying that, theoretically, the minimum spot size can be as small
as a few nanometers, which can provide more possibilities for
metamaterial applications in the visible range, including optical
activity. Chirality has been demonstrated in either planar [51-55]
or three-dimensional metamaterials [56-58] using this fabrication
technique. Planar metasurfaces generally exhibit much weaker
chiral effects than three-dimensional geometries, because an infin-
itesimallythinsurfaceisinherentlyachiral,andexcitationatoblique
incidence or nonreciprocal responses is required to distinguish
between left-handed circular polarization (LCP) and right-handed
circular polarization (RCP).
In this chapter, we theoretically discuss how individual and
stacked planar metasurfaces may provide strong chiral effects and
effectivelyrespondasabulkthree-dimensionalchiralmetamaterial.
We first consider a single, optically thin metasurface located in the
z
=
0 plane, formed by arbitrarily shaped plasmonic nanoparticles
embedded in a rectangular lattice with periods
d
x
and
d
y
.We
assumehereandinthefollowingthatthelatticeconstantsaremuch
smaller than the wavelength of operation so that only the
zero
-th
diffraction order can propagate away from the metasurface plane,
andthattheinclusionsarenottoodenselypackedsothattheoptical
wave interaction can be modeled using the dipolar approximation
with good accuracy. We further assume that the inclusions are
su
ciently thin in the direction normal to the array to ensure that
only an optical displacement current tangential to the surface may
beinduced.
These assumptions ensure that, when excited by an external
plane wave with arbitrary polarization and incidence angle
,the
inclusions are well described by an electric dipole moment parallel
to the surface and a magnetic dipole moment normal to it. In the
planar array, the local fields impinging on each inclusion are given
by the superposition of the impinging fields and the radiation from
other dipoles, due to the coupling with the inclusions in the array.
Therefore, the metasurface response may be compactly described
through a generalized array polarizability tensor
α
s
that relates the
induced dipole moments at the origin
p
00
,
m
00
z
to the impinging
fields
E
inc
(electric) and
H
inc
(magnetic), including the full dynamic
θ
