Biomedical Engineering Reference
In-Depth Information
thediscrepancybecomesnoticeable,indicatinganoticeableamount
of near-field coupling involving higher-order evanescent diffraction
orders.
In [63], we have compared more inclusion shapes made of gold
using experimentally retrieved permittivity values [64], and excited
at optical frequencies, also taken into account plasmonic effects.
Our results show that the surface impedance approach can deal
with inclusions with arbitrary shapes and rotations, validating this
approachaslongasthedistancebetweenneighboringmetasurfaces
is larger than a quarter of the transverse metasurface period.
3.3.2
Optimized Pair of Rotated Metasurfaces for Maximal
Chiral Response
The transmission-line approach using the surface-impedance en-
ables an easy and e
cient analysis of arbitrary stacks of metasur-
faces, including arbitrary rotation of the inclusions. This model can
provide closed-form relations to optimize the design parameters of
finite stacks of metasurfaces, in order to maximize the operational
bandwidth and extinction ratio for different circular polarization
inputs [58].
As an example, Fig. 3.4 describes how the performance of two
metasurfaces composed of PEC dipoles, as in Fig. 3.2, can be
optimized for operation as a circular polarizer by simultaneously
varying the distance and the rotation angle to maximally enhance
its overall chiral response. We show the operational bandwidth
of this stack, defined as the largest continuous wavelength range
over which the ratio of transmittances between opposite circular
polarizations is larger than two. Figure 3.4a refers to the range for
which
|
T
RR
/
T
LL
|
2
2
>
2.
Figure 3.4 shows that both bandwidth variations have a periodic
nature with respect to the distance, but with a reduced envelope
when the separation distance is increased, due to dynamic effects.
The results show only positive rotation angles, corresponding
to a ”right-handed” discrete helix introduced in the cascaded
structure and symmetrical results are obtained for the other
rotation. Depending on the design parameters, we can select either
preferred
T
RR
or
T
LL
for the same rotation. As the plots do not
>
2, Fig. 3.4b to the dual case
|
T
LL
/
T
RR
|
