Biomedical Engineering Reference
In-Depth Information
Figure 3.1
Comparison of
T
LL
(solid curves) and
T
RR
(dotted curves) at
normal (black) and oblique incidence (45
◦
, red), for (a) PEC and (b) silver
split-ring resonator metasurfaces. The unit cell element and direction of
propagation is shown in the insets. In both cases, the metasurface lies on
the
xy
plane and lacks mirror symmetry along the
y
-direction, creating a
nonzero
em
xz
term, which is responsible for the difference between
T
LL
and
T
RR
at oblique incidence.
α
3.2.4
Surface Impedance Model
It is obvious from the previous results that a single, ultrathin
metasurface can hardly provide chiral response over a broad
bandwidth. For this reason, we aim to apply our results to model
stacks of cascaded multilayered structures, which we can analyze
using the transmission-line approach based on the results of the
previous sections [58, 63]. This approach assumes that the main
couplingmechanismamongparallelmetasurfacesisassociatedwith
the
zero
-th order diffraction from the surface, consistent with the
previous analysis. This assumption is valid as long as the distance
between neighboring surfaces is larger than the period among
inclusions in each plane [63]. For simplicity, we will limit our
analysis to normal incidence, which ensures that the magnetic
current density does not contribute to radiation, consistent with
(3.15), (3.23). This ensures that our metasurface can be modeled
as a 2
2 shunt admittance element with surface impedance
Z
s
or
admittance
Y
s
:
×
Z
xx
Z
xy
Z
yx
Z
yy
=
Y
−
s
.
Z
s
=
(3.31)
Each element in the matrix,
Z
mn
=
ix
mn
is a complex quantity
with real part
r
mn
denoting the resistance and imaginary part
x
mn
r
mn
+
