Biomedical Engineering Reference
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which the individual elements themselves are not chiral, although
the complete array of such cells is (due to specific tilts existing
between the incident light and the objects [54]). A related scheme
has also been discussed at the level of achiral plasmonic nanohole
arrays [55].
2.4.1
Surface Plasmons and Archimedean Spirals: Planar
Chirality Gives a Twist to Light
As mentioned in the Introduction, the first manifestations of optical
planar chirality were observed by Zheludev and coworkers [42, 43,
47] using fish-scale periodic metal strips on a dielectric substrate.
The chiral structures were first realized at the millimeter scale
for the GHz regime in 2006 [42] and soon after scaled down
to the nanometer scale for studied in the near-infrared regime
in 2008 [47]. Simultaneously with these last studies, we realized
planar chiral gratings for SPs on a gold film. These structure shown
in Fig. 2.3 (b) are Archimede spirals defined by the parametric
equations
x
(
θ
)
=±
ρ
(
θ
)cos(
θ
),
y
(
θ
)
=
ρ
(
θ
)sin(
θ
),
(2.60)
with
P
2
π
θ
ρ
(
θ
)
=
(2.61)
θ
varyingbetween
θ
min
=
π
and
θ
max
=
θ
min
+
18
π
.Thetwopossible
signs
±
define two enantiomers (labelled
L
and
R
on Fig. 2.3(b)),
which are reciprocal mirror images obtained after reflection across
the y-z plane
x
=
0. Such clock wise or anticlockwise spirals
were milled on a 310 nm thick gold film using focus ion beam
methods.ForsuchArchimede'sspirals,thelength
P
760nmplays
obviously the role of radial period, as at each increment by an angle
δθ
=
2
π
, the radius increases by an amount
δρ
=
P
. The structure
looks like the well known “bull eye's” circular antennas, which are
usedtoresonantlycouplemonochromaticlightwithwavelength
λ
P
impinging normally to the structure [56, 57]. We point out that
P
isactuallyveryclosedtotheSPwavelength
λ
SP
(
λ
0
)
=
760nm,which
λ
0
corresponds to the optical wavelength
780 nm [48]. However,
