Biomedical Engineering Reference
In-Depth Information
Incident
field
Incident
field
(
a
)
(
b
)
λ
λ
SPP
SPP
Phase
of SPP
Phase
of SPP
0
π/ 2
π
3π/2
Spiral slit
Spiral slit
Figure 7.2
Schematic diagram of excited SPPs from a circularly polarized
light illuminated from the backside of a spiral metallic slit. The polarization
of the incident light is (a)RCP and (b)LCP.
the transmitted light from this slit pattern also follows that of the
SPPs at the slit. Therefore, the diffracted light from this slit pattern
will have an OAM that coincides with the SAM of the incident field.
ThisisasimpleillustrationoftheinteractionbetweenSAMandOAM
for a plasmonic structure.
Now, instead of the circularly symmetric slit, let us move on to
a chiral slit pattern as shown in Fig. 7.2. In this case, the distance
between the slit and the rotational center (the origin of the pattern)
linearlyincreases,endingwithadistanceof
λ
SPP
toitsstartingpoint
after one cycle of rotation. As the distance increases in the direction
of the CCW rotation, we will refer to this type of helical pattern as a
right-helical (RH) spiral. For this structure, the phase distributions
near the center of the slit pattern do not simply follow the phase of
the incident light. The spatial distributions of the field magnitude
of the responses for the incidences of RCP and LCP light are very
different.
Let us first consider the case in which the RCP is incident on an
RHspiralinFig.7.2(a).Inthiscase,theSPPsexcitedfromtheslitwill
havephasedistributionsfollowingtheSAMoftheincidentfieldasin
thecircularslitpattern;thiscorrespondstotheamountof2
π
phase
delayalongonecycleofrotationintheazimuthaldirection.However,
the propagation distances of the SPPs from the slit to the origin of
the pattern, or the points at equi-distances from the origin, are also
