Biomedical Engineering Reference
In-Depth Information
excitation of SPPs through a narrow slit occurs dominantly when
the direction of polarization is perpendicular to the slit. At the
centersofFig.7.6(a)and(b),wedenotedtherotationoftheincident
polarizationwithacirculararrowrotatingaroundthecenterofeach
graph. Around each circular arrow, four curved arrows are placed
to denote the directions of the time-dependent locations of SPP
excitationsas the incident beam rotates.
Consider just one of the wings—for example, the one positioned
at the upper-part of the gammadion. When the polarization of the
incidentlightisinthehorizontaldirectionasintheupper-leftfigure
in Fig. 7.6(a), the excitation of the SPPs mainly occurs at the inner
segment of that ring. As the direction of the polarization rotates in
CW direction (LCP), the dominantly excited segment of that wing
will subsequently be changed into the outer one as in the upper-
right figure of Fig. 7.6(a). In a similar manner, the direction of the
excitation of SPPs in a wing for an obtuse gammadion under an LCP
incidencewillmovefrom theinnersegments tothe outersegments,
as shown by the four curved arrows in the center of Fig. 7.6(a).
However, if the handedness of the incident beam is in the
opposite direction (RCP), as in Fig. 7.6(b), the direction of the
excitation of the SPs in a wing of the same obtuse gammadion
under an RCP incidence will be inverted and directed inward, as
depicted by four arrows in the center of Fig. 7.6(b). If we define
the direction of this gammadion with the direction of helicity as
in the previous section in which the increasing radius along the
directionofazimuthalangleistheRH,thisgammadionhasLHwings.
Therefore, as discussed in the previous section for the spiral slit,
we can approximately tell that an RCP incidence onto an LH spiral
creates a constructive building-up of fields near the center of the
pattern. For this reason, for LH gammadions with obtuse angles in
Fig. 7.3, the intensity patterns near the center are brighter than
the outer area for the RCP incidences, as shown in Figs. 7.4(d) and
7.5(b).
Now consider the acute gammadion in Fig. 7.7. Contrary to the
case of obtuse wings, the direction of the excitation of SPPs in a
specificwingunderanLCPincidencewillmovefromtheoutertothe
inner segments, as shown in four arrows in the center of Fig. 7.7(a).
FortheRCPincidence,thedirectionsofthesearrowsareinvertedas
