Biomedical Engineering Reference
In-Depth Information
Figure 2.4
(a) Experimental setup: the incoming laser beam is circularly
polarized using half (HWP) and quarter-wave (QWP) plates and weakle
focused by a microscope objective (5
×
,NA
=
0.13). The transmitted beam
is imaged by a second objective (40
200
mm, not shown) on a CCD camera and analyzed in the circular polarization
basis by additional HWP and QWP. (b) Scanning electron microscope image
of an
R
5
spiral milled on the back side of a gold membrane, with
×
, NA = 0.60) and a lens tube (
f
=
λ
SP
=
768
nm. (c) Intensity distribution imaged through a
L
1
−
R
5
structure. Labels
±
±
(
) correspond to the combination of circular polarization preparation
and analysis. The numbers correspond to the corresponding OAM indices.
Figure adapted from [49] with permission.
,
with enantiomorphic changes
L
↔
R
generated by the planar
character of the spirals milledon one side of the membrane.
As displayed in Fig. 2.5 when comparing panels (a) and (c)
and (b) and (d), the OAM measurements however turn out to be
inconsistent with a simple path reversal operation. As we fully
explain in [49], this discrepancy points to the pivotal role of the
central aperture in the process of OAM conservation, inducing
specific OAM selection rules that must be accounted for in the
generation process. This discussion is beyond the scope of this
