Biomedical Engineering Reference
In-Depth Information
7.1 Introduction: Optical Chirality and Plasmonic Chiral
Patterns
Light beams possessing or carrying angular momentum (AM) have
beenintensivelystudiedfromdiverseareassuchasstudiesofcosmic
objects in rotation [1], rotating atoms with photons [2], and many
other applications [3]. AMs that are usually carried by an optical
field can be classified into two types, namely, spin AM (SAM) and
orbital AM (OAM) [3, 4]. Although the separation of these two AMs
is not based on rigorous physical ground [4-6], for paraxial beams,
it is generally accepted that SAM and OAM are strongly associated
with the polarization and spatial distribution of the optical field,
respectively. In other words, for the case of a paraxial beam of light,
the SAM is in the relation with the rotation of the polarization of
light in the basis of left- or right-hand circularity and the OAM is
related on the dependence of the light field to the azimuth angle in
itscomplexamplitude.TheconversionbetweenthesetwoAMsisthe
preferredchoiceinmanystudiesforexplainingopticalsingularityor
vortices in focused beams or scattered light [7-9].
Recently, great interest has developed in light interactions
with materials having chirality [8-12]. The word chirality refers
to the asymmetry or handedness of a configuration that cannot
be superimposed over its mirror image. More precisely, a chiral
structurewhichcontainshelix,screw-likecirculargeometryisoften
called helical structure. When an optical field passes through a
chiral, optically thick material, the polarization of the transmitted
field is rotated from its original state. This phenomenon is
called optical activity [11]. Optical activity is based on a type of
birefringence: polarization-dependent phase retardations enforced
by the structure. Therefore, such optical activity usually occurs
in a three-dimensional object and is rarely observed in the case
of a planar or an optically thin pattern. However, recent studies
have proven that such phenomena can be associated with planar
plasmonic chiral structures [8-10, 12]. Surface plasmon polaritons
(SPPs) are light-coupled collective oscillations of electrons in a
metal surface [13]. When a light field is transmitted through a
planar structure that contains a chiral metallic pattern, the SPPs are
