Biomedical Engineering Reference
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Lorentz, see below), it necessarily implies that reversing the light
paththroughthemediummustrecreatebacktheinitialpolarization
state.However,asthesenseoftwistofa2Dchiralstructurechanges
when looking from the second side, this polarization reversal is
impossible. It would otherwise lead to the paradoxical conclusion
that a left and right-handed structure generates the same optical
activity in contradiction with the definition, finally meaning that
optical activity must vanish in strictly planar chiral systems. This
behavior strongly contrasts with what is actually observed for 3D
chiral objects having a helicoidal structure (like a quartz crystal [9,
12, 13], a twisted jute element [17], or a metal helix [18, 19]), in full
agreementwiththeprincipleofreciprocityofLorentz[1,38-41],as
the sense of twist of an helix is clearly conserved when we reverse
back the illumination direction. The experimental observation of
optical activity in gammadion arrays forces one to conclude that
such systems must present a form of hidden 3D chirality, which
turnsfullyresponsibleforthepresenceofopticalrotationthatrules
overthedominant2Dgeometricalchiralcharacterpossessedbythe
system.
Things could have stopped here, but the understanding of 3D
chirality was recently challenged in a pioneering study in which
it was shown that chirality has a distinct signature from optical
activity when electromagnetic waves interact with a genuine 2D
chiral structure and that the handedness can be recognized [42].
Although the experimental demonstration was achieved in the
GHz (mm) range for extended 2D structures (the so-called fish-
scale structures[42]), the question remained whether this could be
achieved in the optical range, as the optical properties of materials
are not simply scalable when downsizing to the nanometer level.
Theoretical suggestions were provided to overcome this di
culty
by using localized plasmon modes excited at the level of the
nanostructures [43]. Surface plasmons (SPs) [44-46] are indeed
hybrid photon/electron excitations, which are naturally confined
in the vicinity of a metal structure. As evanescent waves, SPs
are very sensitive to local variations of the metal and dielectric
environments [44]. This property was thus used to tune some 2D
chiral metal structure to optical waves. Two series of experiences
made in the near infrared [47] with fishscale structure and in the
