Biomedical Engineering Reference
In-Depth Information
However,amoredetailedstudyshowsthatforchiralmolecules,it
isimpossibletodiscriminateradiationof“right”and“left”molecules
withoutaspeciallyarrangednano-environment.Theproblemisthat
plasmonic nanoparticles of simple shapes and metal island films do
not have noticeable chirality properties, and they can enhance only
local electric fields:
|
E
loc
| |
E
in
|
,
|
H
loc
| ∼ |
H
in
| ⇒ |
E
loc
| |
H
loc
|
(4.6)
As a result, such a plasmonic environment increases the role of an
electricdipolemomentumintheinteractionofchiralmoleculeswith
light because the initially large electric dipole momentum (
|
d
|
|
m
|
) will interact with an enhanced electric field:
H
R
=−
d
·
E
loc
−
m
·
H
loc
≈−
d
·
E
loc
,
H
L
=−
d
·
E
loc
+
m
·
H
loc
≈−
d
·
E
loc
.
(4.7)
Due to this fact, plasmonic nanoparticles of simple shapes and the
metal island films will equally enhance radiation of the “right” and
“left” molecules. So, for specific control of radiation of enantiomers
of chiral molecules, one should arrange such a nano-environment
that enhances the magnetic fields and reduces the electric fields.
Below, we will show that this is possible with making use of
nanoparticles that have both chirality and negative refractive index
ornegativepermeability.Now,suchunusualpropertiesareavailable
due to development of metamaterials technologies [11] and chiral
metamaterials in particular [46].
4.1.2
Chiral Metamaterials
Generally, metamaterials are artificial materials engineered to have
propertiesthatcannotbefoundinnature.Themetamaterialsconsist
of nanoparticles of different shapes that are called “meta-atoms.”
If the meta-atoms have chiral properties, they will form chiral
metamaterials.
The examples of the chiral meta-atoms used in metamaterials
and their twins are shown in Fig. 4.2. Metal spirals conducting
electric current play an important role in development of chiral
metamaterials (Fig. 4.2a). The second pair of chiral elements
(Fig. 4.2b) is a simplified version of a spiral and is formed by split
