252
Y. Yang et al.
confirmed that in the visible-and-NIR spectrum, the optical response was mainly
dominated by the scattering from individual nanohelies (Fig. 10.7).
10.3.3 Unconventional Approaches
Chiral planar or 3D structures with chirality has merits in a way that it can have
chiroptical responses. Conventional spectroscopy cannot easily detect enantiomers,
because they have the same chemical and physical properties. However, CD spectroscopy can distinguish enantiomers. When they interact with circularly polarized
light, the responses depend on the handedness of the molecules. This effect can be
exploited to distinguish enantiomers by their chiroptical responses. However, the EM
coupling of chiral molecules is typically weak, so they are commonly accompanied
by plasmonic NPs. When irradiated by an EM wave of frequency that coincides with
the surface plasmon frequency of the plasmonic NPs, the CD response is generated [31]. The CD response can be further improved when the coalesced plasmonic
particles are also chiral; this is the motivation for creating chiral plasmonic NPs.
Single-layered chiral plasmonic NPs were demonstrated first, but such planar
structures show very weak chiral response and typically require oblique incidence.
Chiral interaction between light and matter is naturally increased in proportion to
the propagation length of light, so a 3D structure that can offer structural variation
along the incident path shows a pronounced chiroptical response.
One possible fabrication method is EBL, which can create 3D geometries with
high precision. Examples include stacked gammadions, twisted crosses, and twisted
layers of nanorods. However, EBL can only generate plasmonic planar and 3D structures on areas of a few square micrometers, and the method is expensive. Commercialization of chiral plasmonic NPs coupled with enantiomers requires large-area
fabrication at low cost.
Hole-mask lithography (HML) combined with tilted-angle rotation evaporation
has been proposed as a fabrication method (Fig. 10.8) [32]. HML is a derivative of
colloidal lithography [33] that patterns surfaces with nanostructures that are created
by evaporation through holes between close-packed colloidal polystyrene beads that
are self-aligned throughout a thin film mask. By rotating the tilt angle through which
the evaporated gold was deposited on the substrate, 3D chiral gold nanostructures can
be fabricated on an area of a few square centimeters. The structure had the average
outer diameter of 260 nm and structure widths from 20 to 90 nm. Rotation of the
tilt angle with gradually increasing angular velocity in either positive or negative
directions yielded nanostructures that had right-handed and left-handed chirality.
The fabricated chiral nanostructures were illuminated with linearly polarized light in
x and y directions, and with RCP and LCP lights. The transmittance spectra showed
frequency modes at 150, 250, and 320 THz, which all matched simulation results.
A bilayered twisted-arc photonic metamaterial has plasmonic chiral structure
that exhibits CD in the NIR spectrum (Fig. 10.8) [34]. The unit cell is composed
of a pair of silver twisted arcs in different azimuthal orientation, situated on two
Y. Yang et al.
confirmed that in the visible-and-NIR spectrum, the optical response was mainly
dominated by the scattering from individual nanohelies (Fig. 10.7).
10.3.3 Unconventional Approaches
Chiral planar or 3D structures with chirality has merits in a way that it can have
chiroptical responses. Conventional spectroscopy cannot easily detect enantiomers,
because they have the same chemical and physical properties. However, CD spectroscopy can distinguish enantiomers. When they interact with circularly polarized
light, the responses depend on the handedness of the molecules. This effect can be
exploited to distinguish enantiomers by their chiroptical responses. However, the EM
coupling of chiral molecules is typically weak, so they are commonly accompanied
by plasmonic NPs. When irradiated by an EM wave of frequency that coincides with
the surface plasmon frequency of the plasmonic NPs, the CD response is generated [31]. The CD response can be further improved when the coalesced plasmonic
particles are also chiral; this is the motivation for creating chiral plasmonic NPs.
Single-layered chiral plasmonic NPs were demonstrated first, but such planar
structures show very weak chiral response and typically require oblique incidence.
Chiral interaction between light and matter is naturally increased in proportion to
the propagation length of light, so a 3D structure that can offer structural variation
along the incident path shows a pronounced chiroptical response.
One possible fabrication method is EBL, which can create 3D geometries with
high precision. Examples include stacked gammadions, twisted crosses, and twisted
layers of nanorods. However, EBL can only generate plasmonic planar and 3D structures on areas of a few square micrometers, and the method is expensive. Commercialization of chiral plasmonic NPs coupled with enantiomers requires large-area
fabrication at low cost.
Hole-mask lithography (HML) combined with tilted-angle rotation evaporation
has been proposed as a fabrication method (Fig. 10.8) [32]. HML is a derivative of
colloidal lithography [33] that patterns surfaces with nanostructures that are created
by evaporation through holes between close-packed colloidal polystyrene beads that
are self-aligned throughout a thin film mask. By rotating the tilt angle through which
the evaporated gold was deposited on the substrate, 3D chiral gold nanostructures can
be fabricated on an area of a few square centimeters. The structure had the average
outer diameter of 260 nm and structure widths from 20 to 90 nm. Rotation of the
tilt angle with gradually increasing angular velocity in either positive or negative
directions yielded nanostructures that had right-handed and left-handed chirality.
The fabricated chiral nanostructures were illuminated with linearly polarized light in
x and y directions, and with RCP and LCP lights. The transmittance spectra showed
frequency modes at 150, 250, and 320 THz, which all matched simulation results.
A bilayered twisted-arc photonic metamaterial has plasmonic chiral structure
that exhibits CD in the NIR spectrum (Fig. 10.8) [34]. The unit cell is composed
of a pair of silver twisted arcs in different azimuthal orientation, situated on two