10 Realization of Artificial Chirality in Micro-/Nano-Scale …
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these moments yields the chiral behaviors. This conceptual explanation of chiral
behaviors can be transferred to plasmonic molecules. In plasmonic chiral systems,
displacement currents are excited inside the plasmonic particles, where free electrons are displaced. Accordingly, a strong chiral response can occur, especially when
the currents are accompanied by a resonant excitation of localized plasmons. One
way to realize plasmonic chiral molecules is to arrange plasmonic structures in a
‘handed’ way [21]. One straightforward method to realize such handed structures
is to use conventional electron beam lithography (EBL) techniques with a precise
overlay process. An accurate EBL overlay method that has sub-20 nm alignment
accuracy has been used to realize three-dimensional chiral structures on scales of a
few nanometers by stacking plasmonic structures layer-by-layer.
To realize sub-20-nm alignment accuracy, a precise patterning of well-designed
alignment mark is necessary. However, the EBL alignment process cannot be seen
directly, even using scanning electron microscope. Instead, the relative position of a
stage can be predicted by using a set of alignment marks. Therefore, precise alignment
marks with sharp corners must be fabricated for use as reference points. Furthermore,
by defining alignment marks first, then using them at the beginning of fabrication
of the first layer, the alignment accuracy can be improved further. Stage movement
is almost identical for a given substrate, so the relative position of first layer and
second layer should be affected only by the alignment process. This EBL-overlay
process can be used to fabricate many different configurations of 3D nanostructures,
such as 3D suspended/connected, interlayered and hierarchical [23]. Artificial chiral
structures composed of mirror-symmetric suspended and connected nanostructures
show strong chirality at near-infrared (NIR) wavelengths of around 2.1–2.5 μm
[23]. The structure has a chiral resonator, in which gold structures form an artificial
LC circuit. A tilted metallic loop and the gap effectively function as EM circuit
components of an inductor and a capacitor, respectively. Inside the LC circuit, the
electric and magnetic dipoles are excited simultaneously because they share the same
structural resonances. As a result, the structures can exhibit strong chiral response
under the same structural resonances. The structures may exhibit negative refraction
if the induced chirality is strong.
The similar idea an also be applied to design planarized chiral helical structures
[24]. Metasurfaces are sequentially stacked with a tailored rotational twist that forms
anisotropic arrays (Fig. 10.4). The structural anisotropy is effectively converted to
strong magneto-electric coupling, which is responsible for artificial chirality. The
twisted metasurface structure provides strong coupling between each pair of spaced
surfaces over a broad range. If the number of stack sequential layers is increased, the
bandwidth broadens, and the reflections of opposite waves increases.
Recently, the concept of chiral molecules to plasmonic structures has been transferred to realize effective chiral molecular structures using a precise EBL overlay.
In [25], four gold nano-disks are closely arranged in a handed fashion to form a
plasmonic oligomer (Fig. 10.5). The first three particles are placed in the first layer
to make an L-shape, and the fourth particle is located in the second layer. The handedness is determined by the location of the last gold particle in the second layer, and
the structure exhibits configurational chirality. In this construction rule, the ligands
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these moments yields the chiral behaviors. This conceptual explanation of chiral
behaviors can be transferred to plasmonic molecules. In plasmonic chiral systems,
displacement currents are excited inside the plasmonic particles, where free electrons are displaced. Accordingly, a strong chiral response can occur, especially when
the currents are accompanied by a resonant excitation of localized plasmons. One
way to realize plasmonic chiral molecules is to arrange plasmonic structures in a
‘handed’ way [21]. One straightforward method to realize such handed structures
is to use conventional electron beam lithography (EBL) techniques with a precise
overlay process. An accurate EBL overlay method that has sub-20 nm alignment
accuracy has been used to realize three-dimensional chiral structures on scales of a
few nanometers by stacking plasmonic structures layer-by-layer.
To realize sub-20-nm alignment accuracy, a precise patterning of well-designed
alignment mark is necessary. However, the EBL alignment process cannot be seen
directly, even using scanning electron microscope. Instead, the relative position of a
stage can be predicted by using a set of alignment marks. Therefore, precise alignment
marks with sharp corners must be fabricated for use as reference points. Furthermore,
by defining alignment marks first, then using them at the beginning of fabrication
of the first layer, the alignment accuracy can be improved further. Stage movement
is almost identical for a given substrate, so the relative position of first layer and
second layer should be affected only by the alignment process. This EBL-overlay
process can be used to fabricate many different configurations of 3D nanostructures,
such as 3D suspended/connected, interlayered and hierarchical [23]. Artificial chiral
structures composed of mirror-symmetric suspended and connected nanostructures
show strong chirality at near-infrared (NIR) wavelengths of around 2.1–2.5 μm
[23]. The structure has a chiral resonator, in which gold structures form an artificial
LC circuit. A tilted metallic loop and the gap effectively function as EM circuit
components of an inductor and a capacitor, respectively. Inside the LC circuit, the
electric and magnetic dipoles are excited simultaneously because they share the same
structural resonances. As a result, the structures can exhibit strong chiral response
under the same structural resonances. The structures may exhibit negative refraction
if the induced chirality is strong.
The similar idea an also be applied to design planarized chiral helical structures
[24]. Metasurfaces are sequentially stacked with a tailored rotational twist that forms
anisotropic arrays (Fig. 10.4). The structural anisotropy is effectively converted to
strong magneto-electric coupling, which is responsible for artificial chirality. The
twisted metasurface structure provides strong coupling between each pair of spaced
surfaces over a broad range. If the number of stack sequential layers is increased, the
bandwidth broadens, and the reflections of opposite waves increases.
Recently, the concept of chiral molecules to plasmonic structures has been transferred to realize effective chiral molecular structures using a precise EBL overlay.
In [25], four gold nano-disks are closely arranged in a handed fashion to form a
plasmonic oligomer (Fig. 10.5). The first three particles are placed in the first layer
to make an L-shape, and the fourth particle is located in the second layer. The handedness is determined by the location of the last gold particle in the second layer, and
the structure exhibits configurational chirality. In this construction rule, the ligands