246
Y. Yang et al.
Chiral structures with sizes of 10–100 μm have been realized by combining
photolithography and electroplating methods. In the terahertz frequency range,
silicon can be used as the photoactive materials, and artificial silicon meta-atoms
may be described using inductor-capacitor (LC) circuits.
A device that had the above characteristics achieved active metasurfaces that
could switch the their handedness [21] (Fig. 10.3a–c); consisted of meta-atoms
composed of gold, silicon, aluminum oxide, and aluminum components that had
been fabricated using a combination of photolithography and electroplating. To fabricate photoactive chiral metamaterials, firstly, a patterned silicon layer was deposited
using photolithography and reactive ion etching. Then aluminum was deposited by
sputtering. The aluminum was a conductive layer for electroplating, as the ITO
layer, which used in DLW. Sputtering was used due to its high step-coverage. Gold
layers were also deposited using photolithography with aligning processes. Gold
columns were fabricated using photolithography to exposure photoresist (PR) and
electroplating to infill vacancies. This process is similar to DLW with electroplating,
but photolithography is more suitable than DLW to fabricate large-area metasurfaces. Finally, the residual resist was removed using oxygen plasma etching, and
the aluminum also removed using aluminum etchant. Oxygen plasma etching is a
common method to remove PR in complex structures and it can be applied to other
processes (e.g. removing PR when helical structures are fabricated using DLW).
These chiral structures can switch the handedness of circularly-polarized light
without geometrical reconfiguration. The handedness modulation is achieved by
photoactive material and well-designed LC circuits (Fig. 10.3e–f). Gold acts as an
ideal conductor, and loops of gold respond as inductors. The gaps between bottom
gold plates function as capacitors, so the meta-atom has a resonance frequency like
an LC (Fig. 10.3e, MetaA). The short length of MetaB causes a resonant shift, which
leads to the chiral response of the meta-atom. The total atom can be interpreted
as fusion of a right-handed meta-atom and frequency-shifted left-handed meta-atom
(Fig. 10.3e). When photoexcitation of silicon is induced by near-infrared laser pulses,
electrons jump from the silicon pad to the conduction bands, so the silicon becomes
conductive. This change cancels the capacitance of MetaA, and including frequency
shifting of MetaB. As a result, the handedness of chirality is switched by photoactive
material property of Si and combination of fabrication methods (Fig. 10.3f).
10.3 Chirality at the Nanometer to Micrometer Scale
10.3.1 Electron Beam Lithography Overlay
The chiral behaviors in a chiral molecule can be explained by movements of electron clouds. Under illumination by circularly-polarized light, electron clouds of the
molecules are displaced; this change induces a magnetic moment. It has components that are parallel to the electric dipole moment, and the interaction between
Y. Yang et al.
Chiral structures with sizes of 10–100 μm have been realized by combining
photolithography and electroplating methods. In the terahertz frequency range,
silicon can be used as the photoactive materials, and artificial silicon meta-atoms
may be described using inductor-capacitor (LC) circuits.
A device that had the above characteristics achieved active metasurfaces that
could switch the their handedness [21] (Fig. 10.3a–c); consisted of meta-atoms
composed of gold, silicon, aluminum oxide, and aluminum components that had
been fabricated using a combination of photolithography and electroplating. To fabricate photoactive chiral metamaterials, firstly, a patterned silicon layer was deposited
using photolithography and reactive ion etching. Then aluminum was deposited by
sputtering. The aluminum was a conductive layer for electroplating, as the ITO
layer, which used in DLW. Sputtering was used due to its high step-coverage. Gold
layers were also deposited using photolithography with aligning processes. Gold
columns were fabricated using photolithography to exposure photoresist (PR) and
electroplating to infill vacancies. This process is similar to DLW with electroplating,
but photolithography is more suitable than DLW to fabricate large-area metasurfaces. Finally, the residual resist was removed using oxygen plasma etching, and
the aluminum also removed using aluminum etchant. Oxygen plasma etching is a
common method to remove PR in complex structures and it can be applied to other
processes (e.g. removing PR when helical structures are fabricated using DLW).
These chiral structures can switch the handedness of circularly-polarized light
without geometrical reconfiguration. The handedness modulation is achieved by
photoactive material and well-designed LC circuits (Fig. 10.3e–f). Gold acts as an
ideal conductor, and loops of gold respond as inductors. The gaps between bottom
gold plates function as capacitors, so the meta-atom has a resonance frequency like
an LC (Fig. 10.3e, MetaA). The short length of MetaB causes a resonant shift, which
leads to the chiral response of the meta-atom. The total atom can be interpreted
as fusion of a right-handed meta-atom and frequency-shifted left-handed meta-atom
(Fig. 10.3e). When photoexcitation of silicon is induced by near-infrared laser pulses,
electrons jump from the silicon pad to the conduction bands, so the silicon becomes
conductive. This change cancels the capacitance of MetaA, and including frequency
shifting of MetaB. As a result, the handedness of chirality is switched by photoactive
material property of Si and combination of fabrication methods (Fig. 10.3f).
10.3 Chirality at the Nanometer to Micrometer Scale
10.3.1 Electron Beam Lithography Overlay
The chiral behaviors in a chiral molecule can be explained by movements of electron clouds. Under illumination by circularly-polarized light, electron clouds of the
molecules are displaced; this change induces a magnetic moment. It has components that are parallel to the electric dipole moment, and the interaction between