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Y. Yang et al.
thin indium-tin-oxide (ITO) is prepared, and then a positive-tone PR is spin-coated
on it. The ITO has electrical conductivity, which is required to allow use of the
electroplating processes. Second, the positive-tone PR is polymerized using focused
photons, and the exposed spot is removed after the development process. Third,
electrochemical deposition is conducted to grow gold structures in the voids in the
polymer. Gold is a typical material for chiral plasmonics due to its high conductivity to
enable electron oscillations, and to its resistance to corrosion. Finally, the remaining
PR is removed using oxygen plasma etching to leave sophisticated gold structures.
The main strength of DLW is that three-dimensional printing induced by multiphoton polymerization yields the highest degree of design freedom [13, 14]. DLW
can fabricate various helical structures, which have different chiroptical activities depending on the helical parameters and design. For example, tapered helical
structures [11] increase extinction ratio and broaden bandwidth simultaneously.
Handedness-converted helical structures achieve spin-conversion efficiency within
the frequency range of 50−90 THz (6 ≥ λ ≥ 3.33 μm) (Fig. 1d) [12]. However,
DLW has the disadvantage of limited resolution, so the wavelength band in which it
works has a lower limit. The method is also slow, so it cannot be used practically to
pattern large areas.
10.2.2 Buckling Process Using Focused Ion Beam
Thin-film buckling process enables curved surfaces with sizes of tens of micrometers;
the curvature enables three-dimensional chiral structures. This thin-film-buckling
method includes cutting and folding flat objects. Some three-dimensional chiral structures have gigantic chiral phenomena, compared to two-dimensional chiral structures
[15]. Furthermore, this buckling process does not require stacking and aligning,
which are necessary steps in multi-layer fabrications. The residual stress can be
induced by various stimuli, including temperature, mechanical forces, and capillary
forces [16–19]. In this subchapter, we will consider focused ion beam (FIB)-induced
buckling which is a recently emerging method.
Residual induced compressive forces have been evaluated as methods to create
chiral structures and strengthen their chiroptical responses [20]. One method to
generate residual stress in a surface of thin film is to use FIB to implant ions. This
technique requires only a single step, and has both high accuracy and high resolution, so the method can fabricate desirable chiral structures. For example [15], one
chiral structure that had a strengthened chiroptical response (Fig. 10.2a); to generate
plasmonic responses, the authors used a gold thin film in which residual stress was
imposed by global ion beam irradiation (Fig. 10.2b). Curvatures of the structures can
be modulated by controlling the dose intensity during irradiation, so chiral structures
can be shaped with quite high accuracy (Fig. 10.2c).
Compared to two-dimensional structures, buckled chiral techniques enable versatile geometries. Chirality is derived from the difference in interaction depending
Y. Yang et al.
thin indium-tin-oxide (ITO) is prepared, and then a positive-tone PR is spin-coated
on it. The ITO has electrical conductivity, which is required to allow use of the
electroplating processes. Second, the positive-tone PR is polymerized using focused
photons, and the exposed spot is removed after the development process. Third,
electrochemical deposition is conducted to grow gold structures in the voids in the
polymer. Gold is a typical material for chiral plasmonics due to its high conductivity to
enable electron oscillations, and to its resistance to corrosion. Finally, the remaining
PR is removed using oxygen plasma etching to leave sophisticated gold structures.
The main strength of DLW is that three-dimensional printing induced by multiphoton polymerization yields the highest degree of design freedom [13, 14]. DLW
can fabricate various helical structures, which have different chiroptical activities depending on the helical parameters and design. For example, tapered helical
structures [11] increase extinction ratio and broaden bandwidth simultaneously.
Handedness-converted helical structures achieve spin-conversion efficiency within
the frequency range of 50−90 THz (6 ≥ λ ≥ 3.33 μm) (Fig. 1d) [12]. However,
DLW has the disadvantage of limited resolution, so the wavelength band in which it
works has a lower limit. The method is also slow, so it cannot be used practically to
pattern large areas.
10.2.2 Buckling Process Using Focused Ion Beam
Thin-film buckling process enables curved surfaces with sizes of tens of micrometers;
the curvature enables three-dimensional chiral structures. This thin-film-buckling
method includes cutting and folding flat objects. Some three-dimensional chiral structures have gigantic chiral phenomena, compared to two-dimensional chiral structures
[15]. Furthermore, this buckling process does not require stacking and aligning,
which are necessary steps in multi-layer fabrications. The residual stress can be
induced by various stimuli, including temperature, mechanical forces, and capillary
forces [16–19]. In this subchapter, we will consider focused ion beam (FIB)-induced
buckling which is a recently emerging method.
Residual induced compressive forces have been evaluated as methods to create
chiral structures and strengthen their chiroptical responses [20]. One method to
generate residual stress in a surface of thin film is to use FIB to implant ions. This
technique requires only a single step, and has both high accuracy and high resolution, so the method can fabricate desirable chiral structures. For example [15], one
chiral structure that had a strengthened chiroptical response (Fig. 10.2a); to generate
plasmonic responses, the authors used a gold thin film in which residual stress was
imposed by global ion beam irradiation (Fig. 10.2b). Curvatures of the structures can
be modulated by controlling the dose intensity during irradiation, so chiral structures
can be shaped with quite high accuracy (Fig. 10.2c).
Compared to two-dimensional structures, buckled chiral techniques enable versatile geometries. Chirality is derived from the difference in interaction depending