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H. Ishihara et al.
surrounding environment. Therefore, to maximize the potential of superchiral fields
for measuring single molecules, we must develop a measuring device for the in-situ
evaluation of the near field.
Recent studies reported the NF-CDs for various nanostructures and obtained characteristic signals that reflected their local geometry [10–13]. However, these studies
measured the 2D projections of the NF-CD, which are the images of the propagated
component of the scattered light in the vicinity of the target. Therefore, it is difficult
to obtain the information regarding the localized longitudinal field, which is a significant component of the near field. Hence, we propose another scheme that uses optical
force under laser illumination. If the targeted sample and the probe tip of the atomic
force microscope are irradiated by light, dipole-dipole interaction occurs between
them, and the near-field profile around the sample can be obtained by detecting the
induced force, which is proportional to the gradient of the field intensity. This technique is called photoinduced force microscopy (PiFM) [26]. Because this scheme is
sensitive to the longitudinal component of the polarization of the chiral near field, it
can simultaneously acquire the 3D profile of the near field, i.e., the transverse and
localized longitudinal components. To examine the validity of the scheme, for the
superchiral field near the specific metallic structure, the force-measurement calculation results are compared with the electromagnetic-field calculation results.
5.2.1 Model and Method
The system under consideration is depicted in Fig. 5.1. In our numerical demonstration, the metallic structure, which comprises four gold gammadions, is set on the
dielectric substrate. This structure exhibits large FF-CD [4] and boosts the FF-CD
of a few molecules [14, 15]. The tip probe is modeled using a gold hemisphere and
is scanned at 5 nm above the metallic structure. The circularly polarized plane wave
lights are illuminated from below the substrate and propagate along the z-axis. We
calculated the total electric field near the metallic gammadion and probe tip to detect
the optical force.
Based on linear response theory, we can express the induced polarization P(r i , ω)
= χ(r i , ω)E(r i , ω) at the position r i , i.e., the i-th cubic cell of the volume V c , where
E(r i , ω) denotes the total electric field. We set the local susceptibility in the substrate
to χ(r, ω) = ε s − 1 with the relative permittivity set as ε s = 2.25. On the other
hand, the gold gammadion structures and the probe tip are composed of cells, whose
dielectric constant is represented using the Drude model [27]. The dielectric function
here is Drude-type, i.e.,
χ metal (ω) = metal (ω) − 1 = b − 1 −
(
pl
)
2
2 ω 2 + iω( bulk +
v f
L eff
)
,
(5.1)
H. Ishihara et al.
surrounding environment. Therefore, to maximize the potential of superchiral fields
for measuring single molecules, we must develop a measuring device for the in-situ
evaluation of the near field.
Recent studies reported the NF-CDs for various nanostructures and obtained characteristic signals that reflected their local geometry [10–13]. However, these studies
measured the 2D projections of the NF-CD, which are the images of the propagated
component of the scattered light in the vicinity of the target. Therefore, it is difficult
to obtain the information regarding the localized longitudinal field, which is a significant component of the near field. Hence, we propose another scheme that uses optical
force under laser illumination. If the targeted sample and the probe tip of the atomic
force microscope are irradiated by light, dipole-dipole interaction occurs between
them, and the near-field profile around the sample can be obtained by detecting the
induced force, which is proportional to the gradient of the field intensity. This technique is called photoinduced force microscopy (PiFM) [26]. Because this scheme is
sensitive to the longitudinal component of the polarization of the chiral near field, it
can simultaneously acquire the 3D profile of the near field, i.e., the transverse and
localized longitudinal components. To examine the validity of the scheme, for the
superchiral field near the specific metallic structure, the force-measurement calculation results are compared with the electromagnetic-field calculation results.
5.2.1 Model and Method
The system under consideration is depicted in Fig. 5.1. In our numerical demonstration, the metallic structure, which comprises four gold gammadions, is set on the
dielectric substrate. This structure exhibits large FF-CD [4] and boosts the FF-CD
of a few molecules [14, 15]. The tip probe is modeled using a gold hemisphere and
is scanned at 5 nm above the metallic structure. The circularly polarized plane wave
lights are illuminated from below the substrate and propagate along the z-axis. We
calculated the total electric field near the metallic gammadion and probe tip to detect
the optical force.
Based on linear response theory, we can express the induced polarization P(r i , ω)
= χ(r i , ω)E(r i , ω) at the position r i , i.e., the i-th cubic cell of the volume V c , where
E(r i , ω) denotes the total electric field. We set the local susceptibility in the substrate
to χ(r, ω) = ε s − 1 with the relative permittivity set as ε s = 2.25. On the other
hand, the gold gammadion structures and the probe tip are composed of cells, whose
dielectric constant is represented using the Drude model [27]. The dielectric function
here is Drude-type, i.e.,
χ metal (ω) = metal (ω) − 1 = b − 1 −
(
pl
)
2
2 ω 2 + iω( bulk +
v f
L eff
)
,
(5.1)