5 Light–Nanomatter Chiral Interaction in Optical-Force Effects
107
Anomalous optical responses of nanostructures due to the nonlocal characteristics
of the response were theoretically proposed [19, 20], and have been experimentally demonstrated as the giant nonlinear response of quadrupole excitons [21], the
optical forbidden electronic transition of a single-wall carbon nanotube [22], and
the ultra-short radiative decay time in the femtosecond regime [23, 24], etc. Chiral
interaction of light–nanomaterials is another frontier wherein the nonlocal optical
response induces anomalous effects beyond LWA or DA.
Based on the nonlocal scheme, we have focused on the mechanical force (optical
force) associated with the chiral interaction between plasmonic near-field and matter. Specifically, the following two issues are introduced. The first issue is the threedimensional (3D) near-field CD during the optical-force measurement. As aforementioned, the aperture-type SNOM is a powerful tool to unveil the chiral near field [13].
However, it is difficult to elucidate the 3D structure of a superchiral field, especially
around the edges of metallic structures, as the longitudinal component is dominant
there. We show that if we measure the optical-force between the dipoles induced on
the sample and the probe tip irradiated by light, we can observe the 3D distribution of
the electric-field intensity to evaluate the 3D near-field CD (3D NF-CD). The other
issue is the optical manipulation of nanoparticles (NPs) near metallic nanocomplexes
with a high degree of freedom. We show that the flexible rotational optical manipulation, such as the rotation control in nanoscale area and switching of the rotational
direction, of nanoobjects can be achieved. This means that all the basic elements
of nanoobject-motion control (i.e., pushing, pulling, and rotating) are realized in
principle.
The remainder of this chapter is organized as follows: In Sect. 5.2, we discuss the
manner in which we can observe the chiral near field in the vicinity of metallic chiral
structures. In addition, we propose a scheme to measure the 3D NF-CD using the
optical force that visualizes CD with nm resolution by numerically demonstrating
the CD map of the observed force on the gammadion metallic structures. In Sect. 5.3,
we demonstrate that the NPs can be mechanically rotated and manipulated using the
designed chiral field by plasmonic structures [25]. In this demonstration, we specifically consider that utilizing the nonlinear optical response considerably enhances
the degree of freedom to manipulate NPs.
5.2 3D Near-Field CD by Optical-Force Measurement
One of the goals of analytical chemistry is to determine the structures of isolated
single molecules. Accordingly, the single-molecule chiral analysis gains importance.
To that end, the superchiral field is a promising tool [14–17], and its electromagneticfield analysis is essential. However, in the aforementioned studies, the far-field CD
(FF-CD) was evaluated based on the extinction of the light propagating through
the target. The FF-CD signal is a convenient indicator that provides macroscopic
information on the integrated target ensemble. However, nanoscale structures such
as single molecules are averaged together with the information of the microscopic
107
Anomalous optical responses of nanostructures due to the nonlocal characteristics
of the response were theoretically proposed [19, 20], and have been experimentally demonstrated as the giant nonlinear response of quadrupole excitons [21], the
optical forbidden electronic transition of a single-wall carbon nanotube [22], and
the ultra-short radiative decay time in the femtosecond regime [23, 24], etc. Chiral
interaction of light–nanomaterials is another frontier wherein the nonlocal optical
response induces anomalous effects beyond LWA or DA.
Based on the nonlocal scheme, we have focused on the mechanical force (optical
force) associated with the chiral interaction between plasmonic near-field and matter. Specifically, the following two issues are introduced. The first issue is the threedimensional (3D) near-field CD during the optical-force measurement. As aforementioned, the aperture-type SNOM is a powerful tool to unveil the chiral near field [13].
However, it is difficult to elucidate the 3D structure of a superchiral field, especially
around the edges of metallic structures, as the longitudinal component is dominant
there. We show that if we measure the optical-force between the dipoles induced on
the sample and the probe tip irradiated by light, we can observe the 3D distribution of
the electric-field intensity to evaluate the 3D near-field CD (3D NF-CD). The other
issue is the optical manipulation of nanoparticles (NPs) near metallic nanocomplexes
with a high degree of freedom. We show that the flexible rotational optical manipulation, such as the rotation control in nanoscale area and switching of the rotational
direction, of nanoobjects can be achieved. This means that all the basic elements
of nanoobject-motion control (i.e., pushing, pulling, and rotating) are realized in
principle.
The remainder of this chapter is organized as follows: In Sect. 5.2, we discuss the
manner in which we can observe the chiral near field in the vicinity of metallic chiral
structures. In addition, we propose a scheme to measure the 3D NF-CD using the
optical force that visualizes CD with nm resolution by numerically demonstrating
the CD map of the observed force on the gammadion metallic structures. In Sect. 5.3,
we demonstrate that the NPs can be mechanically rotated and manipulated using the
designed chiral field by plasmonic structures [25]. In this demonstration, we specifically consider that utilizing the nonlinear optical response considerably enhances
the degree of freedom to manipulate NPs.
5.2 3D Near-Field CD by Optical-Force Measurement
One of the goals of analytical chemistry is to determine the structures of isolated
single molecules. Accordingly, the single-molecule chiral analysis gains importance.
To that end, the superchiral field is a promising tool [14–17], and its electromagneticfield analysis is essential. However, in the aforementioned studies, the far-field CD
(FF-CD) was evaluated based on the extinction of the light propagating through
the target. The FF-CD signal is a convenient indicator that provides macroscopic
information on the integrated target ensemble. However, nanoscale structures such
as single molecules are averaged together with the information of the microscopic