5 Light–Nanomatter Chiral Interaction in Optical-Force Effects
115
probe-tip diameter, i.e., 100 nm, is greater than the gap spacing between the gammadions in Fig. 5.5 b, c. However, when the spacing between the gammadions and
probe tip is small, the electric-field enhancement is determined only by the structure
near the probe tip. Therefore, the difference between the profile of the electric field
and the one measured using the force is not significant. In actual experiments using
optical-force microscope, the tip-sample spacing is less than 1 nm. Therefore, we
can obtain satisfactory correspondence between the optical force and field gradients
in actual experiments, as well as obtain further enhanced optical force. The information obtained provides significant insight into the spatial structures of the 3D NF-CD.
For the cases in the presence of targeted molecules on the metallic structures, more
detailed analyses of the force map to obtain the 3D NF-CD information are desired.
5.3 Optical Force to Rotate Nano-Particles in Nanoscale
Area
According to Maxwell’s theory of electromagnetism, both traveling and standing
light waves can exert mechanical force on targets because of scattering and absorption. This force is sometimes called the “optical force”. The optical force is classified
as a dissipative force, which arises from the transfer of optical momentum to a substance by absorption and scattering, and a gradient force due to the electromagnetic
interaction between the induced polarization and the incident light. The dissipative
force usually pushes and transports particles, and the gradient force can be used to
attract and trap particles. One of the most impressive applications of this force is an
optical tweezer with a focused laser, as proposed by Ashkin et al. [33]. This optical force is attributed to the transfer of momentum from light to the matter target.
Similarly, the spin and orbital angular momenta of light also can be transferred to
the mechanical motion of small particles. The light with orbital angular momentum,
such as a Laguerre–Gaussian (LG) beam [34], is considered to result in the orbital
rotation of targets. Notably, micro-particles are swirled along a ring-shaped region,
where the field intensity of the LG beam is strong [35]. However, presently, it is not
known how the optical manipulation for the rotational control in a nanoscale area
can be performed.
This section is devoted to discussing the chiral interaction between light and
metallic nanocomplexes, wherein the chiral interaction induces rotational motion
of nano-particles (NPs) in a nanoscale region. In recent years, the target of optical
manipulation has shifted to the nanometer-scale. However, within the Rayleigh scattering regime, the optical force is approximately proportional to the volume of the
object, hence, the induced force is quite weak. To enhance the force to overcome the
disturbance due to the environment of NPs, the use of the evanescent field with a steep
gradient of the electric field has been proposed [36, 37], and recently, the trapping of
NPs associated with LSP resonance has been extensively studied [38–44]. Another
approach is the use of resonance with transitions between the electronic levels in
115
probe-tip diameter, i.e., 100 nm, is greater than the gap spacing between the gammadions in Fig. 5.5 b, c. However, when the spacing between the gammadions and
probe tip is small, the electric-field enhancement is determined only by the structure
near the probe tip. Therefore, the difference between the profile of the electric field
and the one measured using the force is not significant. In actual experiments using
optical-force microscope, the tip-sample spacing is less than 1 nm. Therefore, we
can obtain satisfactory correspondence between the optical force and field gradients
in actual experiments, as well as obtain further enhanced optical force. The information obtained provides significant insight into the spatial structures of the 3D NF-CD.
For the cases in the presence of targeted molecules on the metallic structures, more
detailed analyses of the force map to obtain the 3D NF-CD information are desired.
5.3 Optical Force to Rotate Nano-Particles in Nanoscale
Area
According to Maxwell’s theory of electromagnetism, both traveling and standing
light waves can exert mechanical force on targets because of scattering and absorption. This force is sometimes called the “optical force”. The optical force is classified
as a dissipative force, which arises from the transfer of optical momentum to a substance by absorption and scattering, and a gradient force due to the electromagnetic
interaction between the induced polarization and the incident light. The dissipative
force usually pushes and transports particles, and the gradient force can be used to
attract and trap particles. One of the most impressive applications of this force is an
optical tweezer with a focused laser, as proposed by Ashkin et al. [33]. This optical force is attributed to the transfer of momentum from light to the matter target.
Similarly, the spin and orbital angular momenta of light also can be transferred to
the mechanical motion of small particles. The light with orbital angular momentum,
such as a Laguerre–Gaussian (LG) beam [34], is considered to result in the orbital
rotation of targets. Notably, micro-particles are swirled along a ring-shaped region,
where the field intensity of the LG beam is strong [35]. However, presently, it is not
known how the optical manipulation for the rotational control in a nanoscale area
can be performed.
This section is devoted to discussing the chiral interaction between light and
metallic nanocomplexes, wherein the chiral interaction induces rotational motion
of nano-particles (NPs) in a nanoscale region. In recent years, the target of optical
manipulation has shifted to the nanometer-scale. However, within the Rayleigh scattering regime, the optical force is approximately proportional to the volume of the
object, hence, the induced force is quite weak. To enhance the force to overcome the
disturbance due to the environment of NPs, the use of the evanescent field with a steep
gradient of the electric field has been proposed [36, 37], and recently, the trapping of
NPs associated with LSP resonance has been extensively studied [38–44]. Another
approach is the use of resonance with transitions between the electronic levels in