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K. Imaeda and K. Imura
In order to overcome the dipolar approximation, we induced localized optical
fields in mesostructures with dimensions ranging from several tens of nanometers to
a few micrometers. Mesoscopic structures can confine light fields tightly on subwavelength scales to generate nanoscale light fields in close proximity to the structures.
This light confinement capability paves the way for the development of novel excitation schemes beyond conventional light-matter interactions. For example, confined
fields possess larger electric field gradients and wavevectors than plane waves: these
characteristics surpass conventional frameworks and can potentially induce laddertype transitions, intraband transitions, and indirect band-gap transitions. In addition,
mesoscopic structures amplify the electromagnetic field of light, which can boost
a variety of molecular optical responses, such as nonlinear optical responses and
magnetic dipole responses [8–11]. Furthermore, mesoscopic structures can modulate the electronic states of molecules via near-field interactions and strong coupling
schemes, leading to the active control of molecular functionalities [12–17].
To utilize localized fields for photophysical and photochemical processes, a
detailed understanding of optical fields is absolutely indispensable. For this purpose,
visualization of the optical field distribution on mesoscopic structures is highly desirable. However, the spatial scale of mesoscopic structures is much smaller than the
diffraction limit of light, and thus a conventional optical microscope cannot visualize these fields in real space. The scanning near-field optical microscope (SNOM)
is a powerful tool for the nanoscale optical imaging of near-field distributions of
mesoscopic structures [18, 19]. We used the SNOM in conjunction with a variety of
spectroscopic methods to develop nano-spectral imaging techniques to elucidate the
near-field spatial features of mesoscopic structures. We also explored the near-field
interactions between molecules and mesoscopic optical fields. We demonstrated that
these localized optical fields can be used to induce novel photophysical and photochemical processes beyond the conventional light-matter interaction. In this chapter,
we describe the development of advanced near-field imaging methods, the characterization of localized fields and plasmons excited in mesostructures, and applications
of optical fields to photophysical and photochemical processes.
6.2 Development of Advanced Near-Field Imaging Methods
The optical characteristics of mesostructures are closely related to the elementary
excitations induced by electromagnetic fields of light. For example, unique optical
characteristics of noble metal mesostructures are derived from the coherent oscillation of conduction electrons, which is known as surface plasmon resonance. Plasmon
resonance confines free propagating light into a subwavelength space to generate
intense optical fields in the vicinity of mesostructures [20–23]. To exploit these plasmonic fields as a nanoscale light source, the spatial structures of the fields have
to be elucidated. So far, we have visualized the spatial patterns of the plasmonic
fields induced on mesostructures of various shapes using an aperture-type SNOM
[18]. In most of the studies with the aperture-type SNOM, the transmitted light or
K. Imaeda and K. Imura
In order to overcome the dipolar approximation, we induced localized optical
fields in mesostructures with dimensions ranging from several tens of nanometers to
a few micrometers. Mesoscopic structures can confine light fields tightly on subwavelength scales to generate nanoscale light fields in close proximity to the structures.
This light confinement capability paves the way for the development of novel excitation schemes beyond conventional light-matter interactions. For example, confined
fields possess larger electric field gradients and wavevectors than plane waves: these
characteristics surpass conventional frameworks and can potentially induce laddertype transitions, intraband transitions, and indirect band-gap transitions. In addition,
mesoscopic structures amplify the electromagnetic field of light, which can boost
a variety of molecular optical responses, such as nonlinear optical responses and
magnetic dipole responses [8–11]. Furthermore, mesoscopic structures can modulate the electronic states of molecules via near-field interactions and strong coupling
schemes, leading to the active control of molecular functionalities [12–17].
To utilize localized fields for photophysical and photochemical processes, a
detailed understanding of optical fields is absolutely indispensable. For this purpose,
visualization of the optical field distribution on mesoscopic structures is highly desirable. However, the spatial scale of mesoscopic structures is much smaller than the
diffraction limit of light, and thus a conventional optical microscope cannot visualize these fields in real space. The scanning near-field optical microscope (SNOM)
is a powerful tool for the nanoscale optical imaging of near-field distributions of
mesoscopic structures [18, 19]. We used the SNOM in conjunction with a variety of
spectroscopic methods to develop nano-spectral imaging techniques to elucidate the
near-field spatial features of mesoscopic structures. We also explored the near-field
interactions between molecules and mesoscopic optical fields. We demonstrated that
these localized optical fields can be used to induce novel photophysical and photochemical processes beyond the conventional light-matter interaction. In this chapter,
we describe the development of advanced near-field imaging methods, the characterization of localized fields and plasmons excited in mesostructures, and applications
of optical fields to photophysical and photochemical processes.
6.2 Development of Advanced Near-Field Imaging Methods
The optical characteristics of mesostructures are closely related to the elementary
excitations induced by electromagnetic fields of light. For example, unique optical
characteristics of noble metal mesostructures are derived from the coherent oscillation of conduction electrons, which is known as surface plasmon resonance. Plasmon
resonance confines free propagating light into a subwavelength space to generate
intense optical fields in the vicinity of mesostructures [20–23]. To exploit these plasmonic fields as a nanoscale light source, the spatial structures of the fields have
to be elucidated. So far, we have visualized the spatial patterns of the plasmonic
fields induced on mesostructures of various shapes using an aperture-type SNOM
[18]. In most of the studies with the aperture-type SNOM, the transmitted light or