6 Advanced Function Control of Photochemical Reactions …
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using a confocal optical microscope and near-infrared ultrashort pulses (wavelength:
800 nm, repetition rate: 80 MHz, pulse width <100 fs). The two-photon excitation
image in Fig. 6.13b reveals that the two-photon excitation probability is locally
enhanced on the sample. This result directly indicates that locally enhanced fields
are generated on the assembly. To investigate the influence of the enhanced fields on
the optical response of the molecules, we dispersed R6G molecules on the fabricated
sample and excite Raman scattering from the molecules using a CW laser (785 nm)
as the excitation source. Figure 6.13c shows the Raman scattering spectrum from the
R6G molecules on the sample. We compared the observed Raman signal intensity
with that on the bare glass substrate and found that the Raman enhancement factor
reaches more than 10
8 . We also carried out Raman scattering measurements with
633 and 532 nm CW laser sources and found that the Raman enhancement for these
excitations is comparable to that achieved at 785 nm, indicating that the fabricated
structure exhibits intense optical fields with a broad spectral response. These characteristics are indispensable for the amplification of optical responses in molecular
systems.
6.6 Conclusion
In this chapter, we described the development of advanced near-field imaging
methods and their applications to the spatio- and temporal-characterization of plasmons excited in mesoscopic structures. The excited plasmons in these structures
exhibit periodic wavy features that depend on the size and shape of the structures
and the observed wavelength. The observed spatial features are attributed to the plasmonic standing waves generated in the structures from theoretical simulations. We
demonstrate that out-of-plane and in-plane plasmons are excited in mesoplates and
these modes can be used to amplify and modulate the optical responses of molecules
in a space-resolved manner. To utilize both in-plane and out-of-plane modes, we
fabricated gold nanoparticle trimer assemblies on gold-capped hexagonal structures.
The assemblies provide large optical field amplification, with maintaining a broad
spectral response. Plasmons excited in gold nanorods and plasmonic nanohole arrays
were also utilized to accelerate photochemical processes and the spectral profile of
molecular fluorescence. These observations clearly demonstrate that the near-field
interactions between plasmons and molecular systems modulate the electronic structure of the molecules and provide novel excitation schemes beyond the conventional
light-matter interaction. The localized optical fields generated on the mesoscopic
structures can produce novel photophysical and photochemical processes and thus are
promising for the advanced function control of materials. The use of novel excitation
111
using a confocal optical microscope and near-infrared ultrashort pulses (wavelength:
800 nm, repetition rate: 80 MHz, pulse width <100 fs). The two-photon excitation
image in Fig. 6.13b reveals that the two-photon excitation probability is locally
enhanced on the sample. This result directly indicates that locally enhanced fields
are generated on the assembly. To investigate the influence of the enhanced fields on
the optical response of the molecules, we dispersed R6G molecules on the fabricated
sample and excite Raman scattering from the molecules using a CW laser (785 nm)
as the excitation source. Figure 6.13c shows the Raman scattering spectrum from the
R6G molecules on the sample. We compared the observed Raman signal intensity
with that on the bare glass substrate and found that the Raman enhancement factor
reaches more than 10
8 . We also carried out Raman scattering measurements with
633 and 532 nm CW laser sources and found that the Raman enhancement for these
excitations is comparable to that achieved at 785 nm, indicating that the fabricated
structure exhibits intense optical fields with a broad spectral response. These characteristics are indispensable for the amplification of optical responses in molecular
systems.
6.6 Conclusion
In this chapter, we described the development of advanced near-field imaging
methods and their applications to the spatio- and temporal-characterization of plasmons excited in mesoscopic structures. The excited plasmons in these structures
exhibit periodic wavy features that depend on the size and shape of the structures
and the observed wavelength. The observed spatial features are attributed to the plasmonic standing waves generated in the structures from theoretical simulations. We
demonstrate that out-of-plane and in-plane plasmons are excited in mesoplates and
these modes can be used to amplify and modulate the optical responses of molecules
in a space-resolved manner. To utilize both in-plane and out-of-plane modes, we
fabricated gold nanoparticle trimer assemblies on gold-capped hexagonal structures.
The assemblies provide large optical field amplification, with maintaining a broad
spectral response. Plasmons excited in gold nanorods and plasmonic nanohole arrays
were also utilized to accelerate photochemical processes and the spectral profile of
molecular fluorescence. These observations clearly demonstrate that the near-field
interactions between plasmons and molecular systems modulate the electronic structure of the molecules and provide novel excitation schemes beyond the conventional
light-matter interaction. The localized optical fields generated on the mesoscopic
structures can produce novel photophysical and photochemical processes and thus are
promising for the advanced function control of materials. The use of novel excitation