104
K. Imaeda and K. Imura
exponential functions that are shown as solid lines in Fig. 6.8b. From the figure, we
found that the decay lengths are strongly dependent on the positions on the mesoplate:
the decay length at the apex on the mesoplate is estimated to be 520 nm, whereas the
decay lengths at the center of the mesoplate are 160 and 700 nm. The position dependency of the decay lengths can be interpreted in terms of the spatial patterns of the
eigenmodes. As mentioned above, the spatial features of the near-field transmission
image are consistent with the spatial superposition of the two eigenmodes shown in
Fig. 6.7c, d. From the spatial distribution of these eigenmodes, we found that the
excitation probability of these eigenmodes are strongly dependent on the positions
on the mesoplate: the out-of-plane eigenmode shown in Fig. 6.7c is predominantly
excited at the center of the mesoplate, whereas the in-plane mode in Fig. 6.7d is
predominantly excited near the apices of the mesoplate. Considering this position
dependency of the excitation probability of the eigenmodes, the observed results in
Fig. 6.8b indicates that the decay length of the in-plane mode is longer than that
of the out-of-plane mode. This fact implies that the out-of-plane mode in Fig. 6.7c
confines light fields more tightly compared with the in-plane mode. Therefore, the
near-field distribution induced by the out-of-plane plasmon mode is more promising
for the realization of advanced function control of molecular systems.
6.4 Time-Resolved Near-Field Measurements on Metal
Mesostructures
The steady-state spatial patterns of plasmons in metal mesostructures have been
studied extensively using a variety of high-resolution microscopic imaging methods
[24, 25, 42–45, 48–59]. However, few studies have been conducted on the spatiotemporal dynamics of plasmons due to the ultrashort plasmon lifetimes of several
femtoseconds [64, 65]. Since plasmon lifetime (dephasing time) is directly related
to the enhancement of the local field inside the mesostructure [66–68], a deeper
understanding of the dynamic behavior of plasmons is highly desirable for practical
application of plasmonic fields. We observed real-time plasmon dynamics using
the ultrafast near-field optical microscope shown in Fig. 6.1. This apparatus can
achieve high spatial and temporal resolutions simultaneously, enabling the spaceand time-resolved imaging of plasmons induced in metal mesostructures. First, we
explored plasmon dynamics in a one-dimensional gold nanorod [69]. Figure 6.9a
shows a near-field two-photon excitation image of a chemically synthesized gold
nanorod (length ~240 nm, width ~40 nm). In this image, two bright spots are clearly
observed along the long axis of the nanorod, indicating the resonant excitation of
a second order plasmon mode. We performed time-resolved two-photon induced
photoluminescence (TPI-PL) autocorrelation measurements by changing the delay
time between the two excitation pulses. The red dotted curve in Fig. 6.9b shows the
TPI-PL autocorrelation trace taken near the edge of the nanorod. We also measured
the SHG autocorrelation trace on a BBO crystal, as shown by the blue curve in
K. Imaeda and K. Imura
exponential functions that are shown as solid lines in Fig. 6.8b. From the figure, we
found that the decay lengths are strongly dependent on the positions on the mesoplate:
the decay length at the apex on the mesoplate is estimated to be 520 nm, whereas the
decay lengths at the center of the mesoplate are 160 and 700 nm. The position dependency of the decay lengths can be interpreted in terms of the spatial patterns of the
eigenmodes. As mentioned above, the spatial features of the near-field transmission
image are consistent with the spatial superposition of the two eigenmodes shown in
Fig. 6.7c, d. From the spatial distribution of these eigenmodes, we found that the
excitation probability of these eigenmodes are strongly dependent on the positions
on the mesoplate: the out-of-plane eigenmode shown in Fig. 6.7c is predominantly
excited at the center of the mesoplate, whereas the in-plane mode in Fig. 6.7d is
predominantly excited near the apices of the mesoplate. Considering this position
dependency of the excitation probability of the eigenmodes, the observed results in
Fig. 6.8b indicates that the decay length of the in-plane mode is longer than that
of the out-of-plane mode. This fact implies that the out-of-plane mode in Fig. 6.7c
confines light fields more tightly compared with the in-plane mode. Therefore, the
near-field distribution induced by the out-of-plane plasmon mode is more promising
for the realization of advanced function control of molecular systems.
6.4 Time-Resolved Near-Field Measurements on Metal
Mesostructures
The steady-state spatial patterns of plasmons in metal mesostructures have been
studied extensively using a variety of high-resolution microscopic imaging methods
[24, 25, 42–45, 48–59]. However, few studies have been conducted on the spatiotemporal dynamics of plasmons due to the ultrashort plasmon lifetimes of several
femtoseconds [64, 65]. Since plasmon lifetime (dephasing time) is directly related
to the enhancement of the local field inside the mesostructure [66–68], a deeper
understanding of the dynamic behavior of plasmons is highly desirable for practical
application of plasmonic fields. We observed real-time plasmon dynamics using
the ultrafast near-field optical microscope shown in Fig. 6.1. This apparatus can
achieve high spatial and temporal resolutions simultaneously, enabling the spaceand time-resolved imaging of plasmons induced in metal mesostructures. First, we
explored plasmon dynamics in a one-dimensional gold nanorod [69]. Figure 6.9a
shows a near-field two-photon excitation image of a chemically synthesized gold
nanorod (length ~240 nm, width ~40 nm). In this image, two bright spots are clearly
observed along the long axis of the nanorod, indicating the resonant excitation of
a second order plasmon mode. We performed time-resolved two-photon induced
photoluminescence (TPI-PL) autocorrelation measurements by changing the delay
time between the two excitation pulses. The red dotted curve in Fig. 6.9b shows the
TPI-PL autocorrelation trace taken near the edge of the nanorod. We also measured
the SHG autocorrelation trace on a BBO crystal, as shown by the blue curve in