1 Nanoplasmonics: From Present into Future
31
pronounced hot-spot behavior with inhomogeneous localization, giant fluctuations
in space, where the distributions and intensities of individual hot spots strongly and
randomly change with frequency. These distributions are in a full qualitative agreement with the theoretical predictions for the hot spots of local nanoplasmonic fields
[157, 158]—cf. above Figs. 1.8, 1.9.
We emphasize again that the PEEM-based observation of the plasmonic hot spots
is completely non-perturbing. The photo-emitted electrons that are used in the PEEM
fly away from the metal surface naturally, no matter whether they are used for imaging
or not.
There has also been a series of research dealing with the observation of the plasmonic hot spots using the scanning near-field optical microscope (NSOM or SNOM)
[155, 162, 168]. In fact, the first experimental evidence of the nanoplasmonic hot
spots has been obtained [162] using an aperture-type NSOM, which is a based a
tapered optical fiber with the tip covered by a metal. A general concern about such
observation is that they are perturbative: the tip of NSOM (or nanoscope, as it is
often called) is typically much larger than a hot spot. Made of metal, it can, in principle, modify the host spot by both shifting its resonant frequency and decreasing
the quality factor.
As an example, we present Fig. 1.12 adapted from Ref. [168]. This study is done
on the semicontinuous metal film (random planar composite, or RPC). At relatively
low values of the fill factor, f = 0.36 and f = 0.45, the local intensity distribution
I (r) shows relatively delocalized regions elongated normally to the direction of
propagation (vertical axis in the figure). These are analogous to the caustics of the
usual 3d optics. Relatively close to the percolation point, f = 0.66 and f = 0.73,
the distribution I (r) becomes highly localized exhibiting singular hot spots. The
behavior of I (r) at a relatively high fill factor of f = 0.83 again reminds that for the
low f showing delocalized caustics but not singular hot spots. This is understandable
because in this case the system is basically a smooth film with a few defects. This
film supports SPPs that are weakly scattered by the relatively few defects.
As we have discussed above in this section, NSOM measurements of hot spots are
inherently perturbative. While PEEM is nonperturbative, the spatial resolution so far
has been insufficient (due to aberrations in the electron optics and large spread of the
f=0.45
f=0.65
f=0.73
f=0.83
f=0.36
Fig. 1.12 NSOM images of 4 × 4 µm 2 semi-continuous silver films with different metal filling
fractions f as indicated above the graphs. Local intensity distribution is displayed as a function of
the spatial coordinates in the plane of the film. The white areas correspond to higher intensities.
Adapted from Ref. [168]
31
pronounced hot-spot behavior with inhomogeneous localization, giant fluctuations
in space, where the distributions and intensities of individual hot spots strongly and
randomly change with frequency. These distributions are in a full qualitative agreement with the theoretical predictions for the hot spots of local nanoplasmonic fields
[157, 158]—cf. above Figs. 1.8, 1.9.
We emphasize again that the PEEM-based observation of the plasmonic hot spots
is completely non-perturbing. The photo-emitted electrons that are used in the PEEM
fly away from the metal surface naturally, no matter whether they are used for imaging
or not.
There has also been a series of research dealing with the observation of the plasmonic hot spots using the scanning near-field optical microscope (NSOM or SNOM)
[155, 162, 168]. In fact, the first experimental evidence of the nanoplasmonic hot
spots has been obtained [162] using an aperture-type NSOM, which is a based a
tapered optical fiber with the tip covered by a metal. A general concern about such
observation is that they are perturbative: the tip of NSOM (or nanoscope, as it is
often called) is typically much larger than a hot spot. Made of metal, it can, in principle, modify the host spot by both shifting its resonant frequency and decreasing
the quality factor.
As an example, we present Fig. 1.12 adapted from Ref. [168]. This study is done
on the semicontinuous metal film (random planar composite, or RPC). At relatively
low values of the fill factor, f = 0.36 and f = 0.45, the local intensity distribution
I (r) shows relatively delocalized regions elongated normally to the direction of
propagation (vertical axis in the figure). These are analogous to the caustics of the
usual 3d optics. Relatively close to the percolation point, f = 0.66 and f = 0.73,
the distribution I (r) becomes highly localized exhibiting singular hot spots. The
behavior of I (r) at a relatively high fill factor of f = 0.83 again reminds that for the
low f showing delocalized caustics but not singular hot spots. This is understandable
because in this case the system is basically a smooth film with a few defects. This
film supports SPPs that are weakly scattered by the relatively few defects.
As we have discussed above in this section, NSOM measurements of hot spots are
inherently perturbative. While PEEM is nonperturbative, the spatial resolution so far
has been insufficient (due to aberrations in the electron optics and large spread of the
f=0.45
f=0.65
f=0.73
f=0.83
f=0.36
Fig. 1.12 NSOM images of 4 × 4 µm 2 semi-continuous silver films with different metal filling
fractions f as indicated above the graphs. Local intensity distribution is displayed as a function of
the spatial coordinates in the plane of the film. The white areas correspond to higher intensities.
Adapted from Ref. [168]