Biodetection Based on Fluorescence Quenching and Surface-Enhanced Fluorescence Using Noble Metal Nanoparticles - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 229

the photoluminescence enhances while the excited state lifetime
decreases, showing that the relaxation channels of a single quantum
emitter can be controlled by coupling it to an eficiently radiating
metallic nanoantenna. We point out that the optimum coupling
requires impedance matching, depending on the wavelength and
polarization of the light involved. Because the dielectric function
of metal in the visible regime and in the radio wave regime differs
signiicantly with respect to ohmic losses and plasmon resonance, the
optimized antennas are signiicantly different in the two regimes.
In a mixture of metal NPs of various sizes, localized surface
plasmon (LSP) resonance occurs at various photon energies,
depending on the wavelength and the polarization of the incident
light. Because of different LSP resonances, EM ields couple only
within distances of less than 10 nm. However, coupling of LSP with
long range interactions through the propagative modes of SPP can
occur in random (semicontinuous) ilms. 74,75 The peculiarity of
semicontinuous ilms lies in the range of accessible shapes and sizes
of metal NPs on the surface. They are usually prepared by thermal
evaporation of a metal on a dielectric surface followed by annealing.
Metal evaporation ends around the percolation threshold when the
irst continuous electric path between opposite sides of the substrate
appears. Around the percolation threshold, almost every size of metal
NPs is present in the ilm, giving rise to resonances all over the ilm
from the visible to the far infrared. In such ilms, the propagation of
SPP enhances the resonant effect of LSP, leading to higher intensities
of the propagating SPP waves. Not only does the random nature of
the LSP modes broaden the accessible launching angles for SPP, but
the various shapes of NPs also increase the number of accessible
resonance wavelengths and polarizations.
The localization of light can also be carried out on rough Au ilms,
aggregates, and colloidal ilms. 56-58 The size of the high intensity
peaks, called hot spots, at which light is localized is much smaller
than the wavelength of light. The predicted wavelength dependence
of localized optical excitations has been experimentally shown. 76 A
slight change in the wavelength or in the polarization properties of
light allows changes in the position of these hot spots. Because of
their strong nonlinear properties, random metal ilms are especially
important for nonlinear, optical applications. 54,77 The local
distribution of the EM ield allows dramatic increases in nonlinear
effects 51,53 that are used for multiphoton enhanced luorescence. 78