Biomedical Engineering Reference
In-Depth Information
enhancements can be applied to a variety of optical effects in
which the signal is usually very low, for instance in surface
enhanced Raman scattering (SERS)
25,61
and surface plasmon
ampliication by stimulated emission of radiation.
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(iii) When
d
≥
1 nm, for
ω
≈
ω
SP
, besides the surface plasmon
resonance, the propagating modes of SPPs are excited on a
metal with macroscopic extension. In this case, the metal
system acts as an optical antenna that propagates the
excitation of the luorophore to the far ield, leading to an
increase in the luorescence eficiency and intensity but to a
shorter luorescence lifetime.
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The present section deals with the localization of light around
metal nanomaterials when illuminated by an EM wave and how these
localizations enhance the luorescence. In the following sections, we
will show how single particle and multiple particles interact strongly
with light, leading to increased luorescence. The differences in
luorescence enhancement among single objects, aggregates, and
non-continuous ilms will be discussed.
10.2.2.2.1
Single metal particles
When the localized surface plasmon resonance of metal NPs couples
with the emission of a luorophore (the dipole), four interesting
phenomena arise:
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(i) Close to a plasmon resonance, maximum radiative and
nonradiative emission intensities may occur at various
wavelengths.
(ii) The apparent quantum yield tends to be zero at the plasmon
resonance frequency or when the distance between the
luorophore and the metal particle is close to zero.
(iii) The local ield factor and the luorescence enhancement reach
a maximum at a wavelength slightly longer than the plasmon
wavelength.
(iv) The luorescence enhancement factor is much higher when
the transition dipole is along the main axis of the metal NP
(when the particle is not strictly symmetric).
Some interesting phenomena have been proved experimentally.
It has been suggested that the dipole approximation neither strongly
overestimates the quantum yield at short distances nor predicts
luorescence quenching while giving a fairly good estimate of the
excitation rate.
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In order to obtain a more accurate description
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