Biomedical Engineering Reference
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The eficiency of the light cross-coupling can also be enhanced
by using corrugated thin ilms. The luorescence emission from a
structure with a corrugated thin metal ilm is over 50 times greater
than that from a similar planar structure.
47
Samples containing a
planar metal ilm with a corrugated dielectric overlayer have similar
properties. The strong luorescence emission is mediated via coupled
SPPs, a factor which is promising for devices such as surface-emitting
LEDs.
10.2.2.1.3
Roughened surfaces
The coupling of a luorophore with a corrugated surface is a
simpler approach than that with a roughened surface. In other
words, roughened surfaces can be considered as multi-corrugated
surfaces provided that the roughness is suficiently small.
38
The emission of GaN quantum wells can be enhanced on rough
metallic layers (50 nm thick Ag ilm) mainly because of the eficient
scattering of SPPs into radiative modes.
48
A modulation depth of
the Ag surface is approximately 30-40 nm with a length scale of
a few hundred nanometers. An enhancement of light emission is
generated using a corrugated grating while no enhancement is
observed for either an uncoated GaN or a thick Ag ilm evaporated
surface. This measurement suggests that the size of the metal
structures determines the SP-photon coupling and light extraction.
The presence of metal nanostructures has a dramatic effect on
the redirection of the angular emission of an individual molecule as
well as on the enhancement of the excitation rate of the luorophore.
Researchers have experimentally observed the inluence of nearby
metal NPs on the angular photon emission on a single molecule
(carbocyanine dye) for various dipole orientations.
49
For
z
-oriented
molecules the emitted photons are attracted toward the center of
the metal coating. For
x
-oriented molecules an inluence is observed
only for larger probe-sample separations.
In this section, we show how luorescence enhancement on a
lat surface is improved by surface shaping, additional corrugation,
and roughness. Fluorescence is ampliied by propagating surface
plasmon modes. In the case of highly roughened surfaces, multiple
electron scattering induces localization of the surface plasmons.
The next section deals with SEF using localized and non-propagating
surface plasmons. Successive enhancement processes starting from
the simplest nanometric objects (single particles) to more complex
surfaces will be described.
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