Biomedical Engineering Reference
In-Depth Information
As detailed in [18], the change in the permittivity where the QDs
are located, that is, the gain medium, can be taken into account
with a Maxwell-Garnet-based Lorentz model. Incorporating this
permittivity model into the Mie multipole-representation of the
fields, a variety of designs have been explored. The two designs
whose performance characteristics were noticeably the best were
the Ag/QD/SiO
2
(called the QD-CNP) and QD/Ag/SiO
2
(called the
IO-CNP)configurations[18].Botharethree-layerdesignsconsisting
of the same materials, but differ in the placement of the QD layer.
TheQDlayeroftheQD-CNPdesignliesbetweentheAgshellandthe
SiO
2
core, while the QD layer of the IO-CNP is the outermost layer.
The core and shell sizes were adjustable parameters; the QD size
was fixed by the manufacturer's specifications. Both of the resulting
optimized designs feature a 17.5 nm radius core and a 6.0 nm thick
QD layer. For both cases, a single layer of QDs is su
cient to achieve
a very large resonant response. The optimized thickness of the Ag
shellintheQD-CNPdesignwas6.2nm(29.7nmtotalradius);itwas
6.3 nm thick (29.8 nm total radius) for the IO-CNPdesign.
We have found that placing the active material near the shell
boundary can greatly increase the performance of the active CNP.
In fact, although we have studied designs with the active region
near, but separated from the shell, we have seen that the optimal
location of the active region is directly adjacent to the shell.
Although it is known that placing the active region next to a metal
will enhance the nonradiative decay rate of the active material,
that is, it will quench the emission rate, recent theoretical and
experimental studies have shown that plasmonic-related effects
can actually significantly enhance the radiative decay rates [47-
53]. In particular, by properly designing a multi-layer structure to
retain the unique optical and electronic properties of the QD and
CNP and to provide a much larger density of radiating states to
which the emissions can couple, one can produce active CNPs with
very e
cient florescence behaviors, especially for materials with
high internal quantum e
ciencies that QDs possess. Even though
the metal layer is thin and the QDs are not, which would lead to
lower quenching effects in any event, designs exhibiting the large
radiated power enhancements associated with a super-resonance
state correspond to ones that foster enhanced radiative decays.
