Biomedical Engineering Reference
In-Depth Information
39]illustratedthatthereisasignificantdifferencebetweenmaterial
gain values and the effective gain values in the presence of resonant
structures.
Additional insight into the effective gain behavior of the active
CNPs was given in [17]. In contrast to the plasmon amplification
conceptpromotedin[40],theideawasemphasizedthatdespitethe
CNPs being electrically small, their ability to capture and localize
the excitation fields leads to the enhanced effective gain values.
The plasmons indeed play a crucial role, but it is to provide the
feedback mechanism that creates the resonance. The ENG shell
configuration is resonant and has a very large response even
without the presence of the gain medium [3-10]. The resulting
electrically small resonators foster the localization of the fields,
which in turn promotes their stronger interaction with the gain
material when it is present. Moreover, it has been recognized for
some time that electrically small ENG metallic particles can have
effective absorption cross-sections that are much larger than their
geometrical cross-section values in the vicinity of their plasmon
resonances,forexample,atopticalfrequencieswhere
ε
rel
≈−
2[41,
42]. In particular, visualizations of the power flow near resonant
metallic (plasmonic) spheres have aided in understanding this
effect. More recently, it has been recognized that these power
flows have even more dramatic features, including vortices, and
that they have a significant impact on how energy is transferred
into and out of these ENG particles [43, 44]. We will illustrate
that the flow of energy into the active CNPs has similar behaviors,
that is, the effective cross-sections of the active CNPs are much
larger than their geometrical value and, consequently, localize more
excitation field in the gain region of the resonant cavity. Moreover,
this captured and localized field energy in the gain region is then
amplified by the plasmon-based electrically small resonant cavity.
The flow of this amplified energy out of the gain region is further
facilitated by the resonant geometry. This plasmon-engineered
behavior leads to the smaller actual gain values that are su
cient
to achieve lasing/amplification. Finally, we will contrast the active
CNP geometry with the IO-CNP configuration within this model.
We will then suggest enhanced versions of both systems, which
furtherpromotethelocalizationofthefieldsinthegainregion,thus
