Biomedical Engineering Reference
In-Depth Information
Figure 14.1
Scattering and absorption e
ciencies (a) and extinction
e
ciency (b) versus the effective gain parameter
κ
for the active Ag/SiO
2
CNP [17].
Although the response of an active CNP depends on both the
exciting wavelength and the value of
,aswellasitsgeometrical
parameters, it is observed in Fig. 14.1 that the response of an active
CNP is sensitive to the value of
κ
κ
κ
is such that
the response of the CNP achieves its largest negative value, that is,
it produces its maximum amplification or lasing, we say that the
CNP is in its super-resonant state. The threshold value (i.e., value
of
κ
at which
Q
ext
=
0) occurs near
κ
=−
0.245, as can be
seen from the zoom subplot in Fig. 14.1b. One finds that slightly
belowthisthresholdvalue,the
Q
ext
valueisextremelylargepositive,
whereas slightly above threshold it is extremely large negative. The
active CNP resonance is also sensitive to the incident wavelength.
For
κ
values above and below the one defining the super-resonant
state, the corresponding resonance wavelengths become slightly
detuned from their super-resonant value. For the indicated active
CNP geometry, the super-resonant state occurs at a wavelength of
501.9 nm when
κ
=−
0.25, that is,
Q
scat
has its maximum value for
those parameters, whereas
Q
abs
and
Q
ext
have their minimums, that
is, their largest negative values.
There are strong interactions between the incident field and
a passive or active CNP when either is responding in its super-
resonantstate.Thestrengthandcharacteristicsoftheseinteractions
are revealed by the behavior of the various Poynting vector fields in
. When the value of
