Chemistry Reference
In-Depth Information
Fig. 2.11 Simulated transmission spectra for the holographic sensors as a function of the number
of Ag
0
NP stacks. a 4, b 5 and c 6 stacks of Ag
0
NPs with a lattice constant of 182 nm. d The wave
propagation spectra for the transmission along the photonic structure with 4, 5 and 5 Ag
0
NP
stacks. Stop band is centred at
550 nm. Reproduced from [
83
] with permission from The Royal
*
Society of Chemistry
The position of the re
ection band was at 550 nm, and it did not change by adding
or removing stacks of Ag
0
NP with the same periodicity. As the number of Ag
0
NP
stacks increased, the intensity of the re
fl
fl
ected light also increased. For 6, 5, 4 stacks,
60, 48 and 40 % re
fl
ection was obtained. At 6 stacks the re
fl
ection was stronger
than con
ection, the
wider the trough was. The width at half maximum (FWHM) of the 4 stack curve
was 160 nm, whereas 6 stacks had 110 nm. Consequently, the greater the number of
Ag
0
NP stacks, the deeper the PBG trough and the narrower the bandwidth was.
The concentration of Ag
0
NPs of the holographic sensors was varied from 20 to
80 Ag
0
NPs per stack (Fig.
2.12
a
gurations with fewer stacks. Additionally, the lower the re
fl
d). Comparing the geometries of the models, as the
number of Ag
0
NPs per stack increased, the stacks became more uniform; resem-
bling a continuous medium with fewer voids. Hence, the effective index of refraction
of the stacks differed in each case. 20 Ag
0
NPs per stack produced a weak re
-
fl
ection
(Fig.
2.12
e). With an increasing number of Ag
0
NPs per stack, the re
fl
ection band
ection for 80 Ag
0
NPs
per stack. Increasing the number of Ag
0
NP increased the contrast of the index of
refraction, thus resulting in higher diffraction ef
became stronger, with the deepest one reaching 65 % of re
fl
ciencies. However, as the density of
Ag
0
NPs per stack increased, the net absorption also increased, leading to effectively
lower transmission. The position of the trough shifted to longer wavelengths when
the concentration of Ag
0
NPs per stack increased. At 20 Ag
0
NPs per stack, the dip of
the curve was located at
60 Ag
0
NPs per stack, the dip was
530 nm, but for 40
*
-
located at
555 nm. This shift could be due to a shift in the surface plasmon
resonance caused by the close proximity of Ag
0
NPs, and the overall increase in the
size of the stacks [
96
]. Therefore, an increase in the Ag
0
NP concentration per stack
resulted in an increase in the refractive index contrast of the holographic sensors.
*
