Chemistry Reference
In-Depth Information
simulation, we can predict the optical characteristics such as the periodicity of the
transmission grating, regions that will be patterned at a given transmission or
absorption values and the number of multilayer gratings that will be formed during
fabrication.
2.4.2 Simulations of the Optical Readouts
The operation of the holographic sensor is governed by the periodicity of its lattice
spacing, which controls the propagation of light through the structure. The lattice
periodicity consists of an alternating pattern of mesoscale Ag
0
NP regions organised
in a speci
c direction within a hydrogel [
88
,
89
]. If the absorption of light by the
entire structure is minimum and a contrast is present between the periodic Ag
0
NP
regions, some frequencies are
filtered out as they pass through the photonic
structure. The excluded group of frequencies is called the photonic band gap (PBG).
The dynamic coloration is generally obtained by altering the periodicity of nano-
particle regions either by changing the lattice spacing or the refractive index of the
multilayers through chemical reactions. Dynamic coloration in nature include
sh
(e.g. Paracheirodon innesi)[
90
,
91
], cephalopods (e.g. Euprymna scolopes)[
92
]
and beetles (e.g. Tmesisternus isabellae)[
93
]. Holographic sensors are analogous to
these structures, where the frequency range is designed for a speci
c PBG. For
example, for infrared frequencies, micron dimensions are required for the geometry
of the structure [
94
]. In holographic sensors, the Ag
0
NP-based multilayer structure
that was formed within the hydrogel acts as a dynamic 1D photonic crystal, which
diffracts the frequencies of electromagnetic radiation that fall within the band gap
region. When the band gap region shifts its position to higher or lower frequencies
by changing the geometry of the hydrogel, different frequencies are back scattered.
To present the principle of operation and provide evidence for subsequent opti-
misation of a holographic sensor, a
finite element method based on computational
software COMSOL Multiphysics
®
, was utilised [
82
,
83
,
95
]. The theoretical
diffraction grating consisted of periodic layers of Ag
0
NPs in a hydrogel matrix. The
diffraction grating patterns consisting of stacks of randomly-sized Ag
0
NPs were
generated using a MATLAB
®
code. Since the hydrogel matrix has a refractive
index close to that of water, and the laser wavelength used for the photochemical
patterning was
/2n results in a lattice
constant of l = 176 nm. The 1D periodic array of stacks consisted of Ag
0
NPs,
which were designed as nanospheres with different radii (Fig.
2.8
a).
The simulated geometry consisted of 6 stacks with
λ
= 532 nm, according to Bragg
'
s law,
λ
60 Ag
0
NPs per stack.
Along the vertical axis of each stack, the Ag
0
NPs were uniformly distributed,
whilst in the horizontal axis, the Ag
0
NPs were distributed within the layers de
*
ned
by the laser-induced photochemical patterning. To achieve this, a normal random
distribution was performed with the mean positions of the stacks set to a distance
equal to the lattice constant. Additionally, to obtain a realistic photonic structure in
terms of representing a holographic sensor, a normal random distribution was also
