Chemistry Reference
In-Depth Information
10
−
3
produces a
15
6 nm Bragg peak shift for a sensor originally operating at
600 nm [
80
]. With the grating period and probe angle remaining constant, it is
preferable to record the hologram at a longer wavelength. The absolute change in the
peak wavelength (
×
*
) can be increased by choosing materials with lower initial
effective refractive index (n). Materials with higher porosity have lower effective
refractive index. Moreover, for the detection of larger size analytes, it is preferable to
use recording media with larger pore size, which allow the diffusion of the analyte
into the polymer matrix easily. Both the dimensional and the effective refractive
index effects contribute simultaneously to the change in the spectral response of the
hologram. For example, in gelatin-based sensors the effective refractive index
decreases as the sensor absorbs water and swells; thus, the two factors have opposite
contributions to the spectral shift. However, in some materials, one of the factors is
the main contributor. For example, in humidity sensors recorded in acrylamide-
based photopolymer, the main contributor is the swelling of the polymer matrix due
to absorption of moisture studied at relative humidity up to 80 % [
81
].
ʔλ
2.4 Computational Modelling of Holographic Sensors
in Fabrication and Readout
The principles of laser light interference in the fabrication of responsive diffraction
gratings are discussed. This chapter is divided into two parts; while the
rst part
explains the photochemical patterning during recording of holographic sensors in
Denisyuk re
fl
ection mode, the second part describes the operation of the sensors.
The
first part focuses on the fundamentals of the laser writing in which materials get
physically broken, displaced or removed by means of optical forces and thermal
energy. In order to understand the different phenomena during photochemical
patterning, interference patterns during laser light exposure were simulated. The
second part of this chapter demonstrates computational simulations of a generic
holographic sensor through a
finite element model [
82
,
83
]. To design the sensors
with predictive characteristics, its optical properties due to variation in the pattern
and the characteristics of the NP arrays were evaluated. Various factors including
NP size and distribution within the polymer matrices that directly affect the per-
formance of the sensors were studied computationally.
2.4.1 Photochemical Patterning
In order to predict the interference patterns, which produce the photonic structure in
Denisyuk re
ection mode, the system was modelled during fabrication as arising
from a superposition of different light waves [
84
fl
87
]. Figure
2.7
a shows a schematic
of the experimental setup during laser light exposure. Simplifying the simulation of
-
