Chemistry Reference
In-Depth Information
approximately half the wavelength of the visible light remained a challenge [
16
].
Self-assembly approaches have also been demonstrated, including photonic crys-
talline colloidal arrays [
17
21
], block copolymers [
22
], opals and inverse opals [
23
]
and nanocomposites [
24
]. Bottom-up approaches involved self-assembly of
preformed building blocks such as monodisperse colloidal objects into periodic
gratings. Such building blocks may be silica (SiO
2
), polystyrene microspheres, or
block copolymers. The symmetry, lattice constant of the crystal and the index of
refraction contrast can be controlled to fabricate ordered photonic structures. For
example, block copolymers self-assemble into periodic regions through phase
separation of chemically different polymer blocks [
25
]. In order to achieve visible-
light Bragg diffraction, the diameter of the colloids with ranges from 100 to 1
-
μ
m
may be used to form one, two and three-dimensional photonic structures [
26
28
].
Self-assembled photonic structures reduce materials of fabrication and lower costs
as compared to nano/microfabricated photonic devices.
Stimulus-responsive materials have been incorporated into these photonic devi-
ces to induce a change in their lattice constants or spatial symmetry of the crystalline
array, and refractive index contrast. For example, refractive-index tuneable oxide
materials such as WO
3
,VO
2
, and BaTiO
3
have been incorporated in these matrices
to produce photonic structures that are sensitive to electric
-
field or temperature [
29
].
The crystalline colloidal arrays in
ltrated with liquid crystals optically responded to
an applied external electric field and an increase in the temperature of the device
[
30
32
]. Numerous fabrication strategies and materials science have been developed
to build responsive photonic structures for applications in sensing chemical stimuli,
temperature variation, light, electrical and magnetic
-
fields and mechanical forces
[
33
40
]. However, the challenges included limited tuneability, slow turnaround
times and hysteresis. Another critical fabrication issue has been the narrow response
range due to the limited external stimuli-induced changes in the lattice spacing or the
index of refraction. To overcome these challenges, polymer chemistries, new
building blocks and tuning mechanisms evolve to create practical approaches. These
methods offer potential feasibility for producing diffraction gratings. However, the
control over the material selection, patterning ability, angle of diffraction, three-
dimensional organisation of diffracting gratings and rapid manufacturing have been
limited. To overcome these limitations, generic fabrication approaches have been
developed to improve the capabilities of incorporating 3D images with optical
tuneability [
18
,
21
]. An emerging platform among these approaches is holography,
which allows fabrication of optical sensors with Bragg gratings for applications in
the quanti
-
cation of chemical, biological and physical stimuli [
41
].
2.2 History of Holography
Holography allows recording three-dimensional images of an object or digital
information through the use of a light-sensitive material and laser light, or micro/
nanofabrication techniques [
42
46
]. In 1865, Maxwell had proposed theoretically
-
