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in AR structures,
-II anti-reflectors are mostly found
to depend on GRIN structuring on a single material backbone,
in contrast to the composite materials used in
Generation
Generation
-I
structures. Notably,
-II technology mostly uses tip-like or
tapered nanostructures (Figs. 2.21 and 2.22) and interestingly the
fabrication technique is overwhelmingly tilted in favour of top-down
approaches [74].
Generation
-III AR technology is going to focus on the refinement
of the GRIN technology and mimicking not only the natural but also
the theoretical AR structures that promised ideal AR properties.
Literature comparison of periodic or aperiodic ARCs reveals that the
achieved reflectance values strongly depend on the height/length
(
Generation
H
, Fig. 2.25a) and spacing (
S
, Fig. 2.25b) of GRIN nanostructures.
Figure 2.25
(a, b) Reflectance (
: specular) as a
function of (a) length and (b) spacing for periodic (
: hemispherical;
,
) and
aperiodic (
)nanostructures. Reprinted from Ref. [1],
Copyright 2010, with permission from Elsevier.. The top dashed
line indicates the reflectance (
,
90%) of polished bulk gold
(Au), while the lower dashed line indicates the NIST standard
of reflectance (1.4%). Reprinted with permission from Ref.
[49]. Copyright 2008 American Chemical Society. The arrows
and the shaded regions indicate the future directions for better
AR efficiency and design. Representative source reference
numbers are shown alongside the data. (c) Schematic of
optimal GRIN AR structure, where
∼
denotes the
height of the cylindrical part, tapered part and total structure,
respectively. Here,
h
′,
h
′′, and
H
S
is the inter-structure spacing that controls
its density, and
is the process controlled diameter of the
structure. The apex part (indicated by red colour) can have (c)
straight-tapered, (d) convex-tapered, (e) concave-tapered, (f)
moth eye, or (g) theoretical quintic-index profiles.
L
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