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layers, while the incident wave impinged normally on the surface
with a 45
polarisation. RCWA simulation predicted that reflectivity
should decrease drastically with increasing aspect ratio
°
L
d
/
, where
L
d
represent the height and the periodicity of the SWS layer,
respectively. Reflectance can be suppressed below 1 %, with
and
L
= 100
nm and
= 4.0, over a wide spectral bandwidth from 300 nm to
1000 nm [34]. Another work demonstrated that the transmittance
was increased from 96.2% (the flat surface) to 99.4% at 800 nm
(Fig. 2.6d) for the pyramidal SWS grating with 250 nm grating depth
(D), based on a model consisting of a six-layered SWS with pyramidal
cross-section, assuming the materials of the SWS and the substrate
to be PMMA (
d
/
L
= 200 nm [35].
Southwell et al. [37] have demonstrated that the surface-relief
structures having a modified pyramidal configuration (having
curved rather than flat triangular sides) possess the optimum
gradient-RI profile. Such SWS can be approximated by the quintic-
GRIN profile, presented in Eq. (2.6), with the basic pyramidal shape
differing slightly with a change in the RI of the material (Fig. 2.7a-b).
The surface reflection can be reduced by several orders of magnitude
over the broadest bandwidth. Later, based on 2D EMT, Grann et al.
[20] also pointed out that the shape of continuously tapered SW
grating structures is the best design for broadband AR surface.
n
= 1.49), with
L
Figure 2.7
(a, b) Surface-relief patterns for an equivalent quintic on the
substrate with RI of (a) 1.45 and (b) 3.42. Reprinted from
Ref. [37], with permission from The Optical Society. (c-e)
Biomimetic moth-eye profiles defined by RCWA simulation
and corresponding parameter scan of reflectance, at
l
= 1000
nm, vs. height (
) for a hexagonal array of
pillars with the pillar shapes shown above. The Si RI used is
3.617 + 0.0041i (
h
) and period (
d
= 1000 nm). Reprinted with permission
from Ref. [38]. Copyright 2008, AIP Publishing LLC (http://
dx.doi.org/10.1063/1.2993231).
l
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