Anti-Reflecting Nanostructures - Biomimetic Architectures by Plasma Processing - page 75

In the VIS region, Si-based AR micro-structures could suppress
reflectance upto ~4% (Fig. 2.21a). However, increasing expertise
in nanoscale processing, as evident from Fig. 2.21b,c, leads to a
striking improvement, by reducing the reflectance below 0.1%,
which is predominantly structure determined. Over the 250-400
nm UV regimes, compared with planar Si, AR nanostructures could
routinely reduce the reflectance, specular or hemispherical, in the
range of 1-5% (Fig. 2.22). Notably, there is a trend, that the random
or aperiodic nanostructures, with no specific feature dimension or
spacing, perform better than the periodic ones.
For non-silicon AR structures (Fig. 2.23), the reflectance minima
or the transmission maxima, in VIS or IR region, does not show any
clear evolution with time but impressive numbers were obtained.
Interestingly, while the

-I technology mostly employed
multi-layer of alternate high- and low-RI films, the

Generation

-
II technology relied heavily on the gradient-index structures
inspired biologically through moth's eyes or cicada's wings. In
fact, a comparison of natural and artificial AR nanostructures
(Fig. 2.24) reveals that there is certain tend to push the limits of the
spectral regime of AR. In addition, accounting the material usage

Generation

Figure 2.24 Representative broadband applicability of natural and artificial

AR structures: The extent of the black and coloured lines
indicates the spectral band over which those AR structures
can perform. Representative SEM images and source reference
numbers (in square brackets) are shown alongside the data.
Reprinted from Ref. [1], Copyright 2010, with permission from
Elsevier.