Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 215

Fig. 8.5 Reflection spectra
of materials with
angle-dependent hues

q 2 > q 1 q 1 =

0°

R

q 3 > q 2

λ

R is maximum) occur at the gap wavelengths:
D 2ƒ p n 2 sin 2 =m; (8.2)

where ƒ D l 1 C l 2 is the period of the structure composed of layers with widths l i
and refractive indices n i ;i D 1;2 I is the incidence angle; m is an integer; and n
is the average refractive index given by

D l 1 n 1 C l 2 n 2 =ƒ

n 2 (8.3)

for transverse electric waves and

D l 1 1=n 1 C l 2 1=n 2 =ƒ

1=n 2 (8.4)

for transverse magnetic radiation. In the case of woodboring beetle, for m D 1,this
model predicts D 627 nm (red) at normal incidence . D 0 ı /; D 593 nm for
TE and 588 nm for TM polarized waves (yellow) at D 30 ı ,and D 488 nm
for TE waves and 472 nm for TM radiation (green) at D 75 ı . The values are in
agreement with the observed D 639 nm (red) at normal incidence, D 600 nm
(yellow) at D 30 ı ,and D 505 nm (green) at D 75 ı . An artificial structure
designed according to this simple model, with SiO layers with a refractive index
of 1.9 replacing chitin and Ni layers with a complex index of refraction 1:85 C
i3:48replacing air, was fabricated. The bioinspired structure showed similar angle
dependence of the dominant color since the thicknesses of the SiO and Ni layers
were chosen to obtain almost the same average refractive index as in the beetle
cuticle: 170 nm and 10 nm, respectively. The dominant wavelengths of the artificial
structure consisting of six periods were D 702 nm (red) at normal incidence,
D 662 nm at D 30 ı ,and D 495 nm (green) at D 75 ı when illuminated
with unpolarized white light.

8.2 Bioinspired Devices

Light manipulation by bioinspired devices has many applications in optoelectronics,
especially in nanoscale lasers, photodetectors, and photovoltaic cells. For instance,
broadband antireflection coatings for use in solar cells can be designed by drawing