Bioinspired Devices - Bionanoelectronics: Bioinquiring and Bioinspired Devices - page 217

Not only periodic biological structures achieve light modulation but also pho-
tonic amorphous structures, in which only short-range order but no long-range
order exist. These structures are at the origin of noniridescent colors in nature.
A biomimetic photonic amorphous structure, consisting of holes with a radius
of about 76 nm and an average distance of 255 nm in a freestanding 190-nm-
thick GaAs membrane containing InAs quantum dots as active media, has been
shown to produce laser radiation at frequencies for which the short-range order
enhance optical confinement ( Noh et al. 2011 ). The emission frequency of photonic
amorphous laser structures can be tuned by modifying its structural parameters.
This type of optically pumped laser is intermediate between photonic crystal
lasers, characterized by long-range periodicity and frequency bandgaps used to
confine light and random lasers, in which the feedback mechanism is multiple light
scattering in a disordered medium.
Apart from light modulation, nature teaches us how to efficiently collect light
though highly specialized mechanisms of eye sight which, depending on the animal
species, are optimized for different illumination conditions: near/far, day/night, and
wide/narrow field of view ( Lee and Szema 2005 ). In applications, we often need
to design optical devices with large field of views and small volumes, the design
of which can be inspired from flies since their compound eyes have exactly these
desired features.
The fabrication of a three-dimensional compound eye consisting of 601 chan-
nels/microassemblies of microprisms and microlenses, ultraprecisely machined
with diamond on curved and flat surfaces, respectively, in an area with a 20-mm
diameter, has been reported in Li and Yi ( 2010 ). Cross talking between channels
is eliminated by an aperture array. This system, represented schematically in
Fig. 8.6 , allows for a maximum light deviation angle of 18:43 ı . It is a replica
of the eye of a fly, which is composed of basic imaging unit, called ommatidia,
implemented as individual channels. The optical components on the curved surface
generate an image on a flat plane, which can be sensed with commercially available
photodetectors. All microlenses have an aperture of 0.58 mm and a curvature radius
of 4 mm, while the microprisms arranged on 12 layers/circles have the same width
of 0.6 mm, but different refracting angles depending on their location. The prism at
the vertex is planar and is the only one to be associated with more than one (with
six) microlens(es).
The photodetectors in the previous compound eye could have been placed on
the imaging plane. However, it is possible to fabricate a hemispherical electronic

microprisms

3D compound
eye

Fig. 8.6 Artificial
three-dimensional
compound eye

microlenses

imaging plane