Biomedical Engineering Reference
In-Depth Information
FIGURE 14.10
(a-c) SEM images of diamond-based photonic crystal structures found in exoskeleton scales of the weevils
Eupholus schoenherri
,
Pachyrhynchus moniliferus
, and
Eudiagogus pulcher
, respectively. (d) Calculated dielectric function of the
diamond-based lattice showing three orthogonal planes (air: dark gray; biopolymer: light gray). Adapted from Ref.
26
and
reproduced by permission of The Royal Society of Chemistry.
making this biological structure an ideal candi-
date for biotemplating
[27]
.
gap width; and (4) a variation in replica lattice
within 15-20% of the original structure to assure
the band gap remains within visible frequencies
[27]
.
Even though the calculations and modeling
results give very clear instructions on how an
optimal structure needs to be designed, trans-
lating these results into a solution-based tem-
plating process is rather challenging. Sol-gel
chemistry, with its flexible processing parame-
ters and versatility, is promising; however, it is
necessary to adjust and modify the typical sol-
gel infiltration and processing conditions to
meet all the geometric and dielectric require-
ments for this replica.
The first predicted property, the creation of a
positive replica of the original beetle photonic
crystal structure, requires a double-templating
process since a negative replica of this structure,
which could be obtained by a single replication
step, was predicted to lack a complete band gap.
One way to meet this goal is to fabricate an
14.3.3 Toward New Optical Materials
After the discovery of photonic crystals with
the desirable diamond-based lattice geometry in
weevil scales, the fabrication of the first complete
photonic band gap at visible frequencies became
a real possibility. Using the structure found in
L.
augustus
as a basis, modeling and photonic band
structure calculations revealed the geometric
and dielectric properties needed for a complete
band gap to form in replicated samples: (1) a
positive replica of the original beetle photonic
crystal lattice; (2) a framework compound (high
refractive-index component) refractive index of
at least 2.1 to open a band gap and at least 2.3 for
a 5% wide (gap-to-midgap ratio) band gap; (3)
a high refractive-index component volume frac-
tion between 30% and 40% to optimize the band