Biomedical Engineering Reference
In-Depth Information
spectrum ( Figure 14.12 c) [27] . However, it should
be emphasized that these calculated results were
obtained for a perfect photonic crystal of infinite
size and free of any defects. Therefore, the width
of this calculated band gap is most likely too
narrow to stay open in a real material.
Regardless of whether a complete band gap is
formed or not, these bioreplicated structures are
by far the most efficient photonic crystals cur-
rently available for manipulating light at visible
frequencies. These findings will allow experi-
mental testing of some of the predicted new opti-
cal properties of photonic crystals with visible
light. For example, one of the original motiva-
tions behind photonic crystal research was con-
trolling dynamics of radiative processes;
however, more than 20 years after it was first
proposed, this effect has been rarely observed
due to the lack of appropriate photonic-crystal
samples.
This changed recently when the first experi-
mental studies with titania bioreplica of dia-
mond-based photonic crystals revealed the
enormous potential of this new type of optical
material. In these studies, the spontaneous
emission properties of nanometer-sized light
sources (nanocrystal quantum dots) within tita-
nia replicas were investigated [43] . Unprece-
dented modification of the dynamics of
spontaneous emission at visible frequencies
was observed, showing a variation of the
excited-state lifetime by more than a factor of
10—approximately five times higher than pre-
vious results obtained from the best synthetic
photonic crystals.
These results are extremely promising. Con-
trolling spontaneous emission lies at the heart of
many emerging applications, ranging from solar
energy conversion, solid-state lighting, and las-
ing to quantum information processing. Biorep-
lication has supplied the first efficient photonic
crystals toward these applications, and intrigu-
ing new optical properties will be made possible
through this marriage of biological structure
engineering and materials synthesis.
FIGURE 14.12 (a, b) Cross-sectional SEM images of a
bioreplicated diamond-based photonic crystal made of
titania. (c) Calculated band structure diagram for this pho-
tonic crystal lattice showing a narrow complete band gap
(gray rectangle). Scale bars are 1 μ m. Adapted from Ref. 27 .
Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Repro-
duced with permission.
positive replica of the weevil photonic-crystal
structure made of titania.
This successful replication was confirmed by
structural and optical characterization tech-
niques. For example, Figure 14.12 depicts SEM
images of the positive replica of the L. augustus
photonic crystal structure made of nanocrystal-
line titania with a measured refractive index of
2.3 ± 0.1. Using the structural and dielectric
properties of the replica, band-structure calcula-
tions revealed the formation of a complete pho-
tonic band gap (with a gap-to-mid-gap ratio of
2-5%) in the green portion of the visible
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