Oxide-Based Photonic Crystals from Biological
Templates
Michael H. Bartl, Jeremy W. Galusha, Matthew R. Jorgensen
Department of Chemistry and Department of Physics
University of Utah, Salt Lake City, UT 84112
9.1 Introduction
The ability to organize crystalline and amorphous metal oxide
compounds into periodically ordered three-dimensional structures
has led to a range of novel functional materials. Today, periodically
ordered metal oxide frameworks with periodicities spanning several
orders of magnitude from the microscale (several Ångstroms) to the
mesoscale (2 to 50 nanometers) and to the macroscale are available
[1-6]. Regardless of the framework feature size, it is the combination
of inherent metal oxide properties with three-dimensional structure
organization that gives periodically ordered composites their unique
functionalities and has led to a broad field of new applications.
2
While micro- and mesostructured metal oxides organized by small
molecules [7, 8] and supramolecular self-assembly chemistry [5, 9,
10], respectively, are useful in sorption, separation and catalysis [2,
3, 8], as low-k dielectrics [11], and for various optical, optoelectronic, and energy applications [10, 12-14], macrostructured metal oxides with periodic feature sizes from a few hundred nanometers to
several micrometers [6, 15-17] are prime candidates for so-called
photonic crystals [18, 19].
Photonic crystals, originally proposed by John [20] and Yablonovich [21] in 1987, are an emerging type of optical materials
with the potential to manipulate light in revolutionary new ways.
The defining characteristic of photonic crystals is a periodic variation of the dielectric constant (or refractive index) with periodicities
on the order of the photon-wavelength of interest. Due to this periodic variation of the dielectric constant, the behavior of light in photonic crystals is governed by band structure concepts—similar to
how electrons are affected in crystalline atomic crystals [19]. This
unique, non-classical behavior has the potential to lead to fundamentally new optical principles based on light localization,[20, 22, 23]
3
making photonic band structure materials particularly interesting for
next-generation all-optical information processing and advanced energy technologies [24].
Interestingly, millions of years before we “invented” photonic band structure materials, nature had already made use of these optical concepts for creating spectacular structural colors in many insects, birds, and marine animals. In particular, the strikingly colorful
world of insects is in large part the result of optical interference produced by the interaction of light with precisely ordered, periodic biopolymeric structures, incorporated into their wings and exoskeletons [25-33]. These biological systems have evolved to create
astonishingly complex photonic architectures—structures that are
still far out of our synthetic reach [34-36]. While biological photonic
crystals present exciting structural alternatives to our current limited
photonic engineering capabilities, unfortunately, they do not possess
the high photo- and heat-stability required for most advanced applications. In addition, biological photonic structures are composed of
electrically insulating, low-dielectric biopolymers, which strongly
limits their device integration. These problems can be overcome, by
4
converting biopolymeric photonic structures into inorganic, highdielectric compounds by biotemplating methods [35, 37-43].
In this chapter we will describe and compare different approaches for creating three-dimensional photonic crystals. Particular
focus will be on oxide-based photonic crystals with complex lattice
structures created using replication routes from various biological
photonic templates.
We will cover three main topics, beginning with a brief introduction of photonic crystals, emphasizing the non-classical optical properties of these band structure materials. We will review the
most important top-down and bottom-up fabrication methods for engineering three-dimensional photonic crystals operating in the infrared and visible part of the electromagnetic spectrum and also discuss
current limitations of these approaches.
Next, we will introduce biological systems that employ photonic band structure concepts to create a wide variety of structural
colors, spanning the entire visible range. A new structure evaluation
method based on sequential focused ion beam milling and scanning
electron microscopy imaging will be discussed. This method enables
5
a high-resolution three-dimensional reconstruction of photonic architectures and gives unprecedented insights into the structureoptical properties relationship of biological complex photonic crystal
lattices.
Then, we will discuss the potential of biological photonic
crystal architectures as biotemplates for conversion into oxide-based
high-dielectric replicas. We will briefly introduce and compare different biotemplating methods, including atomic layer deposition,
conformal evaporation films by rotation, and sol-gel chemistry. The
structural and optical properties of oxide-based photonic crystals obtained from these three methods will be addressed. Special focus
will be directed toward sol-gel biotemplating methods that have recently resulted in the fabrication of a photonic crystal with a complete band gap at visible frequencies. We will finish our chapter with
some conclusions and a brief outlook on the potential and promise of
oxide-based three-dimensional photonic crystals for novel optical
phenomena at visible frequencies.
6
9.2 Engineered Photonic Crystals
9.2.1 Characteristics of Photonic Band Structure Materials
In the following section we will give a brief introduction into
characteristics and properties of photonic crystals. It is, however,
beyond the scope of this chapter to provide a detailed discussion of
the physics of photonic crystals and the foundation of photonic band
structure properties and we refer the interested reader to the excellent literature available; see, for example [18, 19, 23, 24] and references therein.
Photonic band structure materials, also known as photonic
crystals, are periodically ordered dielectric composite structures with
periodicities ranging from a few hundred nanometers to several micrometers [18, 19]. The fascination with these materials stems from
their ability to strongly affect the propagation of light in nonclassical ways, making them interesting candidates for nextgeneration optoelectronic applications and all-optical information
processing [24]. The non-classical properties of photonic crystals are
the direct result of their periodic variation of the dielectric constant.
Photons with wavelengths comparable to the variation-periodicity of
7
a given photonic crystal are strongly affected by Bragg diffraction
events—similar to how electrons are affected in atomic crystals. As
a consequence, many concepts of solid state physics can be applied
to describe the properties of photonic crystals, leading to direction
and frequency-dependent photonic band structure diagrams with allowed optical bands and so-called photonic bandgaps (Fig 1).
Fig. 1 Calculated photonic band structure diagram for a photonic crystal consisting of dielectric spheres with a refractive index of 3.6 arranged in a diamond
structure. Adapted from [50]
While photonic crystal can be designed with periodicities in one,
two, or three dimensions, of particular interest are three-dimensional
photonic crystals, since they can have a complete (or omnidirectional) photonic bandgap—a frequency range for which propagation of
light is strictly forbidden [19-21]. Photonic bandgaps are the basis
for several new optical concepts such as control of spontaneous
emission in bulk materials, slow-light enhanced photocatalysis, lowthreshold light amplification and quantum information processing
[22-24, 44-48].
Motivated by the range of new and exciting optical phenomena predicted for photonic bandgap materials, numerous efforts have
8
been undertaken to fabricate photonic crystals with complete
bandgaps in the microwave, infrared and visible regions of the electromagnetic spectrum. In general, the band structure properties of a
photonic crystal are determined by its lattice morphology and the refractive index contrast of its dielectric building blocks. Along with
possessing a high transparency in the wavelength range of interest,
the two different (high and low) dielectric materials building up the
photonic crystal lattice should also have strongly differing refractive
indices; for example, air and silicon (for photonic crystals operating
in the infrared) or air and titanium dioxide (for photonic crystals operating in the visible). In addition to having a high refractive index
contrast, it is also of great importance that these dielectric building
blocks are arranged in an optimal lattice structure. While every periodically ordered structure results in a direction-dependent energy
dispersion of photonic states, only a few selected lattices also give
rise to the formation of complete photonic bandgaps [45, 49-55].
Among them the diamond crystal structure is the clear “champion”
[52]. Moreover, the diamond crystal structure has been shown to be
rather insensitive to deviations from the ideal lattice configuration,
9
resulting in a variety of so-called diamond-based (or diamond-like)
morphologies that allow a complete bandgap to open for refractive
index ratios as low as 2.1 [50, 52, 53].
Fig. 2 Diamond structure models. a) Dielectric spheres arranged in the diamond
lattice. b) Dielectric rods connecting nearest-neighbor sites in the diamond lattice.
Adapted from [52]
To date, tremendous progress in photonic structure engineering has been made in the microwave and infrared regimes. Using
top-down microfabrication and bottom-up colloidal self-assembly
techniques, various three-dimensional photonic crystal structures
have been synthesized with complete bandgaps at infrared frequencies [56-64]. In contrast to the successes in the infrared regime,
complete photonic bandgaps at visible frequencies have proven elusive due mainly to difficulties of creating efficient three-dimensional
photonic lattices with feature sizes in the hundred-nanometer range.
In the following section we will briefly review fabrication strategies
for photonic crystals operating in the infrared and visible regime.
We will discuss advantages and limitations of different top-down
10
and bottom-up fabrication techniques focusing on attainable feature
sizes, crystal lattices and high-dielectric compounds.
9.2.2 Photonic Crystals Operating in the Infrared
The fabrication of photonic band gap crystals operating at infrared
frequencies has benefited tremendously from powerful microprocessing techniques that have been optimized in the semiconductor
industry during the last 50 years. These techniques can generally be
classified into direct and indirect methods. In the former, a desired
photonic crystal structure is formed directly out of a high-dielectric
semiconductor compound. For example, Lin and co-workers used a
comprehensive multi-level stacking process consisting of a repeated
deposition, lithographic patterning, and etching to successfully fabricate dielectric woodpile (a diamond-based lattice structure) photonic crystals with bandgaps in the infrared regime [58]. Subsequently, Noda et al. developed a wafer-fusion based method to
create woodpile structures made out of GaAs with a complete
bandgap at near infrared wavelengths [59], whereas Johnson, Joannopoulos and co-workers designed and fabricated a nine-layer pho-
11
tonic crystal with a wide (up to 25 percent gap-to-mid-gap ratio)
bandgap out of silicon by sequential layer-by-layer scanningelectron-beam lithography [61].
A common disadvantage of these direct methods is that fabrication of high-quality three-dimensional photonic crystals is very
time consuming, expensive and is generally limited to only a few
layers. Indirect methods, on the other hand, use a template structure
created out of inexpensive polymers. This structure serves as a sacrificial mold for templating high-index compounds such as silicon or
germanium. Successfully applied methods to create such polymeric
photonic crystal template structures, including the highly efficient
diamond-based woodpile lattices, are multi-beam holography, multiphoton lithography, and direct laser writing methods [64-66]. An interesting alternative to these light-patterning routes is the direct ink
writing method originally developed by Lewis and co-workers [67,
68]. In this technique, a cylindrical filament approximately 1 micrometer in diameter is formed by deposition of a fluidic polyelectrolyte/water ink into an alcohol-rich reservoir. Braun, Lewis and
coworkers demonstrated that this filament can then be patterned in a
12
layer-by-layer sequence to build a woodpile structure with photonic
crystal feature sizes in the near infrared [57, 69].
All of these polymeric templates can then be converted into
high-dielectric photonic crystals made out of silicon or germanium.
Since the polymeric templates would not withstand the high deposition temperatures required for typical semiconductor deposition
techniques such as chemical vapor deposition, they are first protected by a metal oxide (silica or alumina) coating formed by atomic
layer deposition. For example, Ozin and coworkers showed that depending on the amount of metal oxide deposition (complete backfilling or deposition of a thin coating) it is possible to create highdielectric photonic crystals in the form of a positive replica or an inverse of the original template structure [64, 70]. While a silicon
double-inversion procedure produced a woodpile photonic crystal
with a complete (up to 9 percent wide) bandgap in the infrared [70],
Hermatschweiler et al. showed a silicon inverse woodpile photonic
crystals with a more than 14 percent wide complete bandgap centered at a wavelength of around 2.5 micrometer can be fabricated by
a silicon single-inversion method [64]. Braun, Lewis and co-workers
13
used a similar—although independently developed—technique to
convert direct ink-writing-created woodpile templates into germanium photonic crystals with wide (up to 25 percent) complete
bandgaps centered at a wavelength of around 6 micrometers (Fig. 3)
[57].
Fig. 3 Scanning electron microscopy images of an inverse germanium woodpile
structure fabricated by direct ink writing and a combination of atomic layer deposition and chemical vapor deposition. Adapted from [57]
An interesting—fast, simple, and low-cost—alternative to
these rather labor-intensive routes is colloidal self-assembly [71-73].
In this bottom-up photonic crystal fabrication technique monodisperse microspheres are deposited onto planar substrates by self- or
directed assembly in close-packed face-centered-cubic or hexagonally-close-packed colloidal crystals (also called artificial opals, since
these colloidal crystals closely resemble the microstructure of natural opal gemstones). Similar to the indirect methods described
above, these colloidal crystals are then used as templates and are infiltrated with an infrared-transparent high-dielectric component. After selective removal of the opal template a so-called inverse opal
photonic crystal (a close-packed face-centered-cubic lattice of air
14
spheres in a high dielectric material) is obtained [45]. While inverse
opal photonic crystals are less effective (i.e. less efficient in affecting and controlling the propagation of light) than diamond-based lattices, it was shown that formation of a complete photonic bandgap is
possible provided that the high dielectric material has a refractive of
3 or higher versus air as the low dielectric component [49]. Using
polycrystalline silicon as the high dielectric component (with a refractive index of 3.2 to 3.4) John and co-workers [56] and Norris
and co-workers [62] successfully fabricated inverse opal photonic
crystals with a complete bandgap in the near infrared.
9.2.3 Photonic Crystals Operating at Visible Frequencies
Compared to the enormous progress achieved in fabricating photonic crystals operating in the infrared, photonic structure engineering in the visible is far less advanced—due mainly to the difficulties
in shaping visible-light transparent, high-dielectric materials into efficient morphologies with periodicities at visible wavelengths. Unlike infrared photonic crystals with complete bandgaps of up to 2030 percent gap-to-mid-gap ratios [57-63], enabled by infrared-
15
transparent materials with refractive indices of 3.2 and higher, the
lack of visible-light-transparent dielectrics with comparable refractive indices embosses an enormous challenge for achieving complete
bandgaps at visible wavelengths. The best compounds for photonic
crystals in the visible are cadmium chalcogenides and oxide semiconductors such as zinc oxide and titanium dioxide (titania) with refractive indices in the range of 2.0-2.6. Calculations show the lowered refractive index ratio as compared to infrared compounds not
only reduces the width of potential complete bandgaps to below 10
percent, but also limits photonic crystal morphologies with potentially complete bandgaps at visible frequencies to pyrochlore and diamond-based crystal lattices [50-52].
Unfortunately, successful synthesis of such lattices with feature sizes at visible length scales is extremely challenging. On the
one hand, typical microfabrication methods used to create diamondbased photonic crystal structures in the infrared rely on lithographic/holographic or direct-writing methods and are therefore very difficult to successfully implement at the small length scales required
for three-dimensional structures with bandgaps in the visible.
16
Subramania and co-workers showed that this obstacle could be overcome by electron beam direct-writing of a titania woodpile structure
with lattice parameters in the visible [74, 75]. However, creating
large-scale structures with this technique has proven extremely challenging, limiting the fabricated photonic crystals to 9 alternating
layers. On the other hand, bottom up supramolecular self-assembly
of polymeric building blocks into diamond-based lattices is limited
to features sizes less than 150 nanometers due to the increasingly
slowed assembly kinetics of the required ultra-high molecular
weight monomers, restricting this technique to fabrication of photonic crystals operating in the ultraviolet regime [76].
A fabrication technique that readily bridges the feature-size
gap between microfabrication and supramolecular assembly is colloidal self-assembly of submicrometer spheres [71-73]. Indeed, colloidal crystals can be formed with periodicities spanning the entire
visible range and can be converted into inverse opals by infiltration
with visible-light-transparent dielectric compounds [6, 15, 17, 7779]. This approach has been used to create inverse opal photonic
crystals composed of high-dielectric metal oxide compounds using
17
various infiltration techniques. For example, successful procedures
were reported by Stein and co-workers [6, 80] and Vos and coworkers [17, 81], who used alkoxide-condensation in air to create titania, zirconia and alumina inverse opals and Pine and co-workers
[77, 79], who showed that titania and silica inverse opals can also be
prepared by infiltration of opals with ultrafine colloidal particles of
the oxides. Recently, Bartl and co-workers developed a new titania
sol-gel precursor and combined it with a lift-off/turn-over infiltration/processing technique to fabricate planar titania inverse opals
with an open surface and defined thickness (Fig. 4) [15].
Another powerful method to infiltrate polymeric templates is lowtemperature atomic layer deposition [82]. For example, King et al.
used this method to fabricate titanium dioxide inverse structures
from various self-assembled and holographically-prepared templates
with highly controlled filling fractions and excellent quality [83-85].
Fig. 4 Top (a-c) and side-view (d) scanning electron microscopy images of planar
open-surface titania inverse opal photonic crystals. Images are taken along the
[111] direction (a, b) and the [100] direction (c). Adapted from [15]
18
Despite this intense research in developing high-quality inverse opals, these materials fail to produce complete photonic
bandgaps in the visible. The reason lies in the crystal structure of
opals, which lacks the desired diamond-based symmetry. As mentioned above, spherical building blocks have a natural tendency to
form densely packed lattices. Such opal-based crystals, however, require refractive index ratios greater than 3 to form complete photonic bandgaps [49]. Unfortunately, this is far beyond the reach of visible-light-transparent compounds and consequently, even the best
inverse opal photonic crystals have only directional (incomplete)
bandgaps at visible frequencies.
To conclude, despite promising proposals for directed assembly routes [51, 54] and proof-of-principle electron beam directwriting fabrication of titania woodpile structures [74, 75] and
sphere-by-sphere assembly using a micromanipulator [86], largescale synthetic photonic crystals with complete bandgaps at visible
frequencies have yet to be achieved.
19
9.3 Natural Photonic Crystals
9.3.1 Structural Colors in Biology
In contrast to our current limitations in photonic structure engineering at visible length scales, biological systems have developed a
wealth of photonic structures composed of biopolymeric components such as chitin and keratin. These structures efficiently interact
with visible light for the production of vibrant colors in wings,
feathers, hairs, or as part of insect exoskeleton [25-35, 87-89]. Biological structural colors have been developed for a large variety of
reasons ranging from camouflaging to frightening predators and attracting mates. This requires an array of optical effects: From strongly iridescent to near angle-independent coloration, from shimmering
bright to matte hues, from brilliantly sparkling to pastel-like colors
and various combinations thereof. Moreover, biological photonic
structures were optimized to operate under various illumination conditions; while many species create their optical effects in bright sunlight, others operate under highly scattering conditions produced by
water-droplets and wet leaves in rainforests. Others have to function
under dim illumination at the forest floor or within dense vegetation.
20
These different applications, optical effects and environmental conditions have led to a diversity of photonic structures in biology that
is virtually limitless.
For example, simple one-dimensional multilayer structures
are found in many species of butterflies and beetles, which employ
alternating bio-polymeric layers with slightly different refractive indices [90-92]. More sophisticated examples are found in marine animals such sea urchins [93] and the sea mouse [94], whose skeleton
and hairs, respectively, have an internal two-dimensional periodic
lattice . Two-dimensional photonic structures are also found in the
feathers of several species of birds [87, 88, 95]. In addition to these
simpler one- and two-dimensional structures, many species of butterflies (Lepidoptera) and beetles (Coleoptera) obtain their striking
coloration from various three-dimensional architectures with lattice
constants spanning the entire visible regime (see, for example, Fig.
5). Examples of such cuticular (biopolymeric) photonic crystal architectures range from quasi-periodic lattices to chiral, honeycomb,
and non-close-packed ball-stick structures as well as various cubic
lattice symmetries [28, 29, 31, 34, 35, 89, 96-100].
21
Fig. 5 a) Photograph of the weevil Lamprocyphus augustus. b) Optical micrograph of individual exoskeleton scales of L. augustus under white-light illumination. c) Cross-sectional scanning electron microscopy image of a single scale. d)
Detailed cross-sectional scanning electron microscopy image of a region of a
scale. Adapted from [34].
It is interesting that most of the photonic structures found in
butterflies and beetles display symmetry-breaking strategies to produce tailored optical effects. The role of symmetry in photonic crystals is two-fold, consisting of a delicate interplay between the overall
symmetry of the system given by the crystal lattice structure and the
building blocks occupying the individual lattice sites. While the
symmetry of the overall structure needs to be such that the resulting
Brillouin zone is as spherical as possible, the symmetry of the building blocks need not be high. In fact, lowering the symmetry of the
building blocks (for example from spherical to cylindrical or even
lower) can strongly enhance the photonic properties in these materials [101]. Using designed building block shapes provides a means to
fine-tune photonic properties and engineer photonic crystal structures with specific optical qualities.
22
9.3.2 Structure Evaluation Methods
The concept of symmetry-breaking/lowering is a widely used
strategy in biological photonic crystals, making it difficult to evaluate their exact three-dimensional lattice architecture [34, 89, 91,
102-104]. Typical top-view and cross-sectional scanning electron
microscopy imaging is a powerful technique to investigate structural
features at the relevant tens to hundreds of nanometer length scale.
However, such randomly taken two-dimensional views lack sufficient morphological detail to enable a complete understanding of the
three-dimensional lattice architecture. This difficulty is particularly
exacerbated for biological photonic structures in which the lattice
building blocks often deviate strongly from simple spherical shapes.
The situation is further complicated by the fact that photonic structures in most biological systems are polycrystalline, consisting of
several micrometer sized arrays of differently oriented singlecrystalline domains [29, 34]. These varying hierarchical orderings
and myriad structural fine-features make it exceedingly difficult to
find a single method for solving the lattice geometries of all the different biological photonic architectures. Consequently, a range of
23
structure investigation techniques have been developed and applied
to biological structures. In general, these techniques can be divided
into scattering methods (various scatterometry and spectrophotometry techniques) and structural methods (electron microscopy imaging-based procedures). As an example, we will briefly discuss a
novel electron microscopy-based structure-evaluation technique in
the following section and refer the interested reader to an excellent
review by Vukusic and Stavenga and references therein [104].
To solve hierarchically-ordered three-dimensional photonic
structures hidden within exoskeleton-scales of colored beetles, Galusha et al. developed a high-resolution structure evaluation method
that combines scanning electron microscopy with focused ion beam
milling and subsequent volume rendering/computer modeling [29,
34, 89]. This technique allows a three-dimensional reconstruction of
biological photonic structures with unprecedented resolution (~30
nanometers). In short, serial sectioning was performed by consecutively milling away ~30 nanometer sections of the structure using a
focused ion beam. After each milling step the freshly exposed crosssectional two-dimensional view of the structure was imaged by
24
scanning electron microscopy. This resulted in a three-dimensional
data set consisting of a series or “stack” of two-dimensional images,
each with a “thickness” of ~30 nanometers—significantly less than
the lattice constants of the periodic structures (~300-500 nanometers). This stack of images was smoothed and aligned using a recursive algorithm in ImageJ [105, 106]. The pre-processed stack of
consecutive scanning electron microscopy images was then imported
into the SCIRun visualization package for volume rendering [107].
High-resolution structural information was obtained by comparing
and analyzing a large number of oblique “cuts” through the virtual
reconstruction of the three-dimensional biological photonic structure. For example, Fig. 6 shows as comparison of the calculated/reconstructed structures of different crystal faces with the crystal
faces of the actual three-dimensional photonic crystal lattice of the
weevil Lamprocyphus augustus (details are given in the following
section).
Fig. 6 a) Scanning electron microscopy image of a scale’s top-surface exposed by
focused ion beam milling. b), c), d) Scanning electron microscopy images of individual single-crystalline domains indicated in a) with corresponding calculated
dielectric functions (black-framed insets). Adapted from [34]
25
9.3.3 Examples of Biological Photonic Structures
In this section we will introduce and discuss specific biopolymeric
photonic crystal structures found in the colored wing and exoskeleton scales of butterflies and beetles. From the seemingly infinite
pool of biological photonic architectures, the ones described below
were chosen because of their unique—and in most cases synthetically out-of-reach—lattice structures. These particular biopolymeric
lattices have therefore been the templates of choice in bioinspired
photonic crystal fabrication, as will be discussed later in this chapter.
Among the large group of structural color effects in butterfly
wing scales, those found in the Nymphalidae, Lycaenidae and Papilionidae families are particularly interesting due to their diversity in
structure/coloration and underlying hierarchical architectures, consisting ridges, lamellae and ribs [31, 33, 103, 108-110]. While these
main structural features are found in most colored wing scales, it is
their spatial arrangement, relative size and orientation that results in
the wide variation of optical effects produced by butterflies. The fa-
26
mous tropical Morpho butterflies, for example, obtain their distinct
blue coloration from an elaborate ridge/lamellae/microrib structure
acting as a combination of multi-layer reflector and diffraction grating [29, 33, 108]. In detail, each ridge is built up of alternating
stacked lamellar features with a precise periodic spacing. These lamellar features are connected by microribs and act as a multi-layer
reflector for a particular band of frequencies. In addition, the spacing
between ridges and their relative orientation can be designed to
function as a diffraction grating. As an example, Fig. 7 shows different electron microscopy views of the photonic structure of the butterfly Morpho sulkowskyi. Potyrailo et al. investigated this butterfly
and discovered an interesting phenomenon; namely, the hierarchical
architecture of the scales of M. sulkowskyi also displays a highly selective optical response to various alcoholic vapors and thus can be
used as a photonic gas sensor [111].
Fig. 7 Scanning electron microscopy view of a fractured photonic structure of a
Morpho scale showing a side view of three ridges with their lamellae (l) and associated microribs (mr). Also shown are parts of several crossribs (cr; here fractured) that join the ridges as well as the bottom layer (bl) of the scale and several
pillars (p) that connect the bottom scale layer with the photonic structure.
Adapted from [111]
27
In addition to various ridge-based reflector and grating structures, the wing scales of the Lycaenidae and Papilionidae butterfly
families often possess photonic crystal architectures in which the
bio-polymeric matrix underneath the ridges can be sculpted into
complicated three-dimensional periodic lattices. For example, Michielsen and Stavenga found that the Green Hairstreak butterfly (Callophrys rubi) obtains its distinctive wing coloration from scales with
internal gyroid-structured photonic crystals [100]. Gyroid-based lattices are very interesting photonic structures, since they are known
to have efficient/robust band structure properties. Other interesting
examples of combined ridge-photonic crystal scale structures are
found, for example, in the wings of Parides sesostris and Teinopalpus imperialis in which the ridges and the three-dimensional photonic lattice are connected by a matrix composed of a series of vertical columns organized in so-called honeycomb arrays (Fig. 8) [109].
Additionally, in most cases, the three-dimensional photonic crystal
domains are composites of differently-oriented sub-domains with the
same lattice structure [29]. This structural feature is often also found
28
in another type of structural color in biology, namely in exoskeleton
scales of colored beetles [34].
Fig. 8 Transmission electron microscopy images of the photonic wing scale structure from the butterflies P. sesostris (a) and T. imperialis (b). Adapted from [109]
Bartl and co-workers studied the origin of structural colors in
various colored beetles of the longhorn (Cerambycidae) and weevil
(Curculionidae) families [34, 35, 89, 112]. This group discovered
several interesting bio-polymeric structures with three-dimensional
lattice geometries and multi-domain arrangements. These structures
include highly ordered non-close-packed sphere lattices and quasiperiodic lattices of spheres found in the longhorn beetles Glenea celia and Anoplophora elegans, respectively, as well as elaborate interpenetrating three-dimensional architectures such as the highly desired
diamond-based
structures,
found
in
several
weevils
(Lamprocyphus augustus, Eupholus schoenherri, Eudiagogus pulcher, and Pachyrrhynchus moniliferus).
In particular, the discovered diamond-based architectures are
of great importance, since they resemble some of the most efficient
photonic crystal lattice structures. Galusha et al. performed a de-
29
tailed investigation of the Brazilian weevil L. augustus and discovered its near-angle-independent greenish hue is the result of exoskeleton scales with an interior diamond-based photonic crystal structure
(see also Fig. 5) [34]. Remarkably, a high-resolution reconstruction
revealed that the beetle’s lattice structure is near-identical to a theoretically-derived and property-optimized photonic crystal structure
by Johnson and Joannopoluos—the only major difference being a
higher dielectric filling fraction in the beetle structures [113]. In
more detail, the diamond-based photonic structure of L. augustus
consists of an ABC stacking of layers of hexagonally ordered air
cylinders in a surrounding cuticular matrix (Fig. 9). The air cylinders
have an average radius and height of 0.20 and 0.77, respectively, in
units of the lattice constant, which was found to be 450 nanometers
(with an inherent 10 to 20 percent variation of structural dimensions
within single scales and between scales taken from different parts of
the beetles). The filling fraction of the cuticular matrix was found to
be between 50 and 60 percent. Moreover, the reconstruction over
larger areas revealed each scale is a hierarchically organized photonic structure composed of an array of diamond-based single-
30
crystalline domains selectively oriented with their [100], [110], or
[210] crystal axes normal or slightly off-normal to the scale topsurface. A comparison with photonic band structure calculations and
multi-directional optical reflectance spectroscopy investigations
showed that it is this multi-domain orientation of the diamond-based
crystal lattice that gives L. augustus its angle-independent green coloration.
Fig. 9 a, b, c) Scanning electron microscopy images of the photonic structure of
the weevils E. schoenherri, P. moniliferus, and E. pulcher, respectively. d) Calculated dielectric function, showing three orthogonal planes of the reconstructed diamond-based crystal structure with a 50 % volume fraction (air: dark; dielectric:
light). Adapted from [35]
Subsequently, the same group also discovered diamondbased photonic crystal structures in other weevils. In fact, they found
that pixilated arrays of diamond-based architectures seem to be the
dominant photonic lattices among colored weevils. For example, the
weevils E. schoenherri, P. moniliferus, and E. pulcher all possess
diamond-based photonic crystal lattices (Fig. 9) [35, 89]. While
these crystal structures are isomorphous with the structure originally
found in L. augustus, they display differences in both lattice constant
31
and filling fraction. Such variations in lattice constant as well as filling fraction strongly influence the resulting photonic properties.
While biology uses these strategies to create different structural colors and fine-tune optical effects, they also provide enormous possibilities for photonic materials research. For the first time, some of
the most sought-after photonic crystal structures operating at visible
frequencies are available—lattice symmetries that are not yet available via synthetic structure engineering methods. In the following
sections we will discuss possibilities of using these biological structures as unique templates to create novel oxide-based optical materials.
9.4 Bio-Templated Photonic Crystals
9.4.1 General Considerations
The large variation in crystal structure symmetry, lattice constant
and dielectric filling fraction of biological photonic structures dramatically extends the currently available “synthetic” photonic crystal
32
structures fabricated through microprocessing or bottom-up assembly techniques. Unfortunately, the direct use of biological photonic
crystals for advanced optical/optoelectronic applications is strongly
limited since most of these biological structures are composed of biopolymeric compounds that lack several properties critical to emerging optical applications, such as a high refractive index and structural robustness, as well as heat and photo stability, necessitating
conversion into more stable inorganic materials.
From a materials viewpoint, biological structures are therefore similar to synthetic polymeric photonic crystal templates created by direct ink writing, laser writing, holography, or colloidal selfassembly discussed previously. In these examples, the fabricated
polymeric structures have to be converted into inorganic positive or
inverse structures by infiltration with transparent high-dielectric
compounds. Consequently, similar infiltration methods can be used
to convert biological photonic frameworks into high-dielectric replicas, such as atomic layer deposition, low-temperature evaporation
techniques and sol-gel chemistry routes. Compared to synthetic template structures, however, there are several new considerations that
33
have to be taken into account for successfully converting biological
structures into oxide-based high-dielectric replicas. These considerations include accessibility of the photonic structure for infiltration,
avoidance of lattice shrinkage during processing, and formation of a
dense high-dielectric framework while preserving structural integrity. In the following, we will discuss these general factors in more
detail, before introducing several specific examples of successfully
applied biotemplating methods in the next section.
In contrast to the free-standing, open and easily accessible
lattice framework of synthetic photonic crystal templates such as
polymeric woodpile and opal structures, biological photonic structures are integrated into larger body parts (feathers, wings, hair, exoskeleton). Consequently, biological photonic structures are often
buried, hidden, or embedded within a structure-less matrix. This is
especially true for photonic structures of most butterflies and beetles, which are contained in wing and exoskeleton scales and are
thus partially or completely surrounded by an impermeable biopolymeric shell (see also Fig. 5) [28, 35, 108, 110]. In addition, most
biopolymeric structures are covered by a hydrophobic, wax-like film
34
that can cause problems during infiltration. Successful infiltration/replication of biological photonic crystals thus often requires
pre-processing steps such as treatment with organic solvents or acids
to remove the wax-like layer and cutting or microtoming to provide
access to the encapsulated lattice structures for high-dielectric filling
compounds. The template-infiltration process is generally followed
by densification and/or crystallization of the inorganic replica
framework. This step is of great importance, since achieving a
dense, highly crystalline photonic crystal framework with a high refractive index is of the utmost importance in photonic structure engineering via templating of both synthetic and biological polymeric
structures.
9.4.2 Biotemplating Techniques
9.4.2.1 Deposition and Evaporation Methods
Low-temperature atomic layer deposition is an excellent method
for creating oxide-based inorganic replica of biological photonic
structures [40, 41, 82, 114, 115]. It combines a non-corrosive reaction environment and mild pH conditions with relatively low deposi-
35
tion temperatures of around 100-200 °C. Furthermore, since the infiltrated oxide compound is formed by a layer-by-layer atomic deposition process, the degree of infiltration can easily be tuned by controlling deposition cycles. The precursors used are in general
gaseous compounds and therefore readily infiltrate even complex
three-dimensional frameworks as long as the internal structure is fully accessible. Since the majority of photonic structures found in the
wings of butterflies are open frameworks and require no preinfiltration cutting, they are the templates of choice for most successful atomic layer deposition-based bioreplication attempts.
For example, Wang and co-workers created aluminum oxide
(alumina, Al2O3) replicas of the photonic structure of wing scales
from the butterfly M. peleides by atomic layer deposition [40]. Using a low-temperature atomic layer deposition process at 100 °C and
trimethyl aluminum (Al(CH3)3) and water as precursor sources, the
biopolymeric photonic structure of M. peleides was coated with a
structurally stable layer of amorphous alumina. The thickness of the
alumina coating was gradually increased by about 10 nanometer
steps until a final thickness of 40 nanometers was achieved. Charac-
36
teristic of photonic crystal structures, the infiltration process can be
monitored and precisely controlled by analyzing the color of the reflected light. As shown in Fig. 10a, a red-shift (from blue to pink) of
the reflected light was observed due to a change in periodicity and
effective refractive index of the composite as the thickness of the
coating increased.
Fig. 10 a) An optical microscope image of the alumina coated Morpho peleides
butterfly wing scales, of which the color changed from original blue to pink. b)
Scanning electron microscopy image of alumina replicas of butterfly wing scales.
(c) Energy dispersive X-ray spectrum of alumina replicas shown in part b. d)
Higher magnification scanning electron microscopy image of alumina replicas of
butterfly wing scales and two broken rib tips (e). Adapted from [40]
When a desired layer thickness was obtained, the biopolymeric/alumina composite was heated to 800 °C in air. Under these conditions, the butterfly template completely decomposes and the amorphous alumina coating is transformed into a polycrystalline
framework. The resulting structure is a shell-like copy of the original
M. peleides photonic lattice made out of transparent, polycrystalline
alumina. Scanning electron microscopy images are given in Fig. 10
and show that even nanoscale structural features of the original bio-
37
template were perfectly preserved by this process. Furthermore, optical reflectance spectroscopy studies revealed alumina replica features very similar to that of the original butterfly scales in terms of
reflection peak wavelength position and shape.
Gaillot et al. developed a similar atomic layer deposition
route to fabricate bio-templated organic-inorganic composite photonic crystal structures [41]. In their approach, the photonic scales
covering the wings of the green swallowtail butterfly P. blumei were
used as biological templates. Two different templating methods via
titania low-temperature atomic layer deposition were employed.
While in the first method entire scales were surrounded by a uniformly thin (50 nanometers) layer of amorphous titania, in a second
method both the scale exterior and the interior photonic structure
were covered with a titania layer. The intra-scale deposition of titania was enabled by diffusion and subsequent deposition of the gaseous titania precursors through surface cracks created by razor blades
or sharp tips. Gaillot et al. also conducted detailed structural and optical studies on both organic-inorganic replica types [41]. Experimental results were compared to theoretical modeling and band
38
structure calculations and provided valuable insights regarding the
ability to tune the optical properties of these photonic crystals
through slight variations of the deposited high-dielectric compound.
In addition, analyses of the properties of these different types of oxide-replica butterfly wings indirectly provided new insights into
their structural complexity; with various intersecting nano-channels
and connected chambers in addition to the photonic crystal structure.
Another interesting and elegant bio-templating method is the
so-called conformal-evaporated-film-by-rotation technique developed by Lakhtakia and co-workers [38, 39, 116]. This method,
which evolved from the oblique angle deposition technique, combines thermal evaporation with simultaneous substrate tilting and rotation in a low-pressure chamber. Since the reaction chamber pressure is in the micro-Torr regime, the evaporation/deposition process
can be performed at low temperatures and under a non-corrosive environment, providing excellent conditions for replicating sensitive
biological structures. Additionally, the high-speed rotation of the biological templates during the evaporation process facilitates for-
39
mation of a homogeneous and dense inorganic film, even on highly
curved and structured surfaces.
The first successful application of the conformal-evaporatedfilm-by-rotation technique on biological samples was the highfidelity replication of various body parts of a fruit fly, including the
eyes, head and wings [38]. For example, the replicated eye structures
displayed the same long-range features (micrometer-scale) and finefeatures (nanometer-scale) as found in the original fly eye. The same
group later extended this bio-templating technique to replicate photonic structures found in the wings of butterflies [39, 116]. For example, Fig. 11 shows a high-resolution scanning electron microscopy image of replicated micrometer and nanometer features of the
photonic framework found in the wing scales of the butterfly Battus
philenor. While these initial replicas were all composed of chalcogenide glasses with a nominal composition Ge28Sb12Se60, the conformal-evaporated-film-by-rotation technique can also be used to
create oxide-based compounds [117].
Fig. 11 Scanning electron microscopy image of a replica of the scales of B.
philenor created by the conformal-evaporated-film-by-rotation technique.
Adapted from [39]
40
4.2.2 Sol-Gel Chemistry Methods
Sol-gel chemistry chemistry is an attractive alternative to deposition and evaporation methods for its simplicity enabled by flexible
processing parameters [118]. However, producing oxide-based bioreplicas of similar quality to those obtained from evaporation and
atomic layer deposition in terms of uniformity and degree of template-infiltration, compositional control, and structural integrity requires precise tuning and optimization of processing parameters
along their many degrees of freedom. Among these, the most important are the precursor components and solvents, the sol composition and concentration, and the post-infiltration conditions such as
sol drying time, humidity during the gelation process, and temperature and time of the heat treatment used to induce complete solidification and (in some cases) crystallization of the oxide-based replica
framework.
Similar to evaporation and deposition approaches, the majority of sol-gel bio-templation attempts have focused on replicating the
photonic structures found in the wing scales of various butterflies
due to their open framework. For example, Zhang and co-workers
41
created zinc oxide and titanium dioxide wing scale replicas from the
butterflies Papilio paris and Thaumantis diores [42, 43]. A simple
sol-gel route was developed to infiltrate the biological templates
with ethanol solutions of the respective metal salts followed by heat
treatment at 500 °C in air to induce crystallization of the oxide
framework and remove the bio-polymeric structure. Despite structural shrinkage during the heat-based framework densificationcrystallization treatment, the replicated materials displayed optical
band structure features in the blue-green region. Zhang and coworkers showed that these interesting optical properties can be used
to create new materials for applications in solar cells and light emission. For example, titanium dioxide photonic architectures templated
from butterfly wings can be used as photoanodes with enhanced
light harvesting efficiencies under visible-light illumination [42]. On
the other hand, the patchy blue and black colored wing scales of P.
paris replicated into zinc oxide have interesting room-temperature
cathodoluminescence properties [43]. While replicas from both the
black and the blue patches of the scales showed sharp near-bandedge cathodoluminescence in the UV, the emission from the former
42
also displayed strong green emission, which the authors attributed to
the internal sponge-like “beehive photonic nanostructure” found in
the black patch area of the scales.
Zhu et al. reported an interesting bio-replication approach of
butterfly wings by combining sol-gel templating with sonochemistry
[119]. In their method, the internal bio-polymeric nanostructure of
blue-colored wings from Morpho butterflies was impregnated with
an ethanol-water-based precursor solution followed by highintensity ultra-sonication for several hours at room temperature. Using this approach the authors were able to replicate the photonic
wing structure into a variety of oxide ceramics; including titanium
dioxide, tin dioxide and silicon dioxide. After sonication, the composites were heat-treated to remove the bio-polymeric template and,
in the case of titanium dioxide and tin dioxide, induce the formation
of a polycrystalline framework. All of the replicated samples possessed highly preserved photonic nanostructures stemming from the
original butterfly wings and displayed optical reflectance features in
the visible. In addition, it was found the crystalline tin dioxide replica can be used as a gas sensor, displaying high sensitivity for ethanol
43
vapor with a fast response time (8 seconds) and a short recovery
time (15 seconds).
In addition to replicating the open-scale structures of butterfly wings, sol-gel chemistry can also be used to template photonic
lattices enclosed in the biopolymeric shells typically found in beetles
[35, 37, 112]. Interestingly, small openings of the scale, which naturally form during scale-removal from the insect’s exoskeleton or can
be created by cutting with a razor blade, are sufficient to infiltrate
the entire internal photonic crystal framework. The key here lies in
the liquid nature of the sol-gel precursor, which allows infiltration of
the internal photonic lattice framework through small openings by
capillary forces. Galusha et al. used hybrid organic/inorganic silica
sol-gel chemistry combined with acid-based template removal approach to successfully create glass-like replica of different photonic
crystal structures from a variety of colored beetles [35]. The rationale for using a hybrid (SBA-type) silica sol-gel precursor (originally developed by Stucky and co-workers [120]) as the infiltration
material lies in the superior templating properties of this organic/inorganic compound. The hybrid nature of this organic/inorganic
44
precursor results in enhanced structural stability upon sol drying/solidification and template removal as compared to pure silica.
While the latter shows significant crack formation and structural
long-range order degradation in the replicated structure, the use of
organic/inorganic silica sol not only preserves to a large extent the
macroscopic morphology of the original scale template, but also the
structural fine-features of the photonic structure (compare also Fig.
12).
Fig. 12 a) Scanning electron microscopy image of a sol-gel-derived hybrid silica
inverse structure templated from a scale of G. celia. b) Magnified look of the top
surface of the inverse structure. Adapted from [35]
Another crucial factor in converting bio-polymeric templates
into oxide-based replicas is template removal after the infiltration/solidification event. A common method is heat treatment under
oxidative conditions at temperatures between 300 and 500 ºC. This
process is effective in complete removal of the organic template;
however, it can also cause significant structural damage and shrinkage with a concomitant reduction of the lattice periodicity of up to
30 percent. Due to the pronounced structure-property-relationship in
45
photonic crystals, shrinkage is accompanied by blue-shifting of the
optical features of the replicated photonic structures, which is problematic for applications that require band gaps at particular frequency ranges in the visible. In synthetic structure-templating, this
shrinkage can be circumvented simply by starting with templates
with larger lattice constants (for example, to fabricate a titania inverse opal crystal with a lattice constant of 500 nanometers, a polymeric opal template with a lattice constant of 700 nanometers is
used). Unfortunately, this strategy cannot be applied for biological
photonic templates, since they have fixed lattice constants and are
designed and optimized to create defined structural colors by interaction with visible light. Lattice shrinkage in the range of 30 percent
of these structures during the infiltration/replication process would
therefore lead to photonic crystals with band gap features shifted out
of the desired visible range into the deep blue or ultraviolet range.
To eliminate this obstacle, Galusha et al. developed a lowtemperature acid-etching technique for bio-polymeric template removal [37]. Treating the infiltrated composite with a mixture of concentrated nitric and perchloric acid led to the complete removal of
46
the biological template while greatly reducing shrinkage and cracking of the silica-based replica framework. Furthermore, the acid
treatment was also beneficial for low-temperature silica sol-gel processing, accelerating the condensation and densification due to its
catalytic effect on these processes. The combined beneficial effects
of this acid treatment resulted in robust replicas with well-preserved
lattice features and minimal structural shrinkage in the range of 5
percent.
Apart from using oxide-based replicas of biological photonic
structures for enhancing optical properties in “conventional” applications such as sensors, emitters and photocatalytic cells, some of
these structures are also attractive for fabricating optical materials
with entirely new—non-classical—optical properties. In the final
section of this chapter, we will describe the sol-gel fabrication and
properties of a bio-templated titania structure that was the first example of a photonic crystal with a complete band gap at visible frequencies—a type of optical material that is still not achievable by
any other engineering technique.
47
9.4.3 Biotemplated Bandgap Crystals
The discovery by Bartl and co-workers that several beetles obtain
their striking coloration from light interacting with diamond-based
photonic crystal lattices built into their exoskeleton [34, 89] (see also
Fig. 9) opened the door to exciting new possibilities in photonic
band gap research. Modeling and photonic band structure calculations showed the diamond-based lattice found in the weevil L. augustus possesses a complete photonic bandgap in the green region of
the electromagnetic spectrum when fabricated out of polycrystalline
titania [37]. However, these calculations also revealed that formation
of a complete bandgap for the replicated structure occurs only within
a narrow window of dielectric and lattice properties. The most important findings of these theoretical studies are: 1) The inverse of the
diamond-based photonic structure produced by a single-templation
process lacks a complete bandgap, regardless of the volume fraction
and refractive index of the high-dielectric component. 2) For the
original beetle structure with a high-dielectric volume fraction larger
than 50 percent, a complete photonic bandgap would require a refractive index of at least 3.4 for the high-dielectric component—far
48
beyond the reach of visible-light-transparent compounds. 3) Reducing the high-dielectric volume fraction significantly lowers the refractive index required for opening of a complete photonic bandgap
and can be as low as 2.1 for a volume fraction between 30 and 40
percent. From these findings it can be concluded that only structures
based on the original (not the inverse!) diamond-based photonic lattice will possess a complete bandgap in the visible when fabricated
out of a material with a refractive index larger than 2.1 and a volume
fraction below 40 percent.
Fig. 13 Illustration of the double-imprint sol-gel biotemplating route for converting photonic scales of the weevil L. augustus into high-dielectric titania replica. a)
Weevil L. augustus and its green colored photonic scales (inset). b, c, d) Scanning
electron microscopy images of a cross-sectional views of b) the original biopolymeric photonic structure, c) the inverse structure made of hybrid silica and d) the
titania replica templated from the intermediary hybrid silica structure. Scale bars
are 200 μm (a) and 1 μm (b, c, d). Adapted from [37]
In response to these predicted requirements Galusha et al.
developed a sol-gel chemistry-based double-imprint biotemplation
method (Fig. 13) [37]. The strategy was to first create an inverse of
the original beetle diamond-structured crystal, which then could be
used as a sacrificial new template to fabricate a high-dielectric copy
49
(“inverse-of-the-inverse”) of the original structure with a reduced
volume fraction. Hybrid organic/inorganic silica was used as the
sacrificial template material and was infiltrated with a titania sol-gel
precursor using a dip-coating approach followed by heating of the
silica/titania composite at 500 C for two hours. This induced titania
nano-crystallization and resulted in the formation of a high-dielectric
framework with a measured refractive index of 2.3  0.1. In addition, the authors were able to tune the volume fraction of infiltrated
titania into the required 30-40 percent range by successively repeating the infiltration-solidification-crystallization cycle. Finally, the
intermediary silica-based template was removed using hydrofluoric
acid-etching, producing a polycrystalline anatase titania framework
with a diamond-based lattice structure and lattice periodicities at visible wavelengths.
The obtained titania photonic crystal was investigated by a
range of structural and optical characterization techniques [37].
Structural studies were performed by focused ion beam milling and
scanning electron microscopy imaging, revealing an excellent
preservation of the diamond-based photonic lattice after the double-
50
templating procedure (Fig. 13). Examples of the final structures are
also given in Fig. 14 and a detailed analysis showed the original diamond-based framework—a lattice of ABC stacked layers of hexagonally ordered air cylinders in a surrounding high-dielectric matrix—was excellently preserved. Moreover, even after two sol-gel
templation steps, it was found that overall shrinkage was kept below
15 percent, giving a final lattice constant of 366  24 nm and volume fractions between 30 and 40 percent. Both values are within the
required range for opening a complete band gap at visible wavelengths. The corresponding calculated band structure diagram is
shown in Figure 14 and reveals a 5 percent wide gap-to-mid-gap ratio.
Fig. 14 a) Scanning electron microscopy cross-sectional view of the biotemplated
diamond-based titania photonic crystal lattice. b) Tilted view of the same titania
structure. c) Corresponding calculated band structure diagram. The complete
photonic band gap between the second and third band is indicated by a gray rectangle. Adapted from [37]
The same authors also showed the calculated complete band
gap agrees excellently with the optical properties of the diamondstructured titania replica determined by multi-directional reflectance
51
micro-spectroscopy measurements [37]. These measurements were
performed in two different ways. In the first method, reflectance
spectra were recorded normal or slightly off-normal to particular
crystal axes of the diamond-based lattice. Taking advantage of the
pixilated multi-domain organization of the photonic lattice within
each scale [34], the optical reflectance properties along various crystal axes ([100], [110], and [210] directions) could be evaluated.
Analysis of the obtained spectra revealed significant overlap of the
directional reflectance peaks from the high-index titania replica, in
contrast to the well-separated reflectance features of the original
low-index bio-polymeric structure. Furthermore, a comparison with
the calculated band structure shows an excellent agreement with the
positions and widths of the [100], [110], and [210] directional gaps,
which are narrow and well-separated for the beetle photonic structure, but are wide and strongly overlapping for the titania replica
photonic crystal. In addition, to cover an even larger range of directions, a series of angle-dependent reflectance spectra covering a 30°
range were collected. The obtained series of intensity-normalized
spectra displayed no significant dependence of the reflectance peak
52
position on the recording-angle. This confirms the band structure
calculations of the first complete photonic bandgap at visible frequencies—obtained by sol-gel replicating a biological diamondbased photonic crystal structure into crystalline titanium dioxide.
9.5 Conclusions
In biological systems, structurally complex architectures with feature sizes covering several lengths scales are engineered under rather
simple environmental conditions and with limited resources—
strategies still largely unmatched by our synthetic abilities. Prime
examples of such elaborate architectures are found in the amazingly
colorful world of insects, birds and fishes in which myriad of hues
and optical effects are in large part the result of structural colors. In
contrast to pigmented colors, structural colors arise from a delicate
interplay of light with periodically organized dielectric lattices with
feature sizes of a few hundreds of nanometers. For example, it is the
periodic variation of biopolymeric compounds embedded into wings
and exoskeletons that lends many butterflies and beetles their irides-
53
cent appearance. While the pure beauty of natural iridescence in the
form of opal gem stones, jewel beetles and feather ornaments has
fascinated and inspired mankind for thousands of years, architectural
colors have recently gained tremendous interest with the introduction of band structure concepts to electromagnetism. Such materials,
termed photonic crystals, offer revolutionary new ways to use and
manipulate light and are the cornerstones for future optical devices
based on light localization and control of spontaneous emission.
In this chapter, we briefly reviewed the basic concepts of
photonic crystals and gave examples of successful synthetic photonic crystal fabrication based on top-down microprocessing techniques
as well as bottom-up writing and assembly-based routes. While
these techniques are very well suited to produce photonic crystals
with complete bandgaps in the infrared regime, they have severe
limitations for engineering robust photonic crystal lattices operating
in the visible. In contrast to our current inability to shape dielectric
compounds into effective photonic architectures with feature sizes at
visible wavelengths, biological systems have developed a wealth of
elaborate structures optimized to efficiently interact with visible
54
light. We discussed several examples of colored butterflies and beetles, demonstrating that they have evolved to create highly efficient
and complex photonic structures embedded into their wings and exoskeletons. The fact that some of the most sought after photonic
structures, including diamond-based lattice geometries, occur—to
date, exclusively—in biological systems not only reflects the ingenuity of structural engineering in nature, but opens new avenues in
optical materials design and fabrication.
One of these highly promising new areas uses biological
templates to produce novel photonic materials. In biotemplating the
best of two worlds—biology and materials science—are combined
in a synergistic approach by merging unique structural engineering
in biology with state-of-the-art materials synthesis and processing.
We reviewed several successful biotemplating strategies using threedimensional photonic lattices from butterflies and beetles as unique
molds for replication into oxide-based inorganic compounds such as
titania, silica, and alumina. While typical deposition and evaporation
based techniques are efficient in infiltrating and replicating opensurface photonic structures such as those found in the wings of but-
55
terflies, sol-gel chemistry routes are particularly well suited to replicate lattices surrounded by biopolymeric shells.
All of these biotemplating methods have been shown to create oxide-based replica with high fidelity, preserving even nanoscale
fine-features of the original biological photonic structures. In addition, the combination of interesting bio-photonic lattice geometries
with the intrinsic properties of the oxide-based replica compounds
creates new functionalities and interesting applications as sensors,
photoanodes, and light emitters. Finally, the recently reported
achievement of a complete photonic bandgap at visible frequencies—the first of its kind—by titania sol-gel replication of a diamond-based photonic structure found in a weevil, underlines the
enormous potential of biotemplating based photonic engineering and
paves the road for novel optical materials and phenomena based on
light localization at visible frequencies.
56
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