# Infrared absorbers inspired by nature

Sébastien R. Mouchet  Department of Physics, Namur Institute of Structured
Matter (NISM) & Institute of Life, Earth and Environment (ILEE), University of
Namur, Rue de Bruxelles 61, 5000 Namur, Belgium  School of Physics, University
of Exeter, Stocker Road, Exeter EX4 4QL, United Kingdom
<EMAIL_ADDRESS><EMAIL_ADDRESS>ORCID:
0000-0001-6611-3794

###### Abstract

Efficient energy harvesting, conversion, and recycling technologies are
crucial for addressing the challenges faced by modern societies and the global
economy. The potential of harnessing mid-infrared (mid-IR) thermal radiation
as a pervasive and readily available energy source has so far no been fully
exploited, particularly through bioinspiration. In this article, by reviewing
existing photon-based strategies and the efficiency of natural systems in
harnessing light and thermal radiation, I highlight the promising role of
bioinspiration in enhancing energy capture, conversion, and recycling. Natural
photonic structures found in various organisms, including insects, birds, and
plants, exhibit sophisticated optical properties that can be leveraged for
energy-efficient applications. These developments pave the way for future
research and innovation in bioinspired energy solutions. Ultimately, they
contribute to the pursuit of a sustainable and environmentally conscious
future by harnessing the beauty of nature’s designs to meet humankind’s energy
needs.

Keywords: Light absorption; Infrared absorber; Solar energy; Clean energy;
Energy efficiency; Renewable energy; Sustainable energy; Bioinspiration;
Photonics

##  1 Introduction

Addressing the significant challenges faced by modern global society and the
world economy, the advancement of efficient energy harvesting and recycling
technologies [1, 2, 3] stands as a prominent area of research on a global
scale. Mid-infrared (mid-IR) thermal radiation represents a pervasive and
readily available energy source. This is not only due to the long illumination
of some parts of the Earth by the Sun but also because many machinery,
engines, and industrial processes dissipate energy in the form of heat
radiation, distinct from thermal conduction or convection mechanisms.

While the primary energy source may vary in its environmental impact, the
recycling of this "wasted" energy presents a sustainable approach to
converting radiative heat losses into diverse forms of energy. Numerous
mechanical components found in machinery, engines, industrial processes, and
even household systems generate mid-IR thermal emissions at moderately
elevated temperatures, typically ranging from 150°C to 950°C. These emissions
are an intrinsic byproduct of these components’ regular functioning and
constitute an unavoidable energy loss. The prospect of harnessing this
radiative heat loss is compelling, as it offers the opportunity to transform
it into electrical power, effectively enabling devices to utilise their own
recycled radiative heat loss for enhanced functionality.

Photon-based strategies have already played a crucial role in harnessing solar
energy, enhancing the performance of energy conversion devices [4, 5, 6, 7, 8,
9, 10, 11, 12]. For instance, devices designed for solar light trapping have
effectively increased the efficiency of photovoltaic (PV) cells and thermal
photovoltaic (TPV) cells. Similar photonic devices are instrumental in
augmenting the efficiency of solar thermal panels, or in energy harvesting for
thermoelectric generators (TEG), artificial photosynthesis, and
photocatalysis.

In nature, numerous biological organisms have developed highly efficient
mechanisms to harness thermal radiation, a crucial adaptation for their
survival. Over millions of years of evolution, these natural systems have
honed specialised characteristics to maximise their radiation harvesting
abilities [13, 14]. Consequently, certain structures within their integuments
have become increasingly inspiring for the development, design, and production
of energy-efficient materials [15, 16, 17, 14]. Bioinspiration emerges as a
powerful and promising strategy in this context.

Natural photonic structures found in various animals, including insects,
birds, and fish, are example of effective thermal radiation collectors [15,
17, 18]. In addition, this type of structures exhibit a diverse array of
properties, such as structural colours (resulting from light interference in
nanostructures) [19, 20, 13, 14], antireflection features [18, 21, 16],
thermoregulation mechanisms [22, 23, 24, 25, 26], light-trapping capabilities
[27, 28, 29, 30], and enhanced light-extraction methods [31]. These properties
emerge from the interaction between radiation and structures composed of
biopolymers like chitin, keratin, collagen, or cellulose, sometimes in
combination with pores.

The existence of these naturally occurring radiation management systems
challenges human imagination. While human beings have access to a wide range
of materials, human designs sometimes fall short in complexity compared to
these remarkable natural structures. Identifying and comprehending these
natural photonic devices not only expand human understanding but also empowers
engineers and materials scientists to conceptualise new ideas and explore
potential technological applications through bioinspired principles [15, 16,
17, 14]. These exciting possibilities have captivated the attention of
researchers worldwide. Despite the development of artificial intelligence,
bioinspiration remains a guiding force in the quest for novel technological
applications. In fact, the convergence of both approaches holds promise for
unprecedented advancements in this field.

In this article, I first review previously investigated cases of photonic
structures enhancing electromagnetic-wave absorption (also known as structural
absorption) in natural organisms across the UV, visible and infrared (IR)
range. This is because the dimensions of a visible light absorber occurring in
nature may be adjusted to another range such as IR through a bioinspiration
approach due to the scalability of Maxwell’s equations. Finally, I review
examples of bioinspired IR absorbers from the literature.

##  2 Light absorption enhanced by photonic structures in natural organisms

The management of electromagnetic radiation and thermoregulation are pivotal
functions essential for the survival or benefice of various natural organisms,
including plants, insects, and birds [23, 32, 33, 34, 35, 24, 36, 25, 26, 37],
whether endotherms (organisms able to maintain their body temperature through
their metabolisms), mesotherms (organisms with some metabolic strategies of
heat production without any proper metabolic heat control), or ectotherms
(organisms requiring external heat sources) [38]. For instance, photosynthesis
implies absorbing visible radiation from the Sun whereas thermoregulation of
ectothermic animals involves a subtle trade-off between radiation absorption
and thermal emission in the near-infrared (near-IR) part of the
electromagnetic spectrum. Photonic structures may play roles in the management
of such thermal radiation. For instance, iridescent butterflies were reported
to exhibit in general an absorptance higher than the one of non-iridescent
species [39]. Other striking illustrations are the photonic structures
occurring in the super-black feathers of the bird of paradise (as depicted in
figure 7) [36], as well as in the scales covering of the black wings of
insects like the Magellan birdwing and the Meander prepona butterflies [23,
27]. These feathers and wings exhibit remarkably high energy absorption
properties within the spectral range of solar irradiance, encompassing the
mid-IR spectrum in some instances. Often, in such natural integument, incident
light is absorbed by pigments including melanin [40, 41, 42, 43].
Nanostructures, operating at micro- and nanometer length-scales, yield
fascinating opportunities for both light and thermal radiation harvesting.

###  2.1 Wings of lepidopterans

Insects such as lepidopterans, the taxonomic order encompassing the ethereal
beauty of butterflies and the enigmatic allure of moths, are ectotherms, which
means that their metabolisms rely on environmental heat sources. Harvesting
incident energy appears crucial for these species. The phylogenetic diversity
of structures giving rise to ultra-black colouration occurring in the order
Lepidopetra was recently analysed in detail (figure 1) [44]. All these
structures present some longitudinal ridges connected by crossribs in the
upper lamina of the scales, forming 2D networks of quasi-periodic holes. The
resulting high-surface area was described as increasing light absorption by
underlying cuticular melanin and reducing reflection [45, 44]: whatever the
size and shape of the holes -honeycomb, chevrons, or rectangles- scales giving
rise to ultra-black visual appearances exhibit steeper ridges as well as
deeper and wider trabeculae (namely, pillars connecting the upper and basal
laminae of a scale) than scales with some regular black or brown colour.
Through numerical modelling, these features were shown to play a significant
role in reducing light reflection [45, 44]. Furthermore, coating these
structures with gold does not lead to an increase in light reflectance, unlike
regular black or brown butterfly wings. This experimentally demonstrates the
photonic origin of the related light harvesting [44].

Figure 1: Diverse structures of scales exhibit ultra-black colouration within
the order Lepidopetra. They typically comprise holes located in between the
scales’ ridges with various sizes and shapes: honeycomb (a), chevrons (b), and
rectangles (c-f), as observed by SEM with wings of Trogonoptera brookiana male
papilionid (a), Eunica chlorocroa nymphalid (b), Catonephele antinoe nymphalid
(c), Heliconius doris nymphalid (d), Euploea dufresne nymphalid (e), and
Euploea klugi nymphalid (f). Scale bars: 1 $\mu$m (a-f). This figure was
reproduced from ref. [44], License CC-BY-4.0.

In the scales covering the wings of the male Papilio ulysses butterfly,
similar complex photonic structures were found to play a role in intensifying
the darkness areas [28], in addition to the porous multilayer structure
producing the bright blue colour through interference [46]. They were
described to scatter light towards the ridges and the interior of the scale,
leading to a longer optical path, resulting in a higher light absorption by
the pigments distributed within the scale material. For matt black scales, the
absorption reduced from 95% to 55%, upon contact with bromoform serving as
index-matching fluid, while for lustrous black scales, it decreased from 90%
to 70% (figure 2). Ridges were however demonstrated through simulation to have
neglected role in visible-light absorption of the ultra-black scales of
Pachliopta aristolochiae [47].

Figure 2: The male P. ulysses butterfly (a) exhibits some black areas on its
wings. Upon contact with index-matching fluid (here, bromoform), the
absorption spectra exhibit significantly lower intensities (b). They were
measured at normal incidence with both lustrous (circles) and matt (triangles)
black scales. Electron microscopy (c,d: SEM; e,f: TEM) allowed to observed the
structures of the lustrous (c,e) and matt (d,f) black scales. Scale bars: 3
$\mu$m (c), 2 $\mu$m (d); 2 $\mu$m (e,f); inset, 300 nm. These figures were
reproduced from W. van Aken (a),
https://commons.wikimedia.org/wiki/File:CSIRO_ScienceImage_3831_Ulysses_Butterfly.jpg,
License CC-BY-3.0, and from ref. [28] (b–f), with permission from the Royal
Society.

The wings of Troides magellanus butterfly, the Magellan birdwing, present a
captivating spectacle of optical properties including light diffraction and
controlled fluorescence emission on their hindwings [48, 49, 50, 51]. It
inhabits the Philippines and Taiwan’s Orchid Island. Renowned for their
impressive size and striking appearance, the forewings showcase a remarkable
98% absorption of visible light as well as reveal two distinctive peaks in the
IR spectrum [27]. The presence of chitin imparts the wings with these robust
absorption peaks at 3 $\mu$m and 6 $\mu$m due to C = O vibrations,
strategically positioned within the wavelength range where a black body emits
radiation at 40°C, enabling radiative cooling. The architecture of the
Magellan birdwing consists of five major elements [27]: a roof-like structure
on which are located a series of ridges, holes in the separating structures
and pillars, joining the upper membrane to the lower membrane of the wing. A
similar structure was also found in the case of the related Troides aeacus
[52]. Comparison of numerical simulations between the photonic structure and a
non-structured flat slab with equal volume of material showed a 10% increase
in electromagnetic radiation absorption and a 17% increase in emissivity at
40°C [27]. This unique combination of optical characteristics suggests that
the Magellan birdwing has evolved to manage efficiently both visible and IR
light, underscoring the sophisticated adaptation of these butterfly wings for
light and thermal radiation purposes.

A related thermoregulation was demonstrated in the case of Archaeoprepona
meander, the Meander prepona, a tropical butterfly species (figure 3) [23], as
well as later on in various butterfly species [53]. These insects employ a
sophisticated mechanism to manage its body temperature effectively within a
given range such as 36°C-40°C [23] or 20°C-50°C [53], depending on the
species. The intricate structure of their wings such as the black wings of
Meander prepona serves as a remarkable example of nature’s engineering
prowess. The scale structures on the wings, in addition to melanin pigments,
play a crucial role in harnessing solar energy efficiently, absorbing
approximately 95% of the visible solar spectrum (figure 3c) [23], akin the
phenomenon described in the cases of Papilio ulysses and the Magellan birdwing
hereabove. In the near-IR range, the absorptance intensity decreases down to
less than 2%, ensuring low thermal emissivity, apart from the absorptance
peaks at 3 $\mu$m and 6 $\mu$m. The 6-$\mu$m emissivity peak plays a crucial
role in thermoregulation [23]. At temperatures below 40°C, the black body peak
is located at a longer wavelength (figure 3c). It allows the wing to harvest
heat effectively while maintaining low thermal emission. When temperatures
exceed 40°C, a significant overlap between the black body spectrum and the
6-$\mu$m peak appears (figure 3c), leading to higher thermal emission (namely,
radiative cooling) and contributing to the butterfly’s fine-tuned response to
thermal challenges in its habitat. This thermoregulation mechanism could be
employed in applications such as solar energy harvesting as it can help
maintaining the devices within an optimal temperature range.

Figure 3: Radiative cooling plays a key role in the thermoregulation of
certain butterfly wings, exemplified by Archaeoprepona meander, the Meander
prepona. It arises from the morphology of the scales covering the black area
of the wings, as observed here by SEM. A clear difference in reflectance and
transmittance was measured between the near-IR and visible parts of the
electromagnetic spectrum (c). Black body emission spectra at 40°C and 50°C,
solar irradiance, and the absorptivity of a wing (d) allow to explain the
thermoregulation enabled by the C = O absorption peak of chitin at 6 $\mu$m.
Figure (a) was reproduced from Notafly,
https://commons.wikimedia.org/wiki/File:Archaeoprepona_meander_(Cramer,_-1775-).JPG,
License CC-BY-SA-3.0. Data in figures (b,c) are from ref. [23].

The role of ultra-black colours in butterflies remains the subject of
speculation. However, it was hypothesised that they enhance the contrast in
the visual signals, as ultra-black areas are always located next to bright
areas [44]. Such visual contrast would have implications in terms of
aposematism or intraspecific communication.

In addition to structured scales covering the ventral and dorsal sides of
lepidopteran wings, many species including butterflies Greta spp., the moth
Cacostatia ossa, and the moth Cephonodes hylas display some highly transparent
scale-less wings with antireflection properties through photonic structuring
of the wing membranes [54, 55, 18, 56, 57, 16, 58, 59, 60]. This structuring
curtails reflection of incident light to levels below 2% across the entire
visible spectrum, through electromagnetic impedance matching. It consists in a
lattice of dome-shaped protuberances, also known as nipples (figure 4). The
underlying principle behind this antireflection effect lies in the gradual
refractive index matching between the air and the wing membrane, typically
composed of chitin. If the protuberances are spaced by less than the incident
wavelength, typically less than 200 nm, the non-zero diffraction orders are
evanescent. The protuberance structure can be regarded as a slow variation of
the effective refractive index along the normal to the wing membrane.
Depending on the species, the protuberance lattice can be very well ordered
such as the hexagonal compact array in the wings of C. hylas [54, 55, 18] and
Hemaris fuciformis [56, 60] hawkmoths or more disordered such as the wings of
C. ossa moth (figure 4) [57, 16] and the ones of Greta spp. glasswing
butterflies [58, 59]. Interestingly, disorder in the protuberance height,
width, and position was found to increase the transparency properties, in the
case of G. oto glasswing butterfly [59]. Beyond the order of lepidopterans,
antireflection structures manifest in the wings of odonatans, such as Aeshna
cyanea dragonfly [61, 60], the American Rubyspot dameselfly Hetaerina
americana [58], and Vestalis amabilis damselfly [60], or even in hemipterans
like cicadas [62, 63, 64, 16, 65, 60]. In addition, such nipple arrays were
observed on the surfaces of compound-eye corneas of several arthropods [66,
67, 68, 56, 21]. They are often referred to as moth-eye structures. A
comparative study of 19 species of butterflies led to the classification of
the arrays into three categories according to their morphologies: conical,
paraboloidal, and Gaussian. The paraboloid profile with protuberances almost
touching each other was found to exhibit the lowest reflectance, with the
effective refractive index varying quasi-linearly with depth [21]. Highly
antireflective wings in insect are often reported to play a likely role in
crypsis [63, 64]. Similarly, nipple arrays on the surfaces of compound eyes
are assumed to improve camouflage under daylight and improve night vision [66,
67]. In general, some of such lattices of protuberances were reported to
combine antireflection with hydrophobic properties [63, 64, 16, 69, 70, 71,
72], bactericidal activity [73, 74, 75, 76, 72], and fluorescence emission
[77, 78, 79, 80, 60]. In the case of cicadas, it was shown that the
protrusions could be approximated by truncated cones under hemispheres [64,
16]. The cones gave rise to impedance matching and high antireflection whereas
the cones favoured hydrophobicity. Fluorescence arises from fluorescent
proteins -typically resilin- embedded withing the membrane material [77, 78,
79, 80, 60].

Figure 4: Cacostatia ossa moth’s wings exhibit increased transparency due to a
unique disordered structure in nipple array covering them (a). This structure
was modelled by truncated cones for reflectance simulations (b). Its total
(thin solid curves) and specular (thick solid cuves) reflectance spectra (c)
at different angles of incidence are much lower than the specular reflectance
of a unstructured flat wing (dashed curves). These figures were reproduced
from ref. [57] with permission from the American Physical Society.

###  2.2 Elytra of beetles

The blue-grey elytra of Rosalia alpina longhorn beetle (family Cerambycidae)
exhibit large black spots (figure 5). The micro- and nanostructured setae that
cover these elytra contribute, on one side, to the camouflage of this beetle
on beech barks and, on the other side, to thermoregulation by allowing a quick
heating of the body to the optimal temperature and dissipating excess of heat
through IR emission to prevent overheating [81, 82], akin the wings of some
butterflies described in the previous section. The setae occurring in the
black spots enhance visible light absorption by light trapping, whereas the
setae of all the elytra enable the thermoregulation. The former are inclined
scales, touching neighbours at the tips and forming tent-like architectures
with 1-$\mu$m period and 100-nm period grating patterns (figure 5a) [81, 82].
The setae occurring on the blue-grey areas consist in hairs [81, 82] (figure
5b). Through optical modelling based on SEM observations, the light trapping
role of the scales was demonstrated [81, 82]. Several reflections on opposite
inclined patterned scales and high concentration of melanin in these scales
account for the high-absorption properties. In addition, the scales and the
hairs exhibit absorption (and hence emission) enhancement in the mid-IR range
[81, 82].

Figure 5: The elytra of Rosalia alpina longhorn beetle exhibit black patches
on a blue-grey background (a). The black patches are covered by so-called
"tent-shaped" scales (b), whereas hairs cover the blue-grey area (c). These
figures were reproduced from ref. [82] with permission from Elsevier.

More recently, Vasiljević and co-workers unveiled a combination of lenslets
and micron-sized multilayered spherical black elements located within the
elytra of Morimus asper funereus longhorn beetle (family Cerambycidae) [83],
which also display black spots on a grey surface. However, in this case, both
areas, black and grey, look identical when observed with a thermal camera. The
authors concluded from Finite Element Method (FEM) simulations that the
combined action of the lenslets and the multilayered spherical elements
focuses IR radiation on microchannels containing hemolymph.

Finally, arrays of ellipsoidal and randomly located micropillars (figure 6A-C)
were reported on the elytra of Euprotaetia inexpectata scarab beetle (family
Scarabaeidae) [84]. They enhance light absorption by a combination of Mie
scattering and optical focusing. This way, incident light reaches absorbing
pigment -namely melanin- located within the elytra, giving rise to an
absorptance up to 99.5% and a reflectance of 0.1% at 400 nm (figure 6D).

Figure 6: The microstructure occurring on the elytra of Euprotaetia
inexpectata scarab beetle consists in some ellipsoidal and apparently randomly
positioned micropillars (A-C) observed here by SEM with different viewing
angles. Numerical predictions of the spectral reflectance from a flat
absorbing chitin substrate (black solid line), a flat non-absorbing chitin
substrate (red dashed line), and the described absorbing microstructure (blue
solid line) confirm the role of the observed structure in the absorption
enhancement. The latter simulations agree with experimental results (purple
solid line). Scale bars: 15 $\mu$m (A), 5 $\mu$m (B), and 10 $\mu$m (C). These
figures were reproduced from ref. [84], License CC-BY-4.0.

###  2.3 Bird feathers

The ultra-black plumage occurring in some species of birds of paradise within
the family Paradisaeidae has captivated researchers due to its unparalleled
darkness, reaching absorption levels of up to 99.95%. This phenomenon,
elucidated by McCoy and colleagues, is a result of structural absorption
rather than pigmentation (figure 7) [36]. These feathers appear even darker
than typical black feathers due to a significant reduction in specular
reflection, as measured through directional reflectance ranging from a mere
0.05% to 0.31%. The secret lies in the microstructure of the feathers,
featuring barbules curved up that are tilted vertically by ca. 30° with
respect to the normal in the direction of the feathers’ distal tip. This
unique arrangement enhances multiple light scattering, creating regularly
spaced cavities with dimensions of 5–30 $\mu$m in width and 200–400 $\mu$m in
depth. Astonishingly, the super-black effect is most pronounced when looked
from the distal direction, aligning perfectly with the perspective of a female
observing a male. The cavities present a directional reflectance bias, making
the feathers even darker when viewed from the distal direction. This natural
adaptation showcases the fascinating ways in which birds of paradise have
evolved to achieve remarkable visual effects in their plumage.

Figure 7: The ultra-black feathers of the Parotia wahnesi bird of paradise (a)
is characterised by specialised barbules arrays, as observed by SEM (b).
Reflectance spectra (c,d) of standard black (solid lines) and ultra-black
(dotted lines) feathers are compared, showcasing total (namely, sum of diffuse
and specular components) reflectance (c) as well as specular reflectance at
normal incidence (d). Scale bar: 50 $\mu$m (b). These figures were reproduced
from ref. [36], License CC-BY-4.0.

###  2.4 Cuticle of Maratus jumping spiders

Jumping spiders, specifically the male members of the genus Maratus, commonly
referred to as peacock spiders, have evolved a fascinating display strategy to
attract their female counterparts (figure 8a). These spiders exhibit a
striking combination of brilliant colours arising from pigments or photonic
structures [85] and velvety black areas [86] on their bodies. These black
regions, described as ultra-black [86], reflect less than 0.5% of light,
reaching intensities as low as 0.35% in the case of Maratus karrie, due to
microstructures, including densely packed cuticular bumps resembling microlens
arrays (figure 8c,d). In addition, M. karrie displays some black scales
reassembling brushes (figure 8e,f). Optical modelling revealed a delicate
balance between minimising light reflection from the surface and maximising
absorption by melanin (figure 8g). Interestingly, McCoy and co-workers
proposed that this ultra-black followed a convergent evolution for the success
of these spiders and the birds of paradise in the competitive realm of sexual
selection [86].

Figure 8: Some male jumping spiders such as Maratus nigromaculatus exhibit
ultra-black areas along with striking colour for courtship (a). These ultra-
black areas reflect 0.44% and 0.35% of light with a 30°-detection angle (b) in
the cases of Maratus speciosus and Maratus karrie, respectively. This is
significantly less than light reflection from the cuticle of a standard black
spider such as Cylistella sp. (namely, 4.61%) and more than the one from the
black feathers of Drepanornis bruijnii bird of paradise (i.e., 0.17%). The
ultra-blackness of spiders such as M. speciosus arises from microlens arrays
(MLA) covering some striated layers in the cuticle (SLC) and some absorbing
layer of melanin (Mel.) observed in (c,d). Some plate-like scales (PLS) giving
rise to to blue colour are also imaged in (c). In the case of M. karrie (e,f),
some additional brush-like scales (BLS) occur on the surface of the
integument. They enhance further light absorption. McCoy and co-workers
described four mechanisms of light absorption: 1) Light is scattered multiple
times as it interacts with spiny projections of the BLS, gradually being
absorbed as it traverses through the cuticle material and into the absorbing
melanin layer while scattering; 2) Multiple scatterings occur between bump
surfaces, causing light to absorb as it passes through the cuticle materials
and enters the melanin layer; 3) The path length of light within melanin
layers is extended, leading to increased absorption; 4) Light undergoes
diffraction due to a periodic microlens array, resulting in reduced visual
detection by an observer such as a female spider. Scale bars: 30 $\mu$m (c),
10 $\mu$m (d), 50 $\mu$m (e), and 10 $\mu$m (f). These figures were reproduced
from Graham Wise (a),
https://commons.wikimedia.org/wiki/File:Maratus_nigromaculatus_(14585680722).jpg,
License CC-BY-2.0 and from ref. [86] (b-g), License CC-BY-4.0.

###  2.5 Skin of West African Gaboon viper

The Gaboon viper Bitis rhinoceros, native to West Africa, exhibits a stunning
camouflage in its natural habitat, thanks to its intricate skin pattern [87].
The geometrically arranged velvet black spots, interspersed with pale and
light brown regions (figure 9), seamlessly blend into the diverse light and
shade patterns of the forest ground under the canopy. Observations revealed
that the blackness of the viper’s scales is primarily derived from a
hierarchical structure characterised by densely packed, leaf-like
microstructures covered with nanoridges. Under microscopic scrutiny, even the
areas in between black scales exhibit nanoridge striations (figure 9).
Reflectance spectra analysis demonstrates that both black and pale scales have
nearly flat profiles across the visible range, with a notable peak around 880
nm (figure 9). Intriguingly, applying an Au-Pd coating to the black scales
preserves the black colour and further diminishes reflectance (figure 9). This
finding supports the idea that the viper’s original surface works as an
effective light-trapping device, utilising multiple reflections of light. The
metallic coating further enhances light trapping via light reflections on the
metal-coated surfaces. Modelling of diffuse reflection using Lambertian
symmetric V-shaped cavities validated the proposed light-trapping mechanism
and elucidated the angular dependence of reflectance spectra in pale scales
[87]. However, the black scales exhibit a distinct angular characteristic,
lacking a specular reflection peak and displaying a gradual decrease in
reflectance intensity with increasing emerging angle. This unique angular
behaviour imparts a nonglossy visual appearance to the velvet black,
attributed to the more isotropic arrangement of scale structure.

Figure 9: The West African Gaboon viper, Bitis rhinoceros, (a) exhibits an
effective camouflage pattern with velvety black, light brown and pale hue
areas on its skin. Reflectance spectra (b) highlight the characteristics of
black dorsal scales (solid black line), Au-Pd coated black dorsal scales
(dotted black line), pale dorsal scales (solid grey line), Au-Pd coated pale
dorsal scales (dotted grey line), and ventral scales (dashed black line).
Unlike pale scales (solid black line), black scales (solid grey line) do not
specularly reflect light (c): with an 700 nm incident light at 45° with
respect to the normal to the skin surface, light reflectance decreases with
the detection angle (c). The microstructures of the scales can be imaged by
SEM (d-j). The black scales are densely packed and resemble leaves marked with
an arrow (P) in (d). They are covered with small ridges (e-g) with spinules
indicated with an arrow (S) in (g). The pale scales have a more simple pattern
(h), exhibiting pits (i,j). These figures were reproduced from ref. [87], with
permission from Springer Nature.

###  2.6 Plant leaves and petals

In the realm of plants, surfaces of some petals and leaves reveal a
mesmerizing array of structures optimised to enhance light harvesting (figure
10). The interplay of antireflection and light trapping mechanisms unfolds
through the subtle architecture of conical-shaped epidermal cells [88, 89, 34,
90]. As sunlight encounters these structures, a gradual increase in the
effective refractive index occurs, giving rise to antireflection akin to the
cones found on the surfaces of moth eyes and cicada wings. In some plants,
additional nanowrinkles were demonstrated to reduce light reflection [34, 90].
In addition to antireflection, light redirection extends the path length
within plant integuments, contributing to light trapping. The epidermal cells
of flowers were reported to function as lenses and conducting incident light
into the integuments comprising pigments [88, 91, 92]. It was also shown that
the cone shape of their petals varies, reflecting the plant’s strategic
adaptation to either scatter or absorb incident ultraviolet waves, with
shorter cones in the former, and taller ones in the latter, respectively [93].
This adaptive variability plays a pivotal role in enhancing light capture for
crucial processes like photosynthesis and contributes to the vivid coloration
of these botanical wonders, especially in environments with limited light
availability.

Figure 10: The petals of viola flowers (a) feature cone-shaped structures
adorned with nanowrinkles (b,c). The cones and nanowrinkles collectively play
a role in the augmentation of light harvesting. These figures were reproduced
from ref. [90], with permission from the American Chemical Society.

Venturing into the microscopic world, diatoms, unicellular algae encased in
intricate silica frustules (namely, the hard porous structures of diatoms),
offer a different yet equally captivating story of solar energy harvesting
[94]. The case of Coscinodiscus sp. stands out with its frustule comprising
three layers – termed cribellum, cribrum, and the internal plate – each
composed of thin slabs housing hexagonal arrays of disk holes. The size and
spacing of these holes vary from layer to layer, forming a hierarchical
structure that has been finely tuned for optimal light trapping and
photosynthesis.

The blue iridescent epidermal chloroplasts occurring in some plant leaves,
such as those in shade-dwelling Begonia spp. and Selaginella erythropus,
display intricate multilayers that significantly enhance light absorption [35,
95, 96, 97]. Chloroplasts, crucial plant organelles facilitating
photosynthesis by absorbing incident light via chlorophyll, play a pivotal
role in converting light energy into biochemical energy as adenosine
triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).
Particularly, the initial light-dependent phase of photosynthesis takes place
within the absorbing thylakoid tissues of chloroplasts. Two distinct types of
chloroplasts are of interest with respect to photonics and enhanced light
absorption: iridoplasts and bizonoplasts [98]. Iridoplasts, exclusive to some
plants such as the leaves of Begonia, possess photonic structures consisting
in periodic multilayers of thylakoid tissues. Conversely, bizonoplasts have
been identified in a single plant species, S. erythropus (figure 11) [99,
100]. They look like a mix of conventional thylakoid tissue present in typical
irregular chloroplasts across many plants, and very organised iridoplasts. The
unique ordered photonic structures of iridoplasts and bizonoplasts result in
enhanced light absorption in the green part of the electromagnetic spectrum
due to slow-light effect occurring at the red edge of the photonic bandgap of
the multilayered structures. This increased absorption aligns with the
incident light environment of these canopy-adapted plants. It leads to an
enhanced quantum yield in low-light conditions, bolstering photosynthesis when
compared to normal chloroplasts [35, 95, 96, 97].

Figure 11: Bizonoplasts (a) found in the epidermis of the leaves of S.
erythropus contains very organised periodic multilayers (b,c) and conventional
irregular thylakoid tissues, separated by a dashed line in (a). These SEM
observations allow to measure the dimensions of these structures (d). The
period $\Lambda$ of the multilayer comprises thylakoid membranes of thickness
$L_{\textrm{T}}$, lumen layers of thickness $L_{\textrm{L}}$, and strommal
layers of thickness $L_{\textrm{S}}$. Incident light reflects on this
multilayer in the blue, as evidenced by spectrophotometry and simulations (f)
as well as visualised through light microscopy (g). Scale bars: 5 $\mu$m (a),
0.5 $\mu$m (b), 100 nm (c), and 10 $\mu$m (g). These figures were reproduced
from ref. [95], with permission from the Royal Society.

In the ethereal heights of the Alps, the edelweiss flower, Leontopodium
nivale, unveils a captivating defense mechanism against intense UV radiation.
Located within the wooly cover layer of its bracts (namely, the downy-white
“petals” of the edelweiss that actually are specialised leaves), an
intersecting pattern of transparent filaments, displaying some faint
iridescence can be observed by microscopy (figure 12). These filaments exhibit
variations in diameter and morphology of their cross-sections. They are hollow
with parallel corrugation running along the main axis of the filaments with a
period of ca. 180 nm. The spectral reflectance is rather low from 300 to 400
nm and abruptly increases around 400 nm to form a plateau at ca. 65%. This
optical behaviour corresponds to some strong absorption in the near UV range.
This intricate filamentary architecture, akin to a two-dimensional corrugated
dielectric slab, emerges as a UV-selective waveguide coupling device. Fano
resonances within these filaments facilitate the transfer of incident
ultraviolet waves. The filaments, characterised by a broadband angular
response, act as conduits for UV photon energy, efficiently dissipating it
along the hollow guides, as the filament materials absorb UV. This ingenious
strategy protects the delicate cellular tissue beneath.

Figure 12: Protection against UV is a crucial attribute for edelweiss flowers
(L. nivale) as their silver-white bracts are exposed to Sun radiation. These
bracts are enveloped by a downy layer consisting of an intersecting pattern of
transparent filaments (a) with diameters of ca. 10 $\mu$m, as observed by SEM.
These filaments display some corrugation (a, inset) with a period of ca. 180
nm. The total reflectance of a bract at normal incidence remains consistently
high throughout the visible range but sharply diminishes to zero in the near
UV - solid black line represents total reflectance, while the dashed red line
represents the specular reflectance. The curvature of the hollow filament can
be neglected in the models (b). The simulated absorbance spectrum through the
corrugated-slab model for transverse-electric light polarisation exhibits a
corresponding high intensity in the 300-450 nm range (c). These figures were
reproduced from ref. [101], with permission from the American Physical
Society.

##  3 Infrared absorbers inspired by natural photonic structures

The underlying mechanisms of various natural structures have been elucidated
thus far, as presented in the previous section. If their potential for energy
harvesting remains largely untapped, several devices have been suggested
through a bioinspiration approach [15, 17, 14]. In addition to their eco-
friendly and sustainable materials composition and fabrication, these natural
structures present other benefits with respect to current alternative such as
their thinness and lightness. They can clearly improve the energy yield of PV
cells and solar panels [102, 29, 47], passive radiative cooling [103, 104,
105, 106, 37], photocatalysis [107, 108, 45], or even the efficiency of
electromagnetic camouflage, and the capture of stray light in telescopes.

###  3.1 Bioinspired antireflective coatings: from moth-eye and cicada-wing
templates to functional applications

As described in section 2.1, insects employ antireflective features as crucial
characteristics in their crypsis strategy. The nipple arrays found on some of
their eye and wing surfaces have been replicated to create bioinspired
antireflective coatings applicable across various uses, including solar
panels, antiglare glasses, screens, light-sensitive detectors, telescopes,
thermochromic smart windows and camera lenses [69, 109, 102, 110, 111, 112,
113, 114, 115, 14, 116].

Bottom-up nanofabrication approaches such as self-assembled spherical
nanoparticles in non-close-packed hexagonal arrays have been used to mimic
moth-eye structures (figure 13) [109]. Simulations indicated that non-close-
packed structures result in lower reflectance at wavelengths longer than the
nipple interdistance, affirming the suitability of the natural moth-eye design
for highly efficient antireflective devices. Spin-coating deposition of
colloidal suspensions of silica particles (360 nm in diameter) and their shear
alignment allowed to fabricate a template that served as a mould for casting
some polydimethylsiloxane (PDMS). This PDMS mould was subsequently pressed
onto a ethoxylated trimethylolpropane triacrylate (ETPTA) [109] or a
perfluoroacrylate polymer [117] layer laying on glass substrate (figure
13a-d). These monomer films were then polymerised with a pulsed UV light
curing system. Such biomimetic structures demonstrated outstanding low
reflectance of less than 0.5% across the visible spectrum (figure 13e) [109,
117]. Similarly, such spin-coated monolayer silica colloids were utilised as a
mask in a reactive ion etching (RIE) process of silicon wafers with SF6 [110]
and of gallium antimonide (GaSb) substrates with Cl2 [102], reducing
reflectance to less than 5% in the visible-near-IR range (with respect to ca.
40% for unstructured wafers) [110, 102]. Such biomimetic structures appeared
easy to fabricate on solar and TPV cells.

Figure 13: Nipple array structures observed on moth eye were replicated using
360-nm size silica particles, observed in (a-c) by atomic force microscopy
(AFM) without etching (a) as well as with 20-s (b) and 45-s (c) RIE etching
times. This etching process gave rise to diffent profiles (d). This
bioinspired structuring gives rise to significant reduction in light
reflection at normal incidence (e). Solid and dotted curves are respectively
experimental and simulated reflectance spectra with a flat unstructured
poly(ethoxylated trimethylolpropane triacrylate) (PETPTA) surface (black
curves), 110-nm size hemispherical caps (light grey curves) fabricated with
20-s RIE etching, and 180-nm size hemispherical caps (dark grey curves)
fabricated with 45-s RIE etching. These figures were reproduced from ref.
[109] with permission from AIP Publishing.

Similarly, top-down synthesis techniques such as nanoimprint lithography (NIL)
were employed to develop efficient and cost-effective antireflective coatings
inspired from nature. For instance, cicada wings were directly utilised as
natural stamps, leveraging the wing chitinous material with commendable
thermomechanical characteristics (figure 14a) [69, 111]. This material can
indeed be heated up to 200°C without any damage. After heating, a poly(methyl
methacrylate) (PMMA) film may be pressed onto the biological template (figure
14b-d) [69]. The negative replica was transferred to a silicon substrate using
the array of nanowells in PMMA as a mask for RIE. Upon removal of PMMA, the
resulting patterned surface demonstrated antireflective properties, evident
from its dark visual appearance [69]. If the structured PMMA film was employed
as a mould for gold thermodeposition instead of a mask for RIE, a perfect
replica of gold hexagonal nanopilars was produced (figure 14e,f) [69].
Alternatively, a PMMA replica was fabricated through a modified procedure
[111]: a first negative replica was obtained from the thermodeposition of gold
onto the natural photonic structures of the wings. It was used as a mould to
cast PMMA, which was subsequently peeled off [111]. The antireflective
capability of the replicated PMMA film was notable. The PMMA film’s
reflectance was found to be decreased from approximately 6% to about 2% across
the visible-near-IR range due to the unique nipple array [111].

Figure 14: Nipple array structures were replicated in gold from cicada wings
by NIL (a) [69]. A negative replica was also fabricated in silicon (a). The
process involved using cicada wings as natural stamp, giving rise to a PMMA
negative replica observed by SEM (b,c) and AFM (d). Depositing gold on this
PMMA mould allowed the fabrication of a perfect replica of the gold hexagonal
nanopilars (e) with respect to the natural-wing structure (f). These figures
were reproduced from ref. [69] with permission from John Wiley and Sons.

###  3.2 Advances in bioinspired solar light harvesting: beyond petals, leaves
and butterfly wings

The photonic structures found in the petals and leaves occurring in the
integuments of certain plants have informed the development of improved
bioinspired light-absorbing structures (figure 15a,b) [107, 108, 34, 90, 118,
119]. From enhancing the efficiency of organic solar cells to improving
photocatalysis and contributing to artificial photosynthesis systems, these
bioinspired designs continue to pave the way for sustainable and innovative
energy solutions.

For instance, the microstructures of the epidermal cells of some rose species
inspired a polymer thin film replica that was integrated in a solar cell [34].
The biomimetic coating demonstrated a significant reduction in reflectance
over the entire spectral range, particularly at grazing incidence, with
remarkable 13% and 44% increase in the solar cell’s short-circuit current at
normal and grazing incidence, respectively (figure 15c,d). These properties
are of high interest for solar cell efficiency with respect to the Sun
movement throughout the day. The dome profile of the micropapillae on the
petal surface was demonstrated to play the role of microlenses, lengthening
the optical path of light rays within the plant integuments. This dual
functionality of efficient antireflection and light trapping is crucial for
enhancing the performance of thin-film organic solar cells, addressing issues
of low optical absorption and spectral drops due to Fabry-Pérot interferences.

Figure 15: The microstructures occurring on the integument of rose petals were
replicated in e.g., PMMA for application onto solar cells (a,b). They are
observed here by SEM. The reflection factors from a glass substrate, featuring
a black absorber on the rear and the replicated rose structure on the front
side with light incident at angles of 20° and 80° (solid lines) are
significantly lower than flat unstructured references (dash-dotted lines) (c).
Consequently, the corresponding external quantum efficiencies (EQE) of the
structured thin-film organic solar cell are higher than ones of the flat
references (d). These figures were reproduced from ref. [119] (a,b), License
CC-BY-4.0. Data for (c,d) were sourced from ref. [34].

While the petal of roses offer a compelling template for biomimetic light
harvesting, the sophistication of plant leaves provides an even more complex
blueprint. The thin and soft leaves of Vallisneria spp., aquatic grass plants,
also known as eelgrass are an very informative study case for bioinspiration
[107]. The hierarchical architecture of these leaves includes lens-like
epidermal cells, a so-called palisade parenchyma functioning as optical
waveguides, and a spongy disordered layer with intertwined veins giving rise
to optical multiscattering and extending the optical path length. Utilising a
sol-gel method, a silica and titania mimic of these leaves was templated for
photocatalitic application [107]. The resulting Ti-Si catalyst exhibited a
threefold higher rate constant for the degradation reaction of methylene blue
exposed to UV compared to a commercial TiO2 catalyst. The macroporosity and
enhanced light scattering properties of Vallisneria leaf structure made it an
ideal template for photocatalysis, showcasing the potential for biomimicry in
advancing solar-driven environmental applications.

As one could have expected, butterfly wings have naturally inspired the design
of visible and IR light absorbers such as an SiO2 negative replica of the
black wing scales of Trogonoptera brookiana butterfly that exhibits enhanced
light trapping properties (figure 16I) [29]. The solar energy loss of this
replica, namely, the integrated solar light intensity reflected by the replica
in the 400-900 nm range, was found to be 22.6% of the one of an SiO2 flat
surface. Similarly, a hybrid photonic-plasmonic structure was fabricated by
bio-templating the black forewings of Troides helena [30]. Silver spherical
nanoparticles with various diameters (10 nm, 20 nm, 40 nm, 60 nm, and 80 nm)
were deposited on the wings before the chitin structure was carbonised. An
enhanced absorption was measured and simulated in the near- and mid-IR ranges.
It results from the plasmon resonance of the silver nanoparticles and the
coherent coupling among adjacent nanoparticles within the photonic
architecture of T. helena. Such a hybrid photonic-plasmonic architecture was
designed for photocataltic applications while taking inspiration from P.
nireus (figure 16II) [45]. It consisted in gold nanoantennas located onto a
bismuth vanadate (BVO) photocatalytic unit with the architecture of P. nireus
black wings. This architecture was fabricated through a sol-gel method. Both
experiments and simulations demonstrated the enhanced photocatalytic activity
arising from the 25%-increased light harvesting within the 700 nm-1200 nm
range and the 3.5-fold enhancement of the electric-field intensity of
localised surface plasmons (figure 16III) [45]. Whereas these three artificial
structures were fabricated by bottom-up methods, a nanostructured absorber
film with disordered holes was synthesised as a mimic of the disordered black
wing scales of P. aristolochiae butterfly (figure 16IV) [47]. Using phase
separation of a two-polymer mixture, a hydrogenated amorphous silicon (a-Si:H)
film was patterned for PV applications. The structure exhibited a relative
integrated absorption over the range 450 nm-800 nm of 93% and 207%, with a 0°-
and a 50°-incidence angle with respect to the normal to the film surface,
respectively [47].

Figure 16:  (I) SiO2 negative replica of the black wing scales of Trogonoptera
brookiana butterfly (a) was synthesised through a sol-gel method [29]. The
scales were infiltrated with a precursor solution (b) that was heated for
solidification (c). The scales were removed by etching (d), leaving a negative
replica. (II) A sol-gel method was also used to fabricate a bismuth vanadate
(BVO) replica (b,d) of the wing scales of Papilio nireus (a,c) [45]. Gold
nanoantennas (abbreviated "AuNR") (e,f) were loaded into the BVO wing scales
(g,h). Insets in (g) are a simplified sketch (top) and a higher-magnification
electron micrograph (bottom). The fabrication process from the butterfly wing
scale to the Au NR-loaded BVO hybrid photonic-plasmonic structure is
summarised in (i). (III) This hybrid structure gave rise to the best
photocatalytic activity [45], as demonstrated by the CO2 evolution of
isopropyl alcohol (IPA) degradation as a function of illumination time for the
Au NR-loaded BVO hybrid photonic-plasmonic structure ("BVO wing+Au NRs"),
unstructured BVO powder ("Powder counterparts"), unstructured BVO slab with Au
NRs ("BVO slab+Au NRs"), and bio-templated BVO photonic structure ("BVO
wing"). (IV) A nanostructured hydrogenated amorphous silicon (a-Si:H) film
inspired by the wing scales of Pachliopta aristolochiae (a) gave rise to
enhanced light harvesting at normal incidence (b) and with no-zero incidence
angles (c) [47]. These figures were reproduced from ref. [29] (I), ref. [45]
(II,III), and ref. [47] (IV), Licenses CC-BY-4.0, CC-BY, and CC-BY-NC,
respectively.

##  4 Conclusions

Photonic structures occurring in the integuments of living organisms such as
arthropods, birds, and plants are very sophisticated optical devices that give
rise to various striking optical effects, including UV, visible, and IR
radiation management and absorption enhancement. They occur in biological
tissues encompassing butterfly wings, beetle elytra, bird feathers, spider
cuticle, viper skin, as well as plant leaves and petals. These phenomena are
often crucial for the survival of animal and plant species. This review
article also showcased the promising potential of bioinspiration in the field
of energy capture and conversion. Exploiting light trapping, impedance
matching or antireflection observed in natural structures is indeed highly
interesting in view of developing bioinspired energy-efficient applications
such as PV and TPV cells, TEG, artificial photosynthesis, and photocatalysis.
With the optimisation of the efficiency of such applications, these advances
inspire future research and innovation in the field of bioinspired energy
solutions. Ultimately, this research paves the way for a more sustainable and
environmentally conscious future by harnessing the beauty of nature’s designs
to meet humankind’s energy needs.

## Acknowledgements

The author thanks Prof. Olivier Deparis from the University of Namur, Belgium
for fruitful discussion and reading the manuscript. The author was supported
by a BEWARE Fellowship (Convention n°2110034) of the Walloon Region (COFUND
Marie Skłodowska-Curie Actions of the European Union #847587).

## Conflicts of interest

The author declares no conflict of interest.

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