Metal-oxide semiconductors molded into photonic structures are highly promising for photocatalysis, thanks to their unique property of slowing down light, thereby improving light harvesting. However, this ‘slow light’ is limited to specific frequencies in a narrow spectral region. To address this, we fabricated bilayer-bimodal inverse opal (IO) TiO2-BiVO4 photonic structures that generated slow photons at multiple spectral regions. We tuned their frequencies by lattice parameter and light incidence-angle variations and achieved an 8-fold and a 2-fold increase in photocatalytic efficiency compared to non-structured and monolayer counterparts respectively. The strategies presented here can be extended to all solar energy conversion applications.
Inverse opal (IO) photonic semiconductors are promising materials for photocatalysis, thanks to their slow photon properties that increase light harvesting. Here, we report, in IO TiO2-BiVO4 photonic structures, the ability not only to generate slow photons in the visible range but also to tune their frequencies and transfer their energy. Angle-resolved photocatalytic experiments revealed a 70% increase in activity in all IO structures compared to non-IO compact films and a further 20% increase when the slow photons were accurately tuned to BiVO4 electronic absorption. The synthesis and tuning strategies presented here can be extended to all solar energy conversion applications.
Blue is often described as the rarest colour in natural organisms. Elucidating the underlying colour mechanism(s) is always very impactful for the understanding of the natural world. In this research, the colour of the blue rounded spots occuring in the skin of Taeniura lymma stingray was unveiled by a combination of experimental and numerical techniques. Our results demonstrated that this blue colour arises from coherent scattering in quasi-ordered photonic structures occuring in the skin of this stingray. These structures made of collagen fibers are mostly unknown in marine species. In addition, structural blue colours had never been reported in elasmobranches.
Every year, billions of birds collide with windows, often resulting in their death, as well as in significant material damage. However, many animal species have developed photonic structures active in the UV due to their perception of light in this wavelength range. These structures allow us to elaborate new optimised coatings for selective UV reflection through a bioinspiration approach. These bioinspired coatings consist of a periodic multilayer of metal oxide layers deposited on soda-lime glass with patterns. The optical response of the multilayers was numerically optimised in terms of morphology and material composition using human and avian colour perception models.
Manipulation of light was proved to be an efficient strategy to improve light harvesting efficiency in solar energy conversion. Inverse opal (IO) photonic structures are among the most promising materials, which permit light manipulation, thanks to their ability to slow down light at specific wavelengths and localize it within the dielectric structure. However, the generation, the control and, in particular, the practical utilization of these narrow spectral range ‘slow photons’ remain highly challenging and relatively underexplored. In this work, we report the ability not only to generate slow photons in the visible range by synthesizing highly ordered IO TiO2 photonic structures, but also to control and tune their wavelengths, by varying lattice parameters (pore sizes), such that they can be efficiently utilized by the composite bismuth (Bi)-based semiconductor for visible light photocatalysis. Photocatalytic experiments revealed a 70% increase in efficiency in all IO structures compared to the corresponding non-structured compact film. In addition, a 20% increase in efficiency was observed when the photonic stop band gap as well as its blue and red edges were accurately tuned to match the electronic absorption of the Bi-based photocatalyst. Our choice of IO synthesis parameters and tuning strategies enabled us to generate, control and transfer the energy of slow photons from IO TiO2 to the composite visible light-responsive photocatalyst for highly amplified photoactivity. This work opens new possibilities for the practical utilization of slow photon effect under visible light in various solar energy conversion applications.
In the fabrication processes of photonic crystals, structural disorder arises in the synthesised structures from the limitations in the resolution of the synthesis processes. Elucidating the structural-disorder influence helps reducing its optical effects and allows using it in technological applications. Natural photonic structures are interesting examples to investigate due to their resilience to structural defects. The benefit and cost of these defects are so far not understood. In this contribution, we investigated the structural disorder in one-dimensional photonic crystals occurring on the wings of beetles. We performed optical simulations allowing to assess the optical benefit and cost of such disorder.
Eggshell is essential for the reproduction of birds. The optical properties of their shells may have an impact on biological functions such as heat and UV protection, recognition by the parents or camouflage. Whereas ultraviolet reflection by bird eggshells has been superficially described in the scientific literature, the physical origin of this phenomenon remains poorly understood. In this article, reflectance peaks in the near UV range were observed by spectrophotometric measurements of hen eggshells. In addition, electron microscopy imaging revealed the presence of pores within the so-called “calcified shell” part (i.e., between ca. 20 μm and ca. 240 μm deep from the outer surface). The average radii of these pores range from 120 to 160 nm. Mercury intrusion porosimetry allowed to highlight a distribution of pore radii around 175 nm. Numerical and analytical predictions using scattering theory indicate that these pores are responsible for the optical response observed in the UV range.
In nature, many natural organisms display very conspicuous visual appearances. Most of these appearances are due to pigments located within the biological tissues. In addition, fluorescence emission is also known to arise from several organisms. Upon contact with liquids, the colours and fluorescence of some species such as a few from the class Insecta were reported to change reversibly. However, these optical effects are so far not totally elucidated. In this study, the colour and fluorescence properties of Euchroea auripigmenta beetle were investigated. This insect exhibits a yellow visual appearance on its head, thorax and elytra when it is illuminated by either visible white light or UV light. After soaking into liquids, both the colour and the fluorescence emission from its integuments are modified. The displayed colour turns from yellow to brown. Using optical, fluorescence and electron microscopy techniques, we morphologically characterised the beetle’s integuments. This allowed to observe spike-like protuberances covering the yellow areas of the beetle and from where the yellow visual appearance originates. These protuberances are thought to give rise to further light scattering in addition to the scattering by pigments. Thanks to spectrophotometry, imaging scatterometry and spectrofluorimetry observations, the reflectance and fluorescence properties of this beetle were characterised. Whereas the liquid- induced colour change is attributed to a change in the scattering pattern, the fluorescence emission is most likely due to a chemical influence of the liquids on the two different types of embedded fluorophores.
Natural photonic structures found on the cuticle of insects are known to give rise to astonishing structural colours. These ordered porous structures are made of biopolymers, such as chitin, and some of them possess the property to change colour according to the surrounding atmosphere composition. This phenomenon is still not completely understood. We investigated the structure found on the cuticle of the male beetle Hoplia coerulea (Scarabaeidae). The structure, in this case, consists in a 1D periodic porous multilayer inside scales, reflecting incident light in the blue. The colour variations were quantified by reflectance spectral measurements using water, ethanol and acetone vapours. A 1D scattering matrix formalism was used for modelling light reflection on the photonic multilayer. The origin of the reported colour changes has to be tracked in variations of the effective refractive index and of the photonic structure dimensions. This remarkable phenomenon observed for a non-open but still porous multilayer could be very interesting for vapour sensing applications and smart glass windows.
Structurally colored nano-architectures found in living organisms are complex optical materials, giving rise to multiscale visual effects. In arthropods, these structures often consist of porous biopolymers and form natural photonic crystals. A signature of the structural origin of coloration in insects is iridescence, i.e., color changes with the viewing angle. In the scales located on the elytra of the Brazilian weevil Entimus imperialis (Curculionidae), three-dimensional photonic crystals are observed. On one hand, each of them interacts independently with light, producing a single color which is observed by optical microscopy and ranges from blue to orange. On the other hand, the color perceived by the naked eye is due to multi-length-scale light effects involving different orientations of a single photonic crystal. This disorder in crystal orientations alters the light propagation in such a way that the crystal iridescence is removed. Entimus imperialis is therefore a remarkable example of additive photonic colors produced by a complex multi-scale organic architecture. In order to study this specific natural photonic structure, electron microscopy is used. The structure turns out to be formed of a single type of photonic crystal with different orientations within each scale on the elytra. Our modeling approach takes into account the disorder in the photonic crystals and explains why the structure displays bright colors at the level of individual scales and a non-iridescent green color in the far-field.
Structurally coloured natural photonic crystals found in several insects are made of ordered porous chitin structures.
In such photonic crystals, colour changes can be induced by relative gas/vapour concentration variations
in a mixed atmosphere. For instance, when the composition of the atmosphere changes, the colour of Morpho
sulkowskyi buttery is modied. Based on this eect, it is possible to identify closely related gases/vapours. In
spite of increasing interests for such sensors, the fundamental mechanisms at the origin of the selective optical response
are still not well understood. The point is that refractive index variations resulting from the introduction
of a specic gas species in the atmosphere are too small to justify the dramatic changes observed in the optical
response. Here, we demonstrate through numerical simulations that indeed gas/vapour-induced refractive index
changes are too small to produce a signicant modication of the spectral reectance in a representative 3D
periodic model of natural porous nanostructures. For this purpose, we used the rigorous coupled wave analysis
(RCWA) method for modelling light scattering from inhomogeneous optical media. The origin of the reported
colour changes has therefore to be found in modications of the porous material and their impact on the photonic
response.
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