Circularly polarized reflection from the scarab beetle Chalcothea smaragdina: light scattering by a dual photonic structure

Nature-Inspired Helicoidal Nanocellulose-Based Multi-Compartment Assemblies with Tunable Chiroptical Properties

Cellulose-based nanocomposites are highly appealing for the development of next-generation sustainable functional materials. Although many advances have been made in this direction, the true potential of fibrillar nanocomposites has yet to be realized because available fabrication approaches are inadequate for achieving precise structural control at the sub-micrometer scale. Here a spray-assisted alignment methodology of cellulose nanofibrils is combined with the layer-by-layer assembly into an additive manufacturing process in which the alignment direction of each cellulose layer is rationally selected to achieve thin films with a helicoidal arrangement of the nanofibrils. The helicoidal structure of the films is verified by measuring the circular dichroism (CD) of the samples. The sign and position of the structural CD peak show that the handedness and the pitch of the chiral structures can be easily tuned by deliberately selecting simple parameters, such as the number of consecutive cellulose layers sprayed in the same direction, and the angle of rotation between successive stacks of layers. To the authors' knowledge, this approach is unique as it offers the possibility to prepare complex nanocomposite architectures with various nanoscale-controlled sub-structures from different anisometric objects, which is enabling novel designs of composite films with damage-resistant and/or optical filtering functionalities.

TiO2 Films with Macroscopic Chiral Nematic-Like Structure Stabilized by Copper Promoting Light-Harvesting Capability for Hydrogen Generation

Cellulose nanocrystals (CNCs) have inspired the synthesis of various advanced nanomaterials, opening opportunities for different applications. However, a simple and robust approach for transferring the long-range chiral nematic nanostructures into TiO2 photocatalyst is still fancy. Herein, a successful fabrication of freestanding TiO2 films maintaining their macroscopic chiral nematic structures after removing the CNCs biotemplate is reported. It is demonstrated that including copper acetate in the sol avoids the epitaxial growth of the lamellar-like structure of TiO2 and stabilizes the chiral nematic structure instead. The experimental results and optical simulation demonstrate an enhancement at the blue and red edges of the Fabry-Pérot reflectance peak located in the visible range. This enhancement arises from the light scattering effect induced by the formation of the chiral nematic structure. The nanostructured films showed 5.3 times higher performance in the photocatalytic hydrogen generation, compared to lamellar TiO2, and benefited from the presence of copper species for charge carriers' separation. This work is therefore anticipated to provide a simple approach for the design of chiral nematic photocatalysts and also offers insights into the electron transfer mechanisms on TiO2/CuxO with variable oxidation states for photocatalytic hydrogen generation.

Hierarchical Assembly of Patternable Chiroptical Biotextiles with Extreme Environment Stability

Flexible photonic textiles constructed by sustainable cholesteric organization are very promising to achieve a combination of chiroptical structural colors, mechanical robustness, sustainability, and environment stability. However, the efficient assembly of well-ordered cholesteric nanoarchitectures on flexible textiles in a scalable and patternable manner remains a grand challenge. In this study, we develop an efficient and scalable approach to construct large area chiroptical biotextiles using renewable and bioenabled cellulose nanocrystals (CNCs) as building blocks. This hierarchical assembly enables cholesteric photonic CNCs "cast" in situ, in a seamlessly tessellated design, onto topography-tailored textiles to form a strong interlocked multilayered structure. The resulting hierarchical architecture not only comprises strong photonic-photonic coupling to synergistically enhance the chiroptical properties with tunable wavelengths but also leads to impressive mechanical and optical stability against external mechanical forces and extreme environments. More importantly, through regulating the localized photonic band of the preformed chiroptical textiles by small molecules (e.g., water and glucose), customized colored patterns can be easily generated in large scale that are highly responsive to multistimuli, including chiral polarized light, view angle, and solvent. This chiroptical biotextile is a promising next-generation biomimetic photonic material for defense, aviation, and marine and aerospace special applications.

Image-based polarization detection and material recognition

We demonstrate a single-shot and image-based polarization detection system for material recognition. The Stokes parameters are measured under a single-shot measurement using 4 electrically tunable liquid crystal wave plates, 4 polarizers, and 4 camera modules. The optical principle is introduced and the experiments are performed. We also use a metallic plate and a glass substrate to demonstrate the material recognition. The impact of this study is to provide a practical way in image-based polarization detection in Advanced Driver Assistance Systems for material recognition which could help in driving safety.

Using structural colour to track length scale of cell-wall layers in developing Pollia japonica fruits

Helicoidally arranged layers of cellulose microfibrils in plant cell walls can produce strong and vivid coloration in a wide range of species. Despite its significance, the morphogenesis of cell walls, whether reflective or not, is not fully understood. Here we show that by optically monitoring the reflectance of Pollia japonica fruits during development we can directly map structural changes of the cell wall on a scale of tens of nanometres. Visible-light reflectance spectra from individual living cells were measured throughout the fruit maturation process and compared with numerical models. Our analysis reveals that periodic spacing of the helicoidal architecture remains unchanged throughout fruit development, suggesting that interactions in the cell-wall polysaccharides lead to a fixed twisting angle of cellulose helicoids in the cell wall. By contrast with conventional electron microscopy, which requires analysis of different fixed specimens at different stages of development, the noninvasive optical technique we present allowed us to directly monitor live structural changes in biological photonic systems as they develop. This method therefore is applicable to investigations of photonic tissues in other organisms.

3D Printing of Bioinspired Biomaterials for Tissue Regeneration

Biological systems, which possess remarkable functions and excellent properties, are gradually becoming a source of inspiration for the fabrication of advanced tissue regeneration biomaterials due to their hierarchical structures and novel compositions. It would be meaningful to learn and transfer the characteristics of creatures to biomaterials design. However, traditional strategies cannot satisfy the design requirements of the complicated bioinspired materials for tissue regeneration. 3D printing, as a rapidly developing new technology that can accurately achieve multimaterial and multiscale fabrication, is capable of optimizing the fabrication of bioinspired materials with complex composition and structure. This review summarizes the recent developments in 3D-printed bioinspired biomaterials for multiple tissue regeneration, and especially highlights the progresses on i) traditional bioinspired designs for biomaterials fabrication, ii) biological composition inspired designs for the 3D-printed biomaterials, and iii) biological structure inspired designs for the 3D-printed biomaterials. Finally, the challenges and prospects for the development of 3D-printed bioinspired biomaterials are discussed.

Mueller matrix characterizations of circularly polarized reflections from golden scarab beetles

Circularly polarized light (CPL) reflections are rare in nature. Only a few animal groups-most notably certain stomatopod crustaceans and certain beetles in the family Scarabaeidae-are known to reflect CPL from incident unpolarized light. Here, we examine five species of metallic scarabs in the genus Chrysina that, to the naked human eye, look remarkably similar. Using a spectropolarimetric reflectometer to characterize the complete Mueller matrix elements of the beetles' elytral surfaces, we found that four of the five species were strongly left-handed circularly polarized (LHCP), and only one scarab species, Chrysina resplendens, had an overall lower degree of polarization and switched from LHCP to right-handed circularly polarized reflectance depending on wavelength.

Polarization Volume Gratings for Near-Eye Displays and Novel Photonic Devices

Liquid crystal-based reflective polarization volume grating (PVG), also known as a linear Bragg–Berry phase optical element or a member of volume Bragg gratings (VBGs), is a functional planar structure with patterned orientation of optical axis. Due to the strong polarization selectivity, nearly 100% diffraction efficiency, large diffraction angle, and simple fabrication process, PVGs have found potential applications in novel photonic devices and emerging near-eye displays. In this review paper, we describe the operation principles, discuss the optical properties, present the fabrication methods, and provide promising applications of PVGs for near-eye displays and novel photonic devices.

When nanocellulose meets diffraction grating: freestanding photonic paper with programmable optical coupling

Photonic crystals based on plasmonic or dielectric periodic structures have attracted considerable interest owing to their capabilities to control light-matter interactions with tailored precision. By using a nanocellulose derived chiral liquid crystal as a building block, here we demonstrate a bio-inspired dual photonic structure that contains the combination of microscopic periodic 1D surface grating and nanoscopic helical organization, giving rise to programmable colour mixing and polarization rotation. We show that a variation in the photonic band-gap in the bulk matrix leads to simultaneous control over the reflection and diffraction of light with controllable iridescence.

Interpretation of backscattering polarimetric images recorded from multiply scattering systems: a study on colloidal suspensions

Extracting a system's physical features from polarimetric experiments constitutes a challenging task, especially in the presence of multiple scattering. This can be attributed to the difficulty in interpreting the polarimetric measurements. In this study, we demonstrate that polarimetric images recorded in the backscattering geometry can be interpreted by analyzing the spatial variations of the backscattered light's Stokes vectors and using symmetry/geometry arguments. To illustrate the applicability of our method, we examine experimental and simulation data collected by probing colloidal suspensions. We present an analytical model based on the coherency matrix and the geometric phase to describe the polarimetric behavior of the probed samples.

Pitch profile across the cuticle of the scarab beetle Cotinis mutabilis determined by analysis of Mueller matrix measurements

Helicoidal structures of lamellae of nanofibrils constitute the cuticle of some scarab beetles with iridescent metallic-like shine reflecting left-handed polarized light. The spectral and polarization properties of the reflected light depend on the pitch of the helicoidal structures, dispersion of effective refractive indices and thicknesses of layers in the cuticle. By modelling the outer exocuticle of the scarab beetle Cotinis mutabilis as a stack of continuously twisted biaxial slices of transparent materials, we extract optical and structural parameters by nonlinear regression analysis of variable-angle Mueller-matrix spectroscopic data. Inhomogeneities in the beetle cuticle produce depolarization with non-uniformity in cuticle thickness as the dominant effect. The pitch across the cuticle of C. mutabilis decreased with depth in a two-level profile from 380 to 335 nm and from 390 to 361 nm in greenish and reddish specimens, respectively, whereas in a yellowish specimen, the pitch decreased with depth in a three-level profile from 388 to 326 nm.