Bioinspired phase-separated disordered nanostructures for thin photovoltaic absorbers

Infrared absorbers inspired by nature

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 not 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.

The actin cytoskeleton plays multiple roles in structural colour formation in butterfly wing scales

Vivid structural colours in butterflies are caused by photonic nanostructures scattering light. Structural colours evolved for numerous biological signalling functions and have important technological applications. Optically, such structures are well understood, however insight into their development in vivo remains scarce. We show that actin is intimately involved in structural colour formation in butterfly wing scales. Using comparisons between iridescent (structurally coloured) and non-iridescent scales in adult and developing H. sara, we show that iridescent scales have more densely packed actin bundles leading to an increased density of reflective ridges. Super-resolution microscopy across three distantly related butterfly species reveals that actin is repeatedly re-arranged during scale development and crucially when the optical nanostructures are forming. Furthermore, actin perturbation experiments at these later developmental stages resulted in near total loss of structural colour in H. sara. Overall, this shows that actin plays a vital and direct templating role during structural colour formation in butterfly scales, providing ridge patterning mechanisms that are likely universal across lepidoptera.

Mechanically-Durable Antireflective Subwavelength Nanoholes on Glass Surfaces Using Lithography-Free Fabrication

Traditional multilayer antireflection (AR) surfaces are of significant importance for numerous applications, such as laser optics, camera lenses, and eyeglasses. Recently, technological advances in the fabrication of biomimetic AR surfaces capable of delivering broadband omnidirectional high transparency combined with self-cleaning properties have opened an alternative route toward realization of multifunctional surfaces which would be beneficial for touchscreen displays or solar harvesting devices. However, achieving the desired surface properties often requires sophisticated lithography fabrication methods consisting of multiple steps. In the present work, we show the design and implementation of mechanically robust AR surfaces fabricated by a lithography-free process using thermally dewetted silver as an etching mask. Both-sided nanohole (NH) surfaces exhibit transmittance above 99% in the visible or the near-infrared ranges combined with improved angular response at an angle of incidence of up to θi = 60°. Additionally, the NHs demonstrate excellent mechanical resilience against repeated abrasion with cheesecloth due to favorable redistribution of the shearing mechanical forces, making them a viable option for touchscreen display applications.

Topological Learning for the Classification of Disorder: An Application to the Design of Metasurfaces

Structural disorder can improve the optical properties of metasurfaces, whether it is emerging from some large-scale fabrication methods or explicitly designed and built lithographically. For example, correlated disorder, induced by a minimum inter-nanostructure distance or by hyperuniformity properties, is particularly beneficial for light extraction. Inspired by topology, we introduce numerical descriptors to provide quantitative measures of disorder with universal properties, suitable to treat both uncorrelated and correlated disorder at all length scales. The accuracy of these topological descriptors is illustrated both theoretically and experimentally by using them to design plasmonic metasurfaces with controlled disorder that we then correlate to the strength of their surface lattice resonances. These descriptors are an example of topological tools that can be used for the fast and accurate design of disordered structures or as aid in improving their fabrication methods.

Taming Multiscale Structural Complexity in Porous Skeletons: From Open Framework Materials to Micro/Nanoscaffold Architectures

Recent developments in the design and synthesis of more and more sophisticated organic building blocks with controlled structures and physical properties, combined with the emergence of novel assembly modes and nanofabrication methods, make it possible to tailor unprecedented structurally complex porous systems with precise multiscale control over their architectures and functions. By tuning their porosity from the nanoscale to microscale, a wide range of functional materials can be assembled, including open frameworks and micro/nanoscaffold architectures. During the last two decades, significant progress is made on the generation and optimization of advanced porous systems, resulting in high-performance multifunctional scaffold materials and novel device configurations. In this perspective, a critical analysis is provided of the most effective methods for imparting controlled physical and chemical properties to multifunctional porous skeletons. The future research directions that underscore the role of skeleton structures with varying physical dimensions, from molecular-level open frameworks (<10 nm) to supramolecular scaffolds (10-100 nm) and micro/nano scaffolds (>100 nm), are discussed. The limitations, challenges, and opportunities for potential applications of these multifunctional and multidimensional material systems are also evaluated in particular by addressing the greatest challenges that the society has to face.

A Bioinspired Bilevel Metamaterial for Multispectral Manipulation toward Visible, Multi-Wavelength Detection Lasers and Mid-Infrared Selective Radiation

Manipulation of the electromagnetic signature in multiple wavebands is necessary and effective in civil and industrial applications. However, the integration of multispectral requirements, particularly for the bands with comparable wavelengths, challenges the design and fabrication of current compatible metamaterials. Here, a bioinspired bilevel metamaterial is proposed for multispectral manipulation involving visible, multi-wavelength detection lasers and mid-infrared (MIR), along with radiative cooling. The metamaterial, consisting of dual-deck Pt disks and a SiO2 intermediate layer, is inspired by the broadband reflection splitting effect found in butterfly scales and achieves ultralow specular reflectance (average of 0.013) over the entire 0.8-1.6 µm with large scattering angles. Meanwhile, tunable visible reflection and selective dual absorption peaks in MIR can be simultaneously realized, providing structural color, effective radiative thermal dissipation at 5-8 µm and 10.6 µm laser absorption. The metamaterial is fabricated by a low-cost colloidal lithography method combined with two patterning processes. Multispectral manipulation performances are experimentally demonstrated and a significant apparent temperature drop (maximum of 15.7 °C) compared to the reference is observed under a thermal imager. This work achieves optical response in multiple wavebands and provides a valuable way to effectively design multifunctional metamaterials inspired by nature.

Hierarchical morphogenesis of swallowtail butterfly wing scale nanostructures

The study of color patterns in the animal integument is a fundamental question in biology, with many lepidopteran species being exemplary models in this endeavor due to their relative simplicity and elegance. While significant advances have been made in unraveling the cellular and molecular basis of lepidopteran pigmentary coloration, the morphogenesis of wing scale nanostructures involved in structural color production is not well understood. Contemporary research on this topic largely focuses on a few nymphalid model taxa (e.g., Bicyclus, Heliconius), despite an overwhelming diversity in the hierarchical nanostructural organization of lepidopteran wing scales. Here, we present a time-resolved, comparative developmental study of hierarchical scale nanostructures in Parides eurimedes and five other papilionid species. Our results uphold the putative conserved role of F-actin bundles in acting as spacers between developing ridges, as previously documented in several nymphalid species. Interestingly, while ridges are developing in P. eurimedes, plasma membrane manifests irregular mesh-like crossribs characteristic of Papilionidae, which delineate the accretion of cuticle into rows of planar disks in between ridges. Once the ridges have grown, disintegrating F-actin bundles appear to reorganize into a network that supports the invagination of plasma membrane underlying the disks, subsequently forming an extruded honeycomb lattice. Our results uncover a previously undocumented role for F-actin in the morphogenesis of complex wing scale nanostructures, likely specific to Papilionidae.

Extending the diatom's color palette: non-iridescent, disorder-mediated coloration in marine diatom-inspired nanomembranes

The intricate, siliceous exoskeleton of many marine diatoms (single-celled phytoplankton) is decorated with an array of sub-micron, quasi-ordered pores that are known to provide protective and multiple life-sustaining functions. However, the optical functionality of any given diatom valve is limited because valve geometry, composition, and ordering are genetically programmed. Nonetheless, the near- and sub-wavelength features of diatom valves provide inspiration for novel photonic surfaces and devices. Herein, we explore the optical design space for optical transmission, reflection, and scattering in diatom-like structures by computationally deconstructing the diatom frustule, assigning and nondimensionalizing Fano-resonant behavior with configurations of increasing refractive index contrast (Δn), and gauging the effects of structural disorder on the resulting optical response. Translational pore disorder, especially in higher-index materials, was found to evolve Fano resonances from near-unity reflection and transmission to modally confined, angle-independent scattering, which is key to non-iridescent coloration in the visible wavelength range. High-index, frustule-like TiO₂ nanomembranes were then designed to maximize backscattering intensity and fabricated using colloidal lithography. These synthetic diatom surfaces showed saturated, non-iridescent coloration across the visible spectrum. Overall, this diatom-inspired platform could be useful in designing tailored, functional, and nanostructured surfaces for applications in optics, heterogeneous catalysis, sensing, and optoelectronics.

Biological and Bioinspired Thermal Energy Regulation and Utilization

The regulation and utilization of thermal energy is increasingly important in modern society due to the growing demand for heating and cooling in applications ranging from buildings, to cooling high power electronics, and from personal thermal management to the pursuit of renewable thermal energy technologies. Over billions of years of natural selection, biological organisms have evolved unique mechanisms and delicate structures for efficient and intelligent regulation and utilization of thermal energy. These structures also provide inspiration for developing advanced thermal engineering materials and systems with extraordinary performance. In this review, we summarize research progress in biological and bioinspired thermal energy materials and technologies, including thermal regulation through insulation, radiative cooling, evaporative cooling and camouflage, and conversion and utilization of thermal energy from solar thermal radiation and biological bodies for vapor/electricity generation, temperature/infrared sensing, and communication. Emphasis is placed on introducing bioinspired principles, identifying key bioinspired structures, revealing structure-property-function relationships, and discussing promising and implementable bioinspired strategies. We also present perspectives on current challenges and outlook for future research directions. We anticipate that this review will stimulate further in-depth research in biological and bioinspired thermal energy materials and technologies, and help accelerate the growth of this emerging field.

A Novel Ultra-Wideband Electromagnetic-Wave-Absorbing Metastructure Inspired by Bionic Gyroid Structures

Traditional honeycomb-like structural electromagnetic (EM)-wave-absorbing materials have been widely used in various equipment as multifunctional materials. However, current EM-wave-absorbing materials are limited by narrow absorption bandwidths and incidence angles because of their anisotropic structural morphology. The work presented here proposes a novel EM-wave-absorbing metastructure with an isotropic morphology inspired by the gyroid microstructures seen in Parides sesostris butterfly wings. A matching redesign methodology between the material and subwavelength scale properties of the gyroid microstructure is proposed, inspired by the interaction mechanism between the microstructure and the material properties on the EM-wave-absorption performance of the prepared metastructure. The bioinspired metastructure is fabricated by additive manufacturing (AM) and subsequent coating through dipping processes, filled with dielectric lossy materials. Based on simulations and experiments, the metastructure designed in this work exhibits an ultrawide absorption bandwidth covering the frequency range of 2-40 GHz with a fractional bandwidth of 180% at normal incidence. Moreover, the metastructure has a stable frequency response when the incident angle is 60° under transverse electric (TE) and transverse magnetic (TM) polarization. Finally, the synergistic mechanism between the microstructure and the material is elucidated, which provides a new paradigm for the design of novel ultra-broadband EM-absorbing materials.

Diffraction-optimized aperiodic surface structures for enhanced current density in organic solar cells

Novel diffractive surface structures are designed, evaluated, and optimized to achieve a significant increase in current density in organic solar cells. The surface relief phase gratings are patterned based on concentric rings, a Fermat's spiral, and an Archimedean spiral, respectively, and thus diffract incident light independent of its azimuthal angle, as they all exhibit circularly symmetric diffraction patterns. The grating pillars are arranged on the rings or spirals according to periodic or deterministic aperiodic Thue-Morse and Rudin-Shapiro sequences, tailoring the desired diffraction patterns. After evaluating the surface patterns against each other and further parameter optimization, a final current density enhancement of 5 % is achieved for one of the most promising patterns, the Thue-Morse sequence on an Archimedean spiral.

Biomimetic reconstruction of butterfly wing scale nanostructures for radiative cooling and structural coloration

A great number of butterfly species in the warmer climate have evolved to exhibit fascinating optical properties on their wing scales which can both regulate the wing temperature and exhibit structural coloring in order to increase their chances of survival. In particular, the Archaeoprepona demophon dorsal wing demonstrates notable radiative cooling performance and iridescent colors based on the nanostructure of the wing scale that can be characterized by the nanoporous matrix with the periodic nanograting structure on the top matrix surface. Inspired by the natural species, we demonstrate a multifunctional biomimetic film that reconstructs the nanostructure of the Archaeoprepona demophon wing scales to replicate the radiative cooling and structural coloring functionalities. We resorted to the SiO₂ sacrificial template-based solution process to mimic the random porous structure and laser-interference lithography to reproduce the nanograting architecture of the butterfly wing scale. As a result, the biomimetic structure of the nanograted surface on top of the porous film demonstrated desirable heat transfer and optical properties for outstanding radiative cooling performance and iridescent structural coloring. In this regard, the film is capable of inducing the maximum temperature drop of 8.45 °C, and the color gamut of the biomimetic film can cover 91.8% of the standardized color profile (sRGB).

Socially-Directed Development of Materials for Structural Color

Advancing a socially-directed approach to materials research and development is an imperative to address contemporary challenges and mitigate future detrimental environmental and social impacts. This paper reviews, synergizes, and identifies cross-disciplinary opportunities at the intersection of materials science and engineering with humanistic social sciences fields. Such integrated knowledge and methodologies foster a contextual understanding of materials technologies embedded within, and impacting broader societal systems, thus informing decision making upstream and throughout the entire research and development process toward more socially responsible outcomes. Technological advances in the development of structural color, which arises due to the incoherent and coherent scattering of micro-and nanoscale features and possesses a vast design space, are considered in this context. Specific areas of discussion include material culture, narratives, and visual perception, material waste and use, environmental and social life cycle assessment, and stakeholder and community engagement. A case study of the technical and social implications of bio-based cellulose (as a source for structurally colored products) is provided. Socially-directed research and development of materials for structural color hold significant capacity for improved planetary and societal impact across industries such as aerospace, consumer products, displays and sensors, paints and dyes, and food and agriculture.

Over 65% Sunlight Absorption in a 1 μm Si Slab with Hyperuniform Texture

Thin, flexible, and invisible solar cells will be a ubiquitous technology in the near future. Ultrathin crystalline silicon (c-Si) cells capitalize on the success of bulk silicon cells while being lightweight and mechanically flexible, but suffer from poor absorption and efficiency. Here we present a new family of surface texturing, based on correlated disordered hyperuniform patterns, capable of efficiently coupling the incident spectrum into the silicon slab optical modes. We experimentally demonstrate 66.5% solar light absorption in free-standing 1 μm c-Si layers by hyperuniform nanostructuring for the spectral range of 400 to 1050 nm. The absorption equivalent photocurrent derived from our measurements is 26.3 mA/cm², which is far above the highest found in literature for Si of similar thickness. Considering state-of-the-art Si PV technologies, we estimate that the enhanced light trapping can result in a cell efficiency above 15%. The light absorption can potentially be increased up to 33.8 mA/cm² by incorporating a back-reflector and improved antireflection, for which we estimate a photovoltaic efficiency above 21% for 1 μm thick Si cells.

λ/30 inorganic features achieved by multi-photon 3D lithography

It’s critically important to construct arbitrary inorganic features with high resolution. As an inorganic photoresist, hydrogen silsesquioxane (HSQ) has been patterned by irradiation sources with short wavelength, such as EUV and electron beam. However, the fabrication of three- dimensional nanoscale HSQ features utilizing infrared light sources is still challenging. Here, we demonstrate femtosecond laser direct writing (FsLDW) of HSQ through multi-photon absorption process. 26 nm feature size is achieved by using 780 nm fs laser, indicating super-diffraction limit photolithography of λ/30 for HSQ. HSQ microstructures by FsLDW possess nanoscale resolution, smooth surface, and thermal stability up to 600 °C. Furthermore, we perform FsLDW of HSQ to construct structural colour and Fresnel lens with desirable optical properties, thermal and chemical resistance. This study demonstrates that inorganic features can be flexibly achieved by FsLDW of HSQ, which would be prospective for fabricating micro-nano devices requiring nanoscale resolution, thermal and chemical resistance.

Stereolithography has progressed over the years but resolution and feature size is still limited by the properties of materials and resins. Here, the authors demonstrate femtosecond laser direct writing of a hydrogen silsesquioxane photoresist using a 780 nm femtosecond laser demonstrating feature sizes of 26 nm.

Bioinspired Porous Anodic Alumina/Aluminum Flake Powder for Multiband Compatible Low Detectability

Continuous development and advancement in modern detection technologies have increased the demand for multiband (e.g., visual and infrared) compatible camouflage. However, challenges exist in the requirements of incompatible structure resulting from the adaptation to different camouflage effects. This study is inspired by the light absorption structure of butterfly wing scales and demonstrates a porous anodic alumina/aluminum flake powder material prepared by a microscopic powder anodic oxidation technique for visual and infrared camouflage. The fabricated structures manipulate a compromise condition for visual camouflage by low reflectance (R̅_400-800nm = 0.32) and dual-band infrared camouflage by low emission (ε̅_3-5μm = 0.081 and ε̅_8-14μm = 0.085). Further, the characteristic of short-range disorder in these bioinspired structures allows maintenance of the camouflage performance under omnidirectional detection (0-60°). This study provides new insight and a feasible method for coordinated manipulation of electromagnetic waves via bioinspired structural design and improved fabrication.

Forest-like Laser-Induced Graphene Film with Ultrahigh Solar Energy Utilization Efficiency

To achieve high solar energy utilization efficiency, photothermal materials with broadband absorption of sunlight and high conversion efficiency are becoming a fast-growing research focus. Inspired by the forest structure with efficient sunlight utilization, we designed and fabricated a graphene film consisting of densely arranged porous graphene though laser scribing on polybenzoxazine resin (poly(Ph-ddm)). This hierarchical structure significantly reduced the light reflection of graphene as a 2D material. With a combination of advanced photothermal conversion properties of graphene, the 3D structured graphene film, named forest-like laser-induced graphene (forest-like LIG), was endowed with a very high light absorption of 99.0% over the whole wavelength range of sunlight as well as advanced light-to-heat conversion performance (reaching up to 87.7 °C within 30 s under the illumination of simulated sunlight and showing an equilibrium temperature of 90.7 ± 0.4 °C). As a further benefit of its superhydrophobicity, a photothermal actuator with quick actuated response and high motion velocity, as well as a solar-driven interfacial desalination membrane with durable salt-rejecting properties and high solar evaporation efficiencies, was demonstrated.

Finite-difference Time-domain (FDTD) Optical Simulations: A Primer for the Life Sciences and Bio-Inspired Engineering

Light influences most ecosystems on earth, from sun-dappled forests to bioluminescent creatures in the ocean deep. Biologists have long studied nano- and micro-scale organismal adaptations to manipulate light using ever-more sophisticated microscopy, spectroscopy, and other analytical equipment. In combination with experimental tools, simulations of light interacting with objects can help researchers determine the impact of observed structures and explore how variations affect optical function. In particular, the finite-difference time-domain (FDTD) method is widely used throughout the nanophotonics community to efficiently simulate light interacting with a variety of materials and optical devices. More recently, FDTD has been used to characterize optical adaptations in nature, such as camouflage in fish and other organisms, colors in sexually-selected birds and spiders, and photosynthetic efficiency in plants. FDTD is also common in bioengineering, as the design of biologically-inspired engineered structures can be guided and optimized through FDTD simulations. Parameter sweeps are a particularly useful application of FDTD, which allows researchers to explore a range of variables and modifications in natural and synthetic systems (e.g., to investigate the optical effects of changing the sizes, shape, or refractive indices of a structure). Here, we review the use of FDTD simulations in biology and present a brief methods primer tailored for life scientists, with a focus on the commercially available software Lumerical FDTD. We give special attention to whether FDTD is the right tool to use, how experimental techniques are used to acquire and import the structures of interest, and how their optical properties such as refractive index and absorption are obtained. This primer is intended to help researchers understand FDTD, implement the method to model optical effects, and learn about the benefits and limitations of this tool. Altogether, FDTD is well-suited to (i) characterize optical adaptations and (ii) provide mechanistic explanations; by doing so, it helps (iii) make conclusions about evolutionary theory and (iv) inspire new technologies based on natural structures.

Single-step-fabricated disordered metasurfaces for enhanced light extraction from LEDs

While total internal reflection (TIR) lays the foundation for many important applications, foremost fibre optics that revolutionised information technologies, it is undesirable in some other applications such as light-emitting diodes (LEDs), which are a backbone for energy-efficient light sources. In the case of LEDs, TIR prevents photons from escaping the constituent high-index materials. Advances in material science have led to good efficiencies in generating photons from electron-hole pairs, making light extraction the bottleneck of the overall efficiency of LEDs. In recent years, the extraction efficiency has been improved, using nanostructures at the semiconductor/air interface that outcouple trapped photons to the outside continuum. However, the design of geometrical features for light extraction with sizes comparable to or smaller than the optical wavelength always requires sophisticated and time-consuming fabrication, which causes a gap between lab demonstration and industrial-level applications. Inspired by lightning bugs, we propose and realise a disordered metasurface for light extraction throughout the visible spectrum, achieved with single-step fabrication. By applying such a cost-effective light extraction layer, we improve the external quantum efficiency by a factor of 1.65 for commercialised GaN LEDs, demonstrating a substantial potential for global energy-saving and sustainability.

Shaping the Color and Angular Appearance of Plasmonic Metasurfaces with Tailored Disorder

The optical properties of plasmonic nanoparticle ensembles are determined not only by the particle shape and size but also by the nanoantenna arrangement. To investigate the influence of the spatial ordering on the far-field optical properties of nanoparticle ensembles, we introduce a disorder model that encompasses both "frozen-phonon" and correlated disorder. We present experimental as well as computational approaches to gain a better understanding of the impact of disorder. A designated Fourier microscopy setup allows us to record the real- and Fourier-space images of plasmonic metasurfaces as either RGB images or fully wavelength-resolved data sets. Furthermore, by treating the nanoparticles as dipoles, we calculate the electric field based on dipole-dipole interaction, extract the far-field response, and convert it to RGB images. Our results reveal how the different disorder parameters shape the optical far field and thus define the optical appearance of a disordered metasurface and show that the relatively simple dipole approximation is able to reproduce the far-field behavior accurately. These insights can be used for engineering metasurfaces with tailored disorder to produce a desired bidirectional reflectance distribution function.

Phase-Separated Nanophotonic Structures by Inkjet Printing

The spontaneous phase separation of two or more polymers is a thermodynamic process that can take place in both biological and synthetic materials and which results in the structuring of the matter from the micro- to the nanoscale. For photonic applications, it allows forming quasi-periodic or disordered assemblies of light scatterers at high throughput and low cost. The wet process methods currently used to fabricate phase-separated nanostructures (PSNs) limit the design possibilities, which in turn hinders the deployment of PSNs in commercialized products. To tackle this shortcoming, we introduce a versatile and industrially scalable deposition method based on the inkjet printing of a polymer blend, leading to PSNs with a feature size that is tuned from a few micrometers down to sub-100 nm. Consequently, PSNs can be rapidly processed into the desired macroscopic design. We demonstrate that these printed PSNs can improve light management in manifold photonic applications, exemplified here by exploiting them as a light extraction layer and a metasurface for light-emitting devices and point-of-care biosensors, respectively.

Butterfly wing architectures inspire sensor and energy applications

Natural biological systems are constantly developing efficient mechanisms to counter adverse effects of increasing human population and depleting energy resources. Their intelligent mechanisms are characterized by the ability to detect changes in the environment, store and evaluate information, and respond to external stimuli. Bio-inspired replication into man-made functional materials guarantees enhancement of characteristics and performance. Specifically, butterfly architectures have inspired the fabrication of sensor and energy materials by replicating their unique micro/nanostructures, light-trapping mechanisms and selective responses to external stimuli. These bio-inspired sensor and energy materials have shown improved performance in harnessing renewable energy, environmental remediation and health monitoring. Therefore, this review highlights recent progress reported on the classification of butterfly wing scale architectures and explores several bio-inspired sensor and energy applications.

Designable structural coloration by colloidal particle assembly: from nature to artificial manufacturing

Structural color attracts considerable scientific interests and industrial explorations in various fields for the eco-friendly, fade-resistant, and dynamic advantages. After the long-period evolution, nature has achieved the optimized color structures at various length scales, which has inspired people to learn and replicate them to improve the artificial structure color. In this review, we focus on the design of artificial structural colors based on colloidal particle assembly and summarize the functional bioinspired structure colors. We demonstrate the design principles of biomimetic structural colors via the precise structure engineering and typical bottom-up methods. Some main applications are outlined in the following chapter. Finally, we propose the existing challenges and promising prospects. This review is expected to introduce the recent design strategies about the artificial structure colors and provide the insights for its future development.

Bioinspired Microstructured Materials for Optical and Thermal Regulation

Precise optical and thermal regulatory systems are found in nature, specifically in the microstructures on organisms' surfaces. In fact, the interaction between light and matter through these microstructures is of great significance to the evolution and survival of organisms. Furthermore, the optical regulation by these biological microstructures is engineered owing to natural selection. Herein, the role that microstructures play in enhancing optical performance or creating new optical properties in nature is summarized, with a focus on the regulation mechanisms of the solar and infrared spectra emanating from the microstructures and their role in the field of thermal radiation. The causes of the unique optical phenomena are discussed, focusing on prevailing characteristics such as high absorption, high transmission, adjustable reflection, adjustable absorption, and dynamic infrared radiative design. On this basis, the comprehensive control performance of light and heat integrated by this bioinspired microstructure is introduced in detail and a solution strategy for the development of low-energy, environmentally friendly, intelligent thermal control instruments is discussed. In order to develop such an instrument, a microstructural design foundation is provided.

Facile Nanoimprinting of Robust High-Aspect-Ratio Nanostructures for Human Cell Biomechanics

High-aspect-ratio and hierarchically nanostructured surfaces are common in nature. Synthetic variants are of interest for their specific chemical, mechanic, electric, photonic, or biologic properties but are cumbersome in fabrication or suffer from structural collapse. Here, we replicated and directly biofunctionalized robust, large-area, and high-aspect-ratio nanostructures by nanoimprint lithography of an off-stoichiometric thiol-ene-epoxy polymer. We structured-in a single-step process-dense arrays of pillars with a diameter as low as 100 nm and an aspect ratio of 7.2; holes with a diameter of 70 nm and an aspect ratio of >20; and complex hierarchically layered structures, all with minimal collapse and defectivity. We show that the nanopillar arrays alter mechanosensing of human hepatic cells and provide precise spatial control of cell attachment. We speculate that our results can enable the widespread use of high-aspect-ratio nanotopograhy applications in mechanics, optics, and biomedicine.

On the multifunctionality of butterfly scales: a scaling law for the ridges of cover scales

The bright colors found on the wings of some butterflies have been widely examined during recent decades because they are frequently caused by nano-structures and not by pigments or dyes. Sometimes it is puzzling to discover the physical origin of these structural colors because the color-causing nano-structures are integrated into a complex structure of scales that densely covers the butterfly wings. While the color of the wings serves purposes ranging from mating to camouflage and thermoregulation, the overall structure of the scales is commonly believed to assist with aerodynamics, self-cleaning, and easy release from spider webs. This multi-functionality of butterfly scales causes various constraints for their evolutionary design. Here, we present a structural analysis of the height and distance of the ridges in cover scales of butterfly species from different families. The subsequent analysis reveals a linear scaling law. The height of the ridges is always less than half of the distance between them. Finally, we discuss possible reasons for this geometrical scaling law.

Bioinspired Disordered Flexible Metasurfaces for Human Tear Analysis Using Broadband Surface-Enhanced Raman Scattering

Flexible surface-enhanced Raman scattering (SERS) has received attention as a means to move SERS-based broadband biosensing from bench to bedside. However, traditional flexible periodic nano-arrangements with sharp plasmonic resonances or their random counterparts with spatially varying uncontrollable enhancements are not reliable for practical broadband biosensing. Here, we report bioinspired quasi-(dis)ordered nanostructures presenting a broadband yet tunable application-specific SERS enhancement profile. Using simple, scalable biomimetic fabrication, we create a flexible metasurface (flex-MS) of quasi-(dis)ordered metal-insulator-metal (MIM) nanostructures with spectrally variable, yet spatially controlled electromagnetic hotspots. The MIM is designed to simultaneously localize the electromagnetic signal and block background Raman signals from the underlying polymeric substrate-an inherent problem of flexible SERS. We elucidate the effect of quasi-(dis)ordering on broadband tunable SERS enhancement and employ the flex-MS in a practical broadband SERS demonstration to detect human tear uric acid within its physiological concentration range (25-150 μM). The performance of the flex-MS toward noninvasively detecting whole human tear uric acid levels ex vivo is in good agreement with a commercial enzyme-based assay.

Bioinspired 2D Nanomaterials for Sustainable Applications

The increasing demand for constructing ecological civilization and promoting socially sustainable development has encouraged scientists to develop bioinspired materials with required properties and functions. By bringing science and nature together, plenty of novel materials with extraordinary properties can be created by learning the best from natural species. In combination with the exceptional features of 2D nanomaterials, bioinspired 2D nanomaterials and technologies have delivered significant achievements. Here, the progress over the past decade in bioinspired 2D photonic structures, energy nanomaterials, and superwetting materials, is summarized, together with the challenges and opportunities in developing bioinspired materials for sustainable energy and environmental technologies.

Tailored disorder: a self-organized photonic contact for light trapping in silicon-based tandem solar cells

We present a process development leading to efficient rear side light trapping structures with the purpose of enhancing the infrared response of a silicon-based tandem solar cell. To this end, we make use of phase separation effects of two immiscible polymers, polystyrene and poly(methyl methacrylate), resulting in a non-periodic polystyrene structure on silicon with a well-defined size distribution. Onto this pattern, we evaporate silver as a scattering rear side mirror and contact layer. Average feature sizes and periods can be tuned by varying material properties (e.g. molar weights or ratios of the polymers) as well as processing conditions during the spin coating. This way a favorable pseudo period of approx. 1 µm for these disordered structure features was realized and successfully implemented into a silicon solar cell. The structure shows a ring-shaped scattering distribution which is beneficial for light trapping in solar cells. External quantum efficiency measurements show that a gain in short circuit current density of 1.1 mA/cm² compared to a planar reference can be achieved, which is in the same range as we achieved using nanoimprint lithography in a record triple-junction III/V on a silicon device.

UiO-67 metal-organic gel material deposited on photonic crystal matrix for photoelectrocatalytic hydrogen production

Robust UiO-67 metal-organic framework nanoparticles have been precisely and uniformly anchored on the surface of a photonic crystal via metal-organic gelation, resulting in a nanoscale UiO-67 composite. Mott-Schottky measurements indicate that UiO-67/B is an n-type semiconductor with electron conduction, and the band gap significantly decreases with the assistance of the photonic crystal matrix with a band gap of 0.75 eV. Benefiting from the abundant photoelectrons trapped from the photonic crystal, good hydrogen evolution reaction performance is achieved under light irradiation. The current density increases from 3.2 to 7.0 mA cm-2 at -0.6 V (vs. RHE) for UiO-67/B. The optimized carrier density obtained from UiO-67/B is apparently increased 2.15 times under light irradiation for 30 min. This work provides a rational strategy to address the photo-capture and energy transfer issues of metal-organic frameworks under visible light irradiation for H2 production in artificial photosynthesis.

Diverse nanostructures underlie thin ultra-black scales in butterflies

Recently, it has been shown that animals such as jumping spiders, birds, and butterflies have evolved ultra-black coloration comparable to the blackest synthetic materials. Of these, certain papilionid butterflies have reflectances approaching 0.2%, resulting from a polydisperse honeycomb structure. It is unknown if other ultra-black butterflies use this mechanism. Here, we examine a phylogenetically diverse set of butterflies and demonstrate that other butterflies employ simpler nanostructures that achieve ultra-black coloration in scales thinner than synthetic alternatives. Using scanning electron microscopy, we find considerable interspecific variation in the geometry of the holes in the structures, and verify with finite-difference time-domain modeling that expanded trabeculae and ridges, found across ultra-black butterflies, reduce reflectance up to 16-fold. Our results demonstrate that butterflies produce ultra-black by creating a sparse material with high surface area to increase absorption and minimize surface reflection. We hypothesize that butterflies use ultra-black to increase the contrast of color signals.

Nature has developed the ability to produce a wide range of optical effects most notably in the butterfly wing. Here, the authors report on the analysis of the structures responsible for ultra-black coloration across different butterflies and combine this with modelling to identify the key characteristics

Structure and pigment make the eyed elater's eyespots black

Surface structures that trap light leading to near complete structural absorption creates an appearance of "super black." Well known in the natural world from bird feathers and butterfly scales, super black has evolved independently from various anatomical structures. Due to an exceptional ability to reduce specular reflection, these biological materials have garnered interest from optical industries. Here we describe the false eyes of the eyed elater click beetle, which, while not classified as super black, still attains near complete absorption of light partly due to an array of vertically-aligned microtubules. These cone-shaped microtubules are modified hairs (setae) that are localized to eyespots on the dorsum of the beetle, and absorb 96.1% of incident light (at a 24.8° collection angle) in the spectrum between 300-700 nm. Filled with melanin, the setae combine structure and pigment to generate multiple reflections and refractions causing light to travel a greater distance. This light-capturing architecture leaves little light available to receivers and the false eyes appear as deep black making them appear more conspicuous to predators.

Heat transfer properties of Morpho butterfly wings and the dependence of these properties on the wing surface structure

The heat transfer properties of a material strongly rely on its surface structure. The wings of the Morpho butterfly have a unique surface structure with features of order and disorder. In this work, the surface temperature and radiative heat flux of Morpho butterfly wings with structural colour when a ceramic heater attached to the opposite surface of the wings was heated to 250 °C were evaluated in terms of their heat transfer properties. Morpho menelaus butterfly (MM) wings and Cithaerias (CE) wings with no periodic structure on their surface, were used as samples. The MM wings had higher surface temperature and radiative heat flux than the CE wings, which is the first report of heat transfer properties of the wings. The surface structure of the MM wings was changed by heat treatment in order to investigate the effect of the surface structural change on their heat transfer properties. The treatment changed the colour of the wings to red and brown, distorting the periodic structure. The radiative heat flux increased due to the change in the structure on their surface. XPS spectra revealed that the treatment leads to a slight change in the chemical structure of the wings. The spectral analyses results showed there was no obvious change in the mid-infrared absorbance. The heat radiative properties of the MM wings were strongly influenced by the surface structural changes due to the heat treatment.

The heat transfer properties of Morpho butterfly wings (red line) were higher than those of Cithaerias wings (blue line) due to their surface structure and emissivity.

Aluminum Metasurface with Hybrid Multipolar Plasmons for 1000-Fold Broadband Visible Fluorescence Enhancement and Multiplexed Biosensing

Aluminum (Al)-based nanoantennae traditionally suffer from weak plasmonic performance in the visible range, necessitating the application of more expensive noble metal substrates for rapidly expanding biosensing opportunities. We introduce a metasurface comprising Al nanoantennae of nanodisks-in-cavities that generate hybrid multipolar lossless plasmonic modes to strongly enhance local electromagnetic fields and increase the coupled emitter's local density of states throughout the visible regime. This results in highly efficient electromagnetic field confinement in visible wavelengths by these nanoantennae, favoring real-world plasmonic applications of Al over other noble metals. Additionally, we demonstrate spontaneous localization and concentration of target molecules at metasurface hotspots, leading to further improved on-chip detection sensitivity and a broadband fluorescence-enhancement factor above 1000 for visible wavelengths with respect to glass chips commonly used in bioassays. Using the metasurface and a multiplexing technique involving three visible wavelengths, we successfully detected three biomarkers, insulin, vascular endothelial growth factor, and thrombin relevant to diabetes, ocular and cardiovascular diseases, respectively, in a single 10 μL droplet containing only 1 fmol of each biomarker.

Bioinspired Metamaterials: Multibands Electromagnetic Wave Adaptability and Hydrophobic Characteristics

Although various photonic devices inspired by natural materials have been developed, there is no research focusing on multibands adaptability, which is not conducive to the advancement of materials science. Herein, inspired by the moth eye surface model, state-of-the-art hierarchical metamaterials (MMs) used as tunable devices in multispectral electromagnetic-waves (EMWs) frequency range, from microwave to ultraviolet (UV), are designed and prepared. Experimentally, the robust broad bandwidth of microwave absorption greater than 90% (reflection loss (RL) < -10 dB) covering almost entire X and Ku bands (8.04-17.88 GHz) under a deep sub-wavelength thickness (1 mm) is demonstrated. The infrared emissivity is reduced and does not affect the microwave absorption simultaneously, further realizing anti-reflection and camouflage via the strong visible light scattering by the microstructure, and can prevent degradation by reducing the transmittance to less than 10% over the whole near UV band, as well as having hydrophobic abilities. The mechanism explored via simulation model is that topological effects are found in the bio-structure. This discovery points to a pathway for using natural models to overcome physical limits of MMs and has promising prospect in novel photonic materials.

Strategies to prevent dopamine oxidation and related cytotoxicity using various antioxidants and nitrogenation

Dopamine (DA) plays several important roles in the brain and body and has recently been used as a bioadhesive precursor for medical applications. However, DA oxidizes immediately when exposed to oxygen and rapidly polymerizes into polydopamine (PDA), leading to oxidative stress, cytotoxicity, and loss of DA functionalities. As a result, preventing rapid oxidation of DA is of paramount importance but still remains a major challenge. Here, we report several strategies to impede DA oxidation in relevant aqueous solutions (i.e., water, PBS, and cell culture media). One strategy is based on using reducing agents or antioxidants such as glutathione in its reduced state (GSH) and sodium tetraborate (commonly known as borax). Another strategy is based on nitrogenation, a method used to preserve DA in its reduced form by creating an oxygen-free environment. Our data suggest that the antioxidant properties of GSH and borax substantially decreased DA oxidation for up to 2 months. Nitrogenation or oxygen removal further prevented DA oxidation, enhancing its shelf life for longer periods of time. When tested with mammalian cells, preventing DA oxidation with GSH dramatically improved viability of 3T3 fibroblasts and T cells. These results demonstrate that the use of antioxidants, alone or in combination with nitrogenation, can help prevent DA oxidation and improve its stability for cell-based studies or for the design and development of biomaterials.

Effects of Photonic Band Structure and Unit Super-Cell Size in Graded Photonic Super-Crystal on Broadband Light Absorption in Silicon

The newly discovered graded photonic super-crystal (GPSC) with a large size of unit cell can have novel optical properties that have not been explored. The unit super-cell in the GPSC can be designed to be large or small and thus the GPSC can have no photonic band gap or several gaps. The photonic band structures in Si GPSC can help predict the light absorption in Si. Photonic resonance modes help enhance the absorption of light in silicon; however, photonic band gaps decrease the absorption for light with a large incident angle. The Si device patterned in GPSC with a unit super-cell of 6a × 6a (a is a lattice constant in traditional photonic crystal) has a broadband high absorption with strong incident-angular dependence. The device with the unit super-cell of 12a × 12a has relatively low light absorption with weak incident-angle dependence. The Si GPSC with a unit super-cell of 8a × 8a combines both advantages of broadband high absorption and weak dependence of absorption on the incident angle.

Capacity for heat absorption by the wings of the butterfly Tirumala limniace (Cramer)

Butterflies can directly absorb heat from the sun via their wings to facilitate autonomous flight. However, how is the heat absorbed by the butterfly from sunlight stored and transmitted in the wing? The answer to this scientific question remains unclear. The butterfly Tirumala limniace (Cramer) is a typical heat absorption insect, and its wing surface color is only composed of light and dark colors. Thus, in this study, we measured a number of wing traits relevant for heat absorption including the thoracic temperature at different light intensities and wing opening angles, the thoracic temperature of butterflies with only one right fore wing or one right hind wing; In addition, the spectral reflectance of the wing surfaces, the thoracic temperature of butterflies with the scales removed or present in light or dark areas, and the real-time changes in heat absorption by the wing surfaces with temperature were also measured. We found that high intensity light (600–60,000 lx) allowed the butterflies to absorb more heat and 60−90° was the optimal angle for heat absorption. The heat absorption capacity was stronger in the fore wings than the hind wings. Dark areas on the wing surfaces were heat absorption areas. The dark areas in the lower region of the fore wing surface and the inside region of the hind wing surface were heat storage areas. Heat was transferred from the heat storage areas to the wing base through the veins near the heat storage areas of the fore and hind wings.

A versatile micro-reflectivity setup for probing the optical properties of photonic nanostructures

The spatial- and spectral-dependent optical reflectivity measurements are essential to characterize various natural as well as artificial micron-scale photonic nanostructures. However, it is onerous to measure spatially and spectrally resolved reflectivity values from such photonic nanostructures due to their size limitations. Here, we discuss the development of a versatile micro-reflectivity setup with an in situ optical microscope combined with high-resolution actuators to measure the reflectivity from areas as small as 25 × 25 µm². We illustrate the reflectivity measurements from natural as well as artificially prepared ordered and disordered photonic nanostructures. The optical features that are hidden in the conventional reflectivity measurements are clearly resolved using the micro-reflectivity measurements. The proposed setup is also capable of measuring the polarization-dependent reflectivity and transmission of light.

Insect-inspired vision for autonomous vehicles

Flying insects are being studied these days as if they were agile micro air vehicles fitted with smart sensors, requiring very few brain resources. The findings obtained on these natural fliers have proved to be extremely valuable when it comes to designing compact low-weight artificial optical sensors capable of performing visual processing tasks robustly under various environmental conditions (light, clouds, contrast). Here, we review some outstanding bio-inspired visual sensors, which can be used for either detecting motion in the visible spectrum or controlling celestial navigation in the ultraviolet spectrum and for attitude stabilisation purposes. Biologically inspired visual sensors do not have to comprise a very large number of pixels: they are able to perform both short and long range navigation tasks surprisingly well with just a few pixels and a weak resolution.

Comparative study on nanostructured order-disorder in the wing eyespots of the giant owl butterfly, Caligo memnon

A characteristic feature of the giant owl butterfly, i.e. Caligo memnon, is its big wing eyespot. This feature could serve as deceiving functionality for the butterfly against predators. As evidenced by scanning electron microscope (SEM) image on black part of eyespot, the scales on wing eyespot contain nanostructured ridges and cross-ribs. Applying direct measurement, statistical method, and Fourier analysis, the authors evidence that these nanostructures display order-disorder in their shape and position. The autocorrelation of SEM image provides average values of characteristic periods of the order-disorder nanostructures together with an estimation of corresponding correlation lengths. Linecuts obtained from the Fourier transform of SEM image were also analysed with the Hosemann function to extract similar information. These analyses indicate that the nanostructured order-disorder may contribute to blackness on wing eyespot. The authors thus conclude that the blackness on wing eyespot of C. memnon could be attributed to contributions from both the nanostructured order-disorder and melanin pigment.

Multifunctional biophotonic nanostructures inspired by the longtail glasswing butterfly for medical devices

Numerous living organisms possess biophotonic nanostructures that provide colouration and other diverse functions for survival. While such structures have been actively studied and replicated in the laboratory, it remains unclear whether they can be used for biomedical applications. Here, we show a transparent photonic nanostructure inspired by the longtail glasswing butterfly (Chorinea faunus) and demonstrate its use in intraocular pressure (IOP) sensors in vivo. We exploit the phase separation between two immiscible polymers (poly(methyl methacrylate) and polystyrene) to form nanostructured features on top of a Si3N4 substrate. The membrane thus formed shows good angle-independent white-light transmission, strong hydrophilicity and anti-biofouling properties, which prevent adhesion of proteins, bacteria and eukaryotic cells. We then developed a microscale implantable IOP sensor using our photonic membrane as an optomechanical sensing element. Finally, we performed in vivo testing on New Zealand white rabbits, which showed that our device reduces the mean IOP measurement variation compared with conventional rebound tonometry without signs of inflammation.

Living Light 2018: Conference Report

Living Light is a biennial conference focused on all aspects of light–matter interaction in biological organisms with a broad, interdisciplinary outlook. The 2018 edition was held at the Møller Centre in Cambridge, UK, from April 11th to April 14th, 2018. Living Light’s main goal is to bring together researchers from different backgrounds (e.g., biologists, physicists and engineers) in order to discuss the current state of the field and sparkle new collaborations and new interdisciplinary projects. With over 90 national and international attendees, the 2018 edition of the conference was strongly multidisciplinary: oral and poster presentations encompassed a wide range of topics ranging from the evolution and development of structural colors in living organisms and their genetic manipulation to the study of fossil photonic structures.

Light trapping in thin film silicon solar cells via phase separated disordered nanopillars

In this work, we have improved the absorption properties of thin film solar cells by introducing light trapping reflectors deposited onto self-assembled nanostructures. The latter consist of a disordered array of nanopillars and are fabricated by polymer blend lithography. Their broadband light scattering properties are exploited to enhance the photocurrent density of thin film devices, here based on hydrogenated amorphous silicon active layers. We demonstrate that these light scattering nanopillars yield a short-circuit current density increase of +33%rel with respect to equivalent solar cells processed on a planar reflector. Moreover, we experimentally show that they outperform randomly textured substrates that are commonly used for achieving efficient light trapping. Complementary optical simulations are conducted on an accurate 3D model to analyze the superior light harvesting properties of the nanopillar array and to derive general design rules. Our approach allows one to easily tune the morphology of the self-assembled nanostructures, is up-scalable and operated at room temperature, and is applicable to other photovoltaic technologies.