Bio-inspired Optics and Photonics – From Metamaterials to Applications

The most brilliant colours in nature are obtained by structuring transparent materials on the scale of the wavelength of visible light. By controlling/designing the dimensions of such nanostructures, it is possible to achieve extremely intense colourations over the entire visible spectrum without using pigments or colorants. Colour obtained through structure, namely structural colour, is widespread in the animal and plant kingdom [1]. Such natural photonic nanostructures are generally synthesised in ambient conditions using a limited range of biopolymers. Given these limitations, an amazing range of optical structures exists: from very ordered photonic structures [2], to partially disordered [3], to completely random ones [4].
In this seminar, I will introduce some striking example of natural photonic structures [2-4] and review our recent advances to fabricate bio-mimetic photonic structures using the same material as nature. Biomimetic with cellulose-based architectures enables us to fabricate novel photonic structures using low cost materials in ambient conditions [6-7]. Importantly, it also allows us to understand the biological processes at work during the growth of these structures in plants.

References [1] Kinoshita, S. et al. (2008). Physics of structural colors. Rep. Prog. Phys. 71(7), 076401.
[2] Vignolini, S. et al. (2012). Pointillist structural color in Pollia fruit. PNAS 109, 15712-15716.
[3] Moyroud, E. et al. (2017). Disorder in convergent floral nanostructures enhances signalling to bees. Nature 550, 469.
[4] Burresi M. et al. (2014) Bright-White Beetle Scales Optimise Multiple Scattering of Light. Sci. Rep.  4,  727
[5] Parker R. et al. (2018) The Self-Assembly of Cellulose Nanocrystals: Hierarchical Design of Visual Appearance. Adv Mat 30, 1704477
[6] Parker R. et al. (2016). Hierarchical Self-Assembly of Cellulose Nanocrystals in a Confined Geometry. ACS Nano, 10 (9), 8443–8449
[7] Liang H-L. et al. (2018). Roll-to-roll fabrication of touch-responsive cellulose photonic laminates, Nat Com 9, 4632

Controlling light through photonic nanostructures are important for everyday optical components, from spectrometers to data storage and readout. In nature, nanostructured materials produce wavelength-dependent colors that are key for visual communication across animals. Here, we investigate two Australian peacock spiders (Maratus robinsoni and M. nigromaculatus) which court females in complex dances using colors reflected from nanostructured grating on their wing scales resulting in either strongly angle-dependent colors or stable coloration. Using FIB-SEM, optical microscopy and optical modelling, we here show that the spiders’ scales have nanogratings on structured 3D surfaces. The difference in angle-dependency originates in the difference in macrostructure of the scales and the grating periods. Our results shed light on the design principle of the peacock spiders’ scales and could inspire novel dispersive components, e.g. used in spectroscopic applications.

Photonic structures occurring in natural organisms such as beetles' elytra are known to display striking colours due to light interference. These ordered porous structures are composed of proteins, lipids and biopolymers such as chitin. In beetles, they are usually found in the cuticles, i.e. the insects’ outer coverings, or in scales, encased by a protecting envelope. In addition, some fluorophores can be embedded within these organisms’ photonic structures. Such a confinement of fluorophores gives rise to modification of the fluorescence emission of light in terms of intensity, decay time and spatial distribution. Interestingly, the colour and fluorescence emission of some beetles were found to change upon contact with liquids and vapours. These optical effects induced by different fluids were quantified by spectrometric measurements. Based on histologic observations, optical simulations were performed in order to reproduce the measured colour change. Depending on the type of fluid (i.e., liquid or vapour), the colour variations were explained by investigating different mechanisms including vapour adsorption, liquid penetration into the photonic structures, filling of the structure pores and swelling of the cuticle material, with respect to the physico-chemical properties of the beetles’ cuticle materials (i.e., porosity, permeability and salt concentration). These remarkable optical effects observed for natural porous photonic structures in beetles are of interest for the development of novel vapour sensing applications and smart glass windows, through a bioinspiration approach.

Luke T. McDonald¹, Maria E. McNamara¹, Vinod Kumar Saranathan², Mariana Verezhak³, Mirko Holler³, Elisabeth Müller⁴ and Pete Vukusic⁵

¹School of Biological, Earth and Environmental Science, University College Cork, Cork, Ireland
²Yale-NUS College, NUS Nanoscience and Nanotechnology Institute, Dept. of Biological Sciences, National University of Singapore, Singapore
³Swiss Light Source, Paul Scherrer Institut, CH-5232 Villigen, Switzerland
⁴Paul Scherrer Institut, CH-5232 Villigen, Switzerland
⁵Dept. of Physics and Astronomy, University of Exeter, Exeter, UK

luke.mcdonald@ucc.ie

Light-scattering photonic nanostructures are abundant in extant insects, underpinning structural colours that play instrumental roles in inter- and intraspecific visual communication strategies. Weevils and longhorn beetles can exhibit spectacular optical effects that are produced by 3D photonic nanostructures within the scale envelope. The evolutionary history of these tissue architectures is, however, poorly resolved; due to the paucity of preserved scales, 3D photonic nanostructures have been reported in only a single fossil weevil. Here we report the discovery of 3D photonic nanostructures in diverse fossil beetles from the Pleistocene of North America and Switzerland. We used small-angle X-ray scattering (SAXS) in tandem with electron microscopy and ptychographic X-ray computed tomography (PXCT) to characterize the ultrastructure of the fossil scales, revealing the preservation of 3D nanostructures that range from highly ordered to amorphous. Further, the more highly ordered nanostructures correspond to a single diamond crystallographic structure, with predicted reflections spanning the entirety of the visible wavelength range. These data will serve as a platform to deliver novel insights into the evolution, development and functionality of scales in beetles.

The Swallow Tanager is a common and sexually dichromatic passerine living in the canopy of the Neotropical forests. Males and females are blue and green, respectively. Male’s plumage reflectance spectra change significantly depending on the relation between the angles of incident light and observation, irrespective of the orientation of the feathers, turning from blue to turquoise-green. We investigated the physical basis of this phenomenon using the Korringa-Kohn-Rostoker (KKR) method to simulate the electromagnetic response of the quasi-ordered nanostructure present in the feather barbs, under different angles of incidence and observation. We measured the size of the air spheres present in the barb’s medulla and its variation using TEM images and employed this information to feed our analysis. In this way we obtained an electromagnetic response strongly dependent on the relation between the angle of incidence and observation that matches the variation observed in the plumage of male Swallow Tanagers. We discuss the functional implications of these changes upon the degree of sexual dichromatism perceived by the birds as well as on the contrast of plumage color against a forest background.

The study of structural colour in brightly coloured animals is an exciting interdisciplinary area of research. Complex photonic bandgap (PBG) structures that occur naturally cross a broad range of animals and plants, suggest broad innovation both in nature’s use of materials and in its manipulation of light and colour. In certain butterflies for instance, ultra-long-range visibility of up to one half-mile is attributed to photonic structures that are formed by discrete multilayers of cuticle and air. This contrasts, in other butterfly species, to photonic structures designed more for crypsis and which not only produce strong polarisation effects but can also create additive colour mixing using highly adapted periodicity. Optical systems also exist that employ remarkable 2D and 3D photonic crystals of cuticle to produce partial PBGs, with the effect that bright colour is reflected, or fluorescence emission is inhibited, over specific angle ranges. From the perspective of modern optical technology, these structures arguably indicate a significant functional advance, since in principle, such 2D and 3D periodicities are potentially able to manipulate the flow of light more completely. This presentation will offer an overview, for a more general audience, of this emerging field of study, as well as describing several of the exciting recent discoveries that reflect nature’s optical design ingenuity, and some technological applications to which they are currently being applied.

Located at the interface of art and science, and drawing on relevant findings from material science, optical physics and evolutionary biology, this paper argues that the scientific field of biomimetics has the potential to lead to, and enable, ‘smarter’ art. Tracing the origin of biomimetics back to the ancient concept of mimesis (defined by Aristotle as ‘imitation of nature’ both via form and material), the emphasis is on latest bio-mimetic colour-technology and its potential via novel colour-shift to introduce ‘the dynamic’ into painting - historically a decidedly static medium. Until now, iridescent hues such as those found on the wings of certain butterflies have never been encountered in the art world. However, here the author illustrates how, facilitated by concerted scientific study of nature’s millennia-old colour optics, she arrives at vital clues on how to adapt and adopt these challenging new nano-materials for painting. And indeed, the resulting artwork, like iridescent creatures, fluctuates in perceived colour and pattern, depending on the light and vantage – thereby extending the canon of art by adding new colour spectra, novel visual realms and modes of expression. Tracing the author’s unique biomimetic approach, this visual, vividly illustrated, account affords an insight into the new colour technology’s evolution. Together with innovative artistic possibilities, the paper advocates that – both for scientists and artists – there remains much to be garnered from nature’s ingenuity.

Colour is a widely used communication channel in the living world for a variety of functions ranging from sexual communication to warning colours. A particularly rich spectrum of colours appears on the wings of many butterflies. The males of polyommatine butterflies often exhibit a conspicuous blue coloration generated by photonic nanoarchitectures on their dorsal wing surfaces. These structural colours were shown to be used for sexual communication, therefore they are species-specific with a well-defined wavelength range and also stable for the strong environmental (cold) stress. We investigated the spatio-temporal variations of this coloration for Polyommatus icarus butterflies using optical spectroscopy considering an interval of more than 100 years and over a geographical range spanning through Europe (west) and Asia (east). The blue coloration in Hungary was found to be very stable both within a year (three broods are typical there) and within the period of 100 years (more than 300 generations). We investigated the east-west geographic variation among more than 300 male P. icarus butterflies. In agreement with earlier genetic and morphometric studies, it was found that the western males are not divided in distinct lineages. In contrast, characteristic differences in coloration were found between the eastern and western groups, with a transition in the region of Turkey. These differences are tentatively attributed to bottleneck effects during past glaciations.

Hummingbird plumage has an exceptional beauty and iss made up of extremely shiny feathers. The iridescence of hummingbirds is presumably the most eye-catching example of structural colouration of birds. The structural colouration exists in the feather barbules, which contain stacks of melanosomes, composed of keratin, melanin and air. The optics of the melanosome stacks has been approximated with that of an ideal multilayer reflector by Greenewalt and coworkers [1], using simplified data for the material components. However, only recently detailed knowledge has been obtained about the refractive indices of keratin and melanin [2,3]. Using the latter data and adequate filling factors for the three composing materials, we have been able to construct more accurate models of the structural colouration of hummingbirds. We could well reproduce experimental reflectance spectra of several species, for example the male Anna’s hummingbird (Calypte anna) [4] and the blue-throated starfrontlet (Coeligena helianthea) [5]. We furthermore studied the genus Coeligena, which comprises 12 species with feather colours throughout the whole visible spectrum. Taking this genus as a model group, we applied spectrophotometrical analysis, morphology and computational modelling and related the feather micro- and nano-structure with the optical properties and the phylogeny.

References
[1] Greenewalt C, Brandt W, Friel D (1960). J Opt Soc Am A 50, 1005–1013
[2] Leertouwer HL, Wilts BD, Stavenga DG (2011) Opt Express 19, 24061–24066
[3] Stavenga DG, Leertouwer HL, Osorio DC, Wilts BD (2015) Light Sci Appl 4, e243
[4] Giraldo MA, Parra JL, Stavenga DG (2018) J Comp Physiol A 204, 965-975
[5] Sosa J, Parra JL, Stavenga DG, Giraldo MA (2019) In prep.

Authors: Barreira Ana S., Bianchimano Luciana, Nuñez-Bustos Ezequiel O. & Pablo L. Tubaro

The Gulf Fritillary Butterfly Agraulis vanillae and its closely related species of the genus Dione have a wide geographic distribution, from southern United States to the Pampas region in Argentina, inhabiting a large range of different environments. These species possess highly reflective silvery structures on the ventral side of their wings that are usually exposed when these butterflies flap their wings while perched. The biological function that these structures fulfill, if they meet any, has not been described yet. These structures could play a role in visual signaling (i.e. mate choice, and/or species recognition) or as an anti-predatory aposematic signal. Another possibility is that these silvery spots intervene in the regulation of body temperature of these butterflies. Surfaces with high percentage of broadband reflectance in butterflies allow reducing the radiation absorbed and therefore could contribute to reduce overheating when the individuals pose in direct sunlight. This effect is more likely to have an important role in species or populations inhabiting more open environments with higher exposure to sunlight. We explore the hypothesis that silvery spots in these species cover a higher percentage of the wings in those species (or populations) that inhabit more open habitats, and vice-versa. For this, we will use spatial color analysis on photographs of museum specimens of Agraulis vanillae and Dione spp. and investigate the patterns of variation in these highly reflective structures among them, particularly in relation to the environmental structure of their habitats.

Lepidopterans grow wing scales with precise structures that confer coloration and other benefits to the animal. These types of structures have been both the inspiration and template for creating new optical materials. However, knowledge about the biological formation of wing scale structures is limited due to challenges of imaging live insect tissue with sufficiently high spatial and temporal resolution.

Here we present novel imaging strategies to capture the in vivo development of scale-forming cells. Implementing advanced techniques such as orientation-independent differential interference contrast microscopy and dynamic speckle illumination wide-field reflection phase microscopy, we are able to follow the growth of scale cells and quantify changes to features such as the ridges and the scale ground plan. This data allows us to determine morphologically critical time points, which has implications regarding the mechanics driving the structural changes.

We believe that in vivo imaging of biological optical materials with high spatial and temporal resolution will lead to new understandings of processes in biological and synthetic material production.

Integrated optical sensors have the potential to empower a suite of novel and compact medical devices including implants, wearables and biosensors. However, in practice, the ease of remote optical readout of these devices is limited by their angle dependent optical behavior. In laboratory settings, precise alignment can be achieved due to having trained users and controlled conditions. However, in the real world for point-of-care applications, the measurement of a device by a potentially untrained user without specialized equipment in an optically unpredictable environment prevents optimal optical alignment. Inspiration for the design of optical devices can be drawn from nature, which has demonstrated a variety of exotic optical phenomena including wide angle optical resonance. Exploiting the interplay of order and disorder in biophotonic nanostructures, we develop nanophotonics-based ocular implants for real-time monitoring of intraocular pressure (IOP) for glaucoma management. The highly miniaturized nanophotonic sensor implants are made of bio-inspired nanostructures (BINS) that behave as a pressure-sensitive optical resonator and delivers IOP readings when interrogated with near-infrared light. Such BINS that combine periodic and amorphous morphology with almost no defects in the millimeter-scale result in a wide-angle resonant reflection ensuring an easy and accurate handheld read-out. Finally, I will demonstrate ongoing in vivo IOP monitoring of our sensors in New Zealand rabbit as well as the in vivo biophysical studies of the nanostructured sensor.

The male peafowl prides himself with a magnificent array of feathers exhibiting a wide variety of structural colours. These originate from a rectangular 2-D photonic structure that consists of melanosomes (solid melanin rodlets) and air channels embedded in keratin [1-3]. We studied the reflectance characteristics of the tail feathers by applying scatterometry, bifurcated-probe- and micro-spectrophotometry. We compared the experimental results with calculated spectra based on published anatomical data. For this we used an effective-medium multilayer model that includes the number and the diameter of the melanosome rodlets, the air channels, the lattice spacing and the keratin cortex thickness. We find that slight variations in the parameter values caused substantial changes in amplitude, spectral position and shape of the reflectance bands and that the latter is especially sensitive to the first layers near the surface [4].

References 1. H. Durrer, "Schillerfarben beim Pfau (Pavo cristatus L.)," Verhandl. der Naturforsch. Ges. Basel 73, 204–224 (1962).
2. J. Zi, X. Yu, Y. Li, X. Hu, C. Xu, X. Wang, X. Liu, and R. Fu, "Coloration strategies in peacock feathers," Proc. Natl. Acad. Sci. 100, 12576–12578 (2003).
3. P. Freyer, B. D. Wilts, and D. G. Stavenga, "Reflections on iridescent neck and breast feathers of the peacock, Pavo cristatus" J. R. Soc. Interface Focus 9, 20180043 (2018).
4. P. Freyer, D. G. Stavenga, “Colour diversity of peacock tail feathers is caused by barbule surface characteristics,” (2019, in preparation).

Frictional effects represent a major constraint on animal locomotion. An animal has to regulate the effects of friction in order to optimize the effort invested in locomotion. One form of terrestrial locomotion dominated by friction is that of snakes. Friction plays a major role in balancing the motion of the snake. One aspect that is never started in literature is the effect of friction-induced heating on the structural integrity of snakeskin. As the snake moves part of the heat generated due to friction will penetrate the ventral scales. For structural integrity of the ventral skin the amount of heat going into the skin has to be relatively minimized (to avoid scoring of the skin). In this work we study the problem of friction heating of a moving snake. Thus, we first apply thermal scan microscopy to determine the thermal conductivity on several locations of the skin of a P.regius. The spots were present locations of various coloration on both the dorsal and the ventral side of the snake. Following measurements, we apply a heat partition model to study heat sharing of friction induced heat among the snake ventral side and the substrate soil. The results show that in a P.regius the ventral skin has the lowest value of thermal conductivity. The slow value helps in minimizing the thermal load acting on the contact spot between the snake and the soil. This prediction is verified through computing the partition factor for the sliding of ventral skin on several different types of soils including sand and clay.

Serge Berthier ¹, Bernd Schöllhorn ², Willy de Marcillac ¹, Nguyen Thi Phuong Lien ³, Camille Aracheloff ¹

(1) Institut des NanoSciences de Paris, Sorbonne Université
(2) Laboratoire d’Electrochimie Moléculaire, Université de Paris
(3) Institute of Ecology and Biological Resources, Vietnam Academy of Science and Technology

Natural fluorescence has long been considered marginal in animal staining. Ether because quantum yields are generally very low and do not change the appearance of the organism; or because many of these organisms are nocturnal and therefore not very subject to energy radiation (UV, Violet ...). The emergence of new sources of UV lighting (LED) has changed this impression.
An extremely bright luminescence in the visible range was observed for the cocoon caps of Vietnamese Polistes species (paper wasps) upon irradiation by UVA light. The intensity of the emitted greenish-yellow light was found to be exceptionally high and Stokes shifts of up to 160 nm were measured. Transmission spectra were registered in order to estimate the effect of the fluorescence at the inside of the cocoons. These interesting fluorescence properties are attributed to fluorophores incorporated in the silk proteins of the cocoon caps fabricated by the larvae prior to its metamorphosis.
In a second part, we will briefly compare the results with the better known, but weaker fluorescence of the silk of Bombyx mori (Lepidopteran, Bombicidae) as well as various tissues, ancient or contemporary made of crude or treated natural silk.

Organisms construct broadband reflectors, tunable photonic crystals, light scatterers and mirrors from assemblies of organic crystals, which have diverse optical functions including the formation of structural colors and light manipulation in vision. The crystal structure, polymorph, size, morphology and superstructural arrangement of the component molecules that determine the optical properties, are controlled from the nanoscale to the millimeter level. The structural features endow the crystals with unusually high refractive indexes for light impinging along specific crystal directions. The controlled assembly of several thin crystals that are exposed to light in those directions creates multilayer reflectors. Guanine crystal multilayer reflectors generate the metallic silvery reflectance of fish scales, the brilliant iridescent colors of some copepods, and the mirrors used for vision in several animal eyes (1). Scallops have tens of eyes, each containing a concave multi-layered mirror perfectly tiled with a mosaic of square guanine crystals, reflecting the light to form images onto the overlying retinas (2, 3). Decapod crustaceans possess reflecting superimposition compound eyes that contain two sets of reflective units, composed of isoxanthopterine crystals. The two reflectors have very different ultrastructures and functions that we can rationalize in terms of the optical performance of the eye (2, 4). Light scattering spherical nanoparticles in the tapetum, a reflective layer behind the retina, are hollow spherical shells, with an outer diameter of ≃300 nm and thickness of 70 nm. The arrangement of the crystalline isoxanthopterin segments in the shell is such that the optic axes of the crystals point radially outward. Using Mie theory we have performed calculations of the scattering amplitudes at optical wavelengths (5). The calculations show that the back-scattering efficiency is significantly higher for the case of the hollow shells with spherulitic birefringence than for the case of amorphous particles, having a single effective refractive index.

References
1) D Gur, BA Palmer, S Weiner, L Addadi, Adv Funct Mater, 1603514 (2017)
2) BA Palmer, D Gur, S Weiner, L Addadi D Oron, Adv. Mater. 2018, 1800006 (2018)
3) BA Palmer, GJ Taylor, V Brumfeld, D Gur, M Shemesh, N Elad, A Osherov, D Oron, S Weiner, L Addadi, Science, 358, 1172–1175 (2017).
4) BA Palmer, A Hirsch, V Brumfeld, ED Aflalo, I Pinkas, A Sagi, S Rozenne, D Oron, L Leiserowitz, L Kronik, S Weiner, L Addadi PNAS, 115, 10, 2299–2304 (2018)
5) Benjamin A. Palmer, Venkata-Jayasurya Yallapragada, Nathan Schiffmann, Eyal Merary Wormser, Nadav Elad, Eliyahu D. Aflalo, Amir Sagi, Steve Weiner, Dan Oron, Lia Addadi, in preparation.

The molecular mechanism for light harvesting in photosynthesis (PS), arguably the most important photo-chemical process for life, is well understood. Surprisingly though the photonic aspects of this process have been largely overlook. In fact, most of the photosynthetic organism known where photonic structures play a key role in the light harvesting process have only been recently described [1][2]. This open many interesting questions. How does micro and nano-structuration at single cell level affect light harvesting in photosynthetic organism?. Has the inner nano-structure of chloroplast (the PS factory in cells) developed non-trivial photonic properties as suggested by recent theoretical approaches [3]?. This are interesting fundamental question but could also be a source of inspiration for new strategies on artificial light energy harvesting. On this paper I will describe three illustrative examples of photonic natural systems that we are currently investigating in our laboratory: lens shaped cells in moss, intracellular photonic crystals in algae and multi-layer arrangements in single chloroplast. Besides I will introduce a new microscopy implementation that allows the measurements of the micro-scattering and photosynthetic activity simultaneously and on a single cell.

References
[1] A. L. Holt, et at, "Photosymbiotic giant clams are transformers of solar flux," J. R. Soc. Interface 11(101), 20140678 (2014).
[2] M. Lopez-Garcia et al., "Light induced dynamic structural color by intracellular 3D photonic crystals in brown algae," Sci. Adv. 4, eaan8917 (2018).
[3] A. Capretti et al., "Nanophotonics of higher-plant photosynthetic membranes," Light Sci. Appl. (2019).

Fiona Sander¹, Dominic Melvin¹, Stefano Angioletti-Umberti¹, and Ahu Gumrah Dumanli¹,²
1 Department of Materials, Imperial College, London, Exhibition Road, SW7 2AZ, London, UK
2 School of Materials, University of Manchester, Oxford Road, Manchester M13 9PL, UK

Chitin structures can be found in a variety of organisms in nature and show unique mechanical and optical properties in combination with biodegradability and nontoxicity. One of those species is Penaeus setiferus (King Prawn) with an exoskeleton made of chitin which is conformed in to a bouligand/helicoidal structure that is a photonic crystal. Penaeus setiferus uses this structure mainly to form a strong network as a part of exoskeleton and the optical appearance of the shrimp seems to do not reveal the interesting chiral optics of such organisation. However, when the proteins and calcium minerals were removed from the shrimp exoskeleton, the residual chitin film show quite remarkable iridescent appearance with strong circular polarization. This isolated helicoidal framework is much more porous than the starting material and provides an exciting foundation as a template for exploring chiral photonics using different deposition methods. With the goal of mimicking this intricate network, the iridescent chitin shell is used in this study to decorate with metal nanoparticles using wet chemical methods and in a separate study conformally coated with TiO2 using atomic layer deposition techniques. It is shown that the helicoidal organization of the iridescent shrimp shells can be conformed and transferred in to higher refractive index materials with distinctive macroscopic combination of optical effects.

Demospongiae and Hexactinellida are two classes of marine sponges that fabricate skeletal elements, the glass spicules, made of amorphous silica. Astonishingly, despite the amorphous nature of the mineral phase, these spicules exhibit a large variety of highly regular three-dimensional branched morphologies that are a paradigm example of symmetry in biological systems. During spicule formation, the process of silica deposition is templated and catalyzed by a scaffold made of organic fibers (axial filaments). Our work shows that these organic filaments are in fact perfect single crystals made of enzymatically active proteins, termed silicateins, surrounded by a glass shell and that these slender hybrid crystals are formed with the assistance of a screw dislocation (via the Burton-Frank-Cabrera mechanism). Moreover, by combining X-ray diffraction and tomography data we demonstrate that crystallographic branching of the protein crystals is responsible for the highly regular morphology of these naturally occurring glass structures. The study reveals a unique biomineralization concept that only recently has been used by man-kind to produce novel technologically relevant nanomaterials.

Cephalopods, such as squid, octopuses, and cuttlefish, have captivated the imagination of both the general public and scientists for more than a century due to their visually stunning camouflage displays, sophisticated nervous systems, and complex behavioral patterns. Given their unique capabilities and characteristics, it is not surprising that these marine invertebrates have recently emerged as exciting models for novel materials and systems. Within this context, our laboratory has developed various cephalopod-derived and cephalopod-inspired materials with unique functionalities. Our findings hold implications for next-generation adaptive camouflage devices, sensitive bioelectronic platforms, and advanced renewable energy technologies.

Nature has a plethora of examples of structurally colored materials that change color either because they are on or within a dynamic structure (e.g. the wing of a butterfly), or because they reversibly respond to their environment (e.g. water infiltration of the porous cuticle of several beetle species). In this talk, I will present several systems recently developed in the lab that either (1) exhibit dynamic color changes in response to a specific stimulus or set of stimuli or (2) are encoded with light-responsive moieties such that they exhibit novel, remote-controllable deformations in response to light. In analogy to their natural counterparts, synthetic structures can be designed to exhibit dynamic color changes as a result of mechanical deformations or infiltration by liquids or gases. Such color changes can be aesthetic, employed to modulate the response to the environment, or can be utilized as a dynamic sensor for a variety of environmental stimuli and analytes. Likewise, our group has focused on emulating nature's versatility, multifunctionality, and multimodal behavior. By doping materials with light-responsive moieties and coupling light absorption to some other form of energy transduction (e.g. mechanical deformation), it is possible to achieve a synthetic system capable of feedback and self-regulatory behavior, important for soft robotics, microfluidic platforms, solar harvesting, and all-optical computing, to name a few.

I will discuss recent advances and efforts to obtain a deeper understanding of the physical parameters that control the transport properties of amorphous photonic materials in 2D and 3D. Our results show that, depending on the frequency of incident radiation, a disordered, but highly correlated, dielectric material can transition from photon diffusion to Anderson localization and a bandgap. In two dimensions, we can also identify a regime, near the gap, dominated by tunneling between weakly coupled states. Our results demonstrate that the observation of exponential decays of the total transmitted intensity of light alone, are generally insufficient to indentify strong Anderson localization in the classical sense.

Farben sind in der belebten Natur für sehr viele Pflanzen und Tieren überlebenswichtig. Durch die Evolution haben sich daher faszinierende Farbeffekte entwickelt. So tarnen sich Tiere durch Farbanpassungen nahezu perfekt, nutzen Farben als Signal oder auch bei der Balz. Unter all diesen Effekten sind insbesondere strukturelle Farben im Fokus aktueller Forschung. Im Unterschied zu Farbstoffen und Pigmenten, entsteht die Farbe hier allein durch die spezifische Nanostruktur eines Materials. Insbesondere Schmetterlinge und Vögel erzeugen auf diese Weise außergewöhnlich brillante Farben, die auch für technische Anwendungen interessant sind. In den letzten Jahren wurden viele Effekte aus der Natur analysiert und deren Farbwirkung technisch imitiert.

The ability to confine light now routinely to of order 10nm3 volumes enables new functionalities for optoelectronic switching at ultralow energies. We present a range of devices in which an active polymer is sandwiched into nm-scale plasmonic gaps that can deliver fast and reversible actuation. Using the thermo-responsive polymer PNIPAM combined with directed nano-assembly using Au or Ag nanoparticles, allows high densities of nano constructs to be created, that change colour either in response to light or heat. High throughput synthesis allows the scale-up of these actuating nano-transducers (ANTs), opening up video-rate large area display technologies. We show measurements on single ANTs which confirm that nN forces are developed, and discuss their application for optically-driven microfluidic valves. Close-packed films of ANTs act to pump water in and out of permeable membranes using light, and produce a switchable metamaterial. The integration of these devices with DNA origami machines is demonstrated for the first time. We also show that lithium can be optically-pumped in and out of perovskite materials, making light-rechargeable batteries.

References:
Dynamic- and Light-Switchable Self-Assembled Plasmonic Metafilms, Adv.Opt.Mat. 1800208 (2018); DOI: 10.1002/adom.201800208
Photo-Rechargeable Organo-Halide Perovskite Batteries, Nano Letters (2018); DOI 10.1021/acs.nanolett.7b05153
Thermo-responsive Actuation of a DNA Origami Flexor, Adv.Func.Mat. 1706410 (2018); DOI 10.1002/adfm.201706410
The Crucial Role of Charge in Thermoresponsive-Polymer-... Au Nanoparticles, Adv.Opt.Mat. 1701270 (2018); DOI 10.1002/adom.201701270
Actuating Single Nano-oscillators with Light, Adv.Opt.Mat. 1701281 (2018); DOI 10.1002/adom.201701281
Light-Directed Tuning of Plasmon ..Polymerization Using Hot Electrons, ACS Phot. 4, 1453 (2017); DOI: 10.1021/acsphotonics.7b00206
Light-induced actuating nanotransducers, PNAS 113, 5503 (2016); DOI 10.1073/pnas.1524209113

The outstanding whiteness observed in the scales of Cyphochilus spp. beetles arises from a well-adjusted network of interconnecting cuticular filaments, which results in efficient light scattering over the visible range and beyond. This prominent example can inspire the design of artificial porous polymeric materials, whose light diffusing properties are tailored according to the optical application targeted.
As discussed in the present contribution, this can be achieved by using the industrially relevant and versatile microcellular foaming method. The latter allows the fabrication of self-standing polymer layers which can display various degrees of translucency or can be made totally reflecting. These different light diffusion regimes are analyzed experimentally and numerically alongside with their corresponding network made of disordered nano-/micrometer scaled pores.
The developed porous layers can improve light management in energy related applications in several ways. For example, they can be directly employed as diffuse photovoltaic reflectors fostering light trapping or, when designed semi-transparent, implemented in tandem solar cells as intermediate reflectors. Additionally, efficient light converting materials can be engineered by introducing quantum dots within the polymer matrix before foaming. In these porous composite layers, we show that the porous network assists the absorption of the exciting photons and helps the outcoupling of the converted light, which is realized by a proper balance between the forward and backward light scattering effects.

M. Renner1, D. T. Meiers1, M. C. Heep1, and G. von Freymann1,2 1Technische Universität Kaiserslautern, Physics Department and State Research Center OPTIMAS, Kaiserslautern, 67663, Germany 2Fraunhofer Institute for Industrial Mathematics ITWM, Kaiserslautern, 67663, Germany email: georg.freymann@physik.uni-kl.de Tailored disorder in photonic structures decides, if light propagates in a ballistic way or if it localizes due to multiple scattering. Tailored disorder also decides, if a structure presents bright non-iridescent color or if it shows brilliant whiteness. Lepitiota Stigma and Cyphochilus beetles possess such brilliant whiteness achieved by light scattering within the inner disordered chitin network of their scales. Due to this evolutionarily optimized network the scales belong to the strongest scattering structures based on low refractive index materials [1,2,3]. In this talk, I will present two model systems, which characteristically demonstrate the influence of tailored disorder. First, I will discuss the influence of deterministic aperiodic disorder on the transport properties of light in three-dimensional woodpile-type photonic crystals [4]. Here, a transition from transport to localization is observed. Second, I will present a model system explaining the optical properties of the white beetle scales’ structure. This model is based on small one-dimensional Bragg reflectors with tailored disorder in layer thickness and filling fraction and is able to also explain the noniridescent colors found, e.g. in certain Morpho butterflies. For both model systems, experiments and numerical calculations are in excellent agreement. References [1] P. Vukusic et al., Science 315, 348 (2007). [2] B. D. Wilts et al., Adv. Mater. 30, 1702057 (2018). [3] M. Burresi et al., Sci. Rep. 4, 6075 (2014). [4] M. Renner et al., Sci. Rep. 5, 13129 (2015).

The ubiquitous pigment melanin strongly absorbs light and thus produces many of the familiar black, brown and grey colors observed in nature. Paradoxically, it is also involved in the production of some of nature's brightest colors such as the brilliant iridescent gorgets of hummingbirds. I will review what is known about melanin's optical properties and how these are used to produce diverse structural colors both in nature and in the lab.

Nocturnal mammals exhibit an evolutionarily adapted, inverted nuclear architecture in their rod photoreceptor cells. This adaptation is currently hypothesized to improve light transmission through the retina. Here, experimental evidence is presented for reorganisation and fusion of nuclear chromatin causing reduction in light scattering in mouse rod nuclei, corroborating existing theory on effects of cellular ultra-structures in tissue transparency. The retinal image transfer is shown to improve by two-fold due to the inversion of chromatin. Modelling and simulation show a drastic reduction in the large angle light scattering in the retina. Finally, behavioural studies confirm the visual benefit of inverted nuclear organisation for motion detection under low light conditions. This work provides a comprehensive understanding of how improved retinal light transmission emerging from the photoreceptor nuclear adaptation aids nocturnal vision.

The arthropod cuticle is an impressively versatile bio-composite material made of chitin fibers embedded in a protein matrix. The cuticle also forms the cornea of the arthropod compound eye – a characteristic feature of the phylum that has largely contributed to its incredible evolutionary success. In the compound eye each photoreceptor unit carries its own optics. The optical structure includes a cornea and a crystalline cone. The former is made of a similar cuticle material as the animal exoskeleton, however, the latter may be similarly made of cuticle or it can be made of a protein, similar to the vertebrate lens-protein, crystalline. The horseshoe crab, Limulus polyphemus, compound eye is thought to be an ancestor structure to modern compound eye structures, as its cornea and cone are both made of cuticle material. Using refractive index mapping based on optical holotomography we find RI index gradients in cross- and long-section of the horseshoe crab cornea, which originate from the chitin fiber orientation as determined from X-ray diffraction tomography and from Br-doping as determined from X-ray fluorescence (XRF) tomography. We use these and other results to describe how the optical properties of the cornea are governed by its underlying architecture.

Light-microscopy and bio-photonics have traditionally been descriptive disciplines in search of structure-function-relationships to explain cellular organization, structural colors in nature, and tissue transparency. Starting from advancements in tissue optics simulations, I will stress opportunities to use light as a tool to control cellular organization, and genetics to alter cellular optics. In particular, I will talk about ‘Focused light-induced cytoplasmic streaming’ (FLUCS), a novel and interactive microscopy method to move the cytoplasm of living cells, which enables to guide the development of early embryos. Furthermore, I will report on directed evolution approaches to change the optical properties of living cells.

Colloidal nanoparticles offer a range of interesting optical and electronic effects. A prominent example is the localized surface plasmon resonance (LSPR) of metal nanoparticles due to resonant excitations of vibrations of the particles’ free-electron cloud by light. Due to the LSPR, plasmonic nanoparticles provide excellent means for controlling electromagnetic near-fields at optical frequencies, which has led to a broad range of applications in various field such as surface enhanced spectroscopy, light harvesting or photonics. While much research is dedicated to understanding nanoparticle synthesis and tailor their LSPR on the single particle level [1], ordering particles on different length scales opens another powerful avenue towards optical and electronic functionality. Plasmonic particles can couple locally, altering their LSPR, but as well collective long range phenomena can give rise to novel effects. In this context, integrating plasmonic nanoparticles into polymeric shells and / or scaffolds opens novel perspectives for solving this technological challenge. In particular we focus on core-shell particles and bioinspired controlled wrinkling as a versatile means for surface patterning [2]. We discuss the underlying physico-chemical effects and perspectives for applications in Surface Enhanced Raman Spectroscopy and Photonics, as well as approaches towards tuning of plasmonic coupling effects by mechanical strains.

[1] Mayer, M.; et al. Nano Letters 2015, 15, 5427-5437; Mayer, M. et al., Angewandte Chem. Int. Ed., 2017, 56 (50), 15866-15870
[2] Steiner, A., et al, ACS Nano 2017, 11(9), 8871–8880, Tebbe, M.; et al. Faraday Discussions 2015, 181, 243-260; Hanske, C.; et al. Nano Letters 2014, 14 (12), 6863 – 6871

Microreactor-Assisted Nanomaterial Synthesis and Deposition process combines the merits of microreaction technology and solution phase synthesis of nanomaterials. This technique uses continuous flow microreactors for the synthesis, assembly and deposition of nanomaterials. In synthesis, microreactor technology offers large surface-area-to-volume ratios within microchannel structures to accelerate heat and mass transport. This accelerated transport allows for rapid changes in reaction temperatures and concentrations leading to more uniform heating and mixing in the deposition process. The possibility of synthesizing nanomaterials in the required volumes at the point-of-application eliminates the need to store and transport potentially hazardous materials, while providing new opportunities for tailoring novel nanostructures and nanoshaped features. MAND processes control the heat transfer, mass transfer and reaction kinetics using well defined microstructures of the active unit reactor cell that can be replicated to produce higher chemical production volumes. This important feature opens a promising avenue in developing scalable nanomanufacturing. Furthermore, the continuous flow microreactor opens up the opportunity to conveniently assemble unique nanostructures and nanostructured thin films. The advantages of the microreactor system can also be realized in the fabrication of nanostructured thin films. There are a large number of innovative solution-based syntheses of nanomaterials partly due to its simplicity and low cost nature. The majority of these syntheses however were carried out using small batch reactors. Results-to-date demonstrate the possibility to control the reacting flux including small intermediate-reaction molecules, nanoclusters, nanoparticles and structured assembly of nanomaterials. In this presentation, recent progress in using continuous microreactors to synthesize bio-inspired nanostructured thin films and the applications of these functional nanostructured thin films for optics and photonics will be discussed.

Colloidal particles can be used as building blocks to create materials with unprecedented properties arising from hierarchical structure across different length scales. For example, the optical properties of a colloidal crystal can be readily tuned via composition, size, and nanoscale ordering of the particles. Yet, methods for assembly of colloidal particles are limited to forming films, patterns on substrates, and in specific confinements such as droplets. If colloidal particles can be assembled in freeform, new possibilities in device design could be enabled. Here, we present a direct-write technique that utilizes a dispensing needle to fabricate freestanding colloidal crystals with complex geometries and centimeter-scale dimensions. We demonstrate tuning of structural color based on particle size and order, and show that the technique can be extended to assembly of diverse particle systems.

Distributed Bragg Reflectors (DBRs) are one-dimensional (1D) multi-layered structures whose morphology enables display of interesting optical features, e.g. a high reflectance or resistance to color fading. The manufacturing of these seemingly simple alternating configurations usually requires exquisite precision. Most industrial techniques only allow deposition of “compact” metal oxides thin films in a vacuum atmosphere. Here, we present a spin-coating deposition technique, which gives us good control over thin film thickness and material composition. We assemble optical multilayers by breaking the multilayers into minimally repeating segments of carefully designed tri-layers to shorten the manufacturing time and reduce the number of possible defects. Usage of a sacrificial material enables release of high concentration of "flakes" into the compatible solvent. With solvent evaporation, tri-layers start to stack together and a high-quality DBR with any desired number of layers is achieved. The resulting materials can be used as proof-of-concept materials, e.g. for gas sensors.

Gas sensors are needed in modern applications for reliable and unobtrusive measurements. Existing gas sensors often degrade their performance because of the loss of the measurement accuracy in the presence of interferences and under a prolonged operation without re-calibration. Thus, new sensing approaches are required with improved sensor selectivity and stability. In this talk, we will present our assessment of capabilities of natural, biomimetic, and bioinspired nanostructures for selective gas sensing. We used nanostructures of Morpho butterfly scales for initial measurements of gases and their mixtures. We further recreated the main features of the observed gas-response functionality of Morpho butterfly scales in our biomimetic nanostructures. Further, we fabricated bioinspired nanostructures to achieve new functionality, well beyond Nature. We will provide details of our approach for selective gas sensing by taking advantage of the photonic nanostructure formed in the scales of Morpho butterfly wings and fabricated in-house nanostructures. Upon interactions with different individual gases and their mixtures, such photonic structures produce remarkably diverse differential reflectance spectra. While we have found that the response selectivity of iridescent scales of the Morpho butterfly wings dramatically outperforms existing sensors, our interest was to fabricate such nanostructures. Upon fabrication of biomimetic and bioinspired nanostructures, we have detected gases and their mixtures that were unrealistic to detect using natural Morpho structures. These colorimetric sensors can be tuned for numerous gas sensing scenarios in ambient and high temperatures, in confined areas or as individual nodes for distributed monitoring.

The conformal-evaporated-film-by-rotation (CEFR) technique has been successfully used to replicate surfaces of biological origin. This has led to the development of the Nano4Bio technique, which is the sequential combination of the CEFR technique, electroforming, plasma ashing, and stamping/casting. The Nano4Bio technique is emerging as a robust and industrially scalable manufacturing process to fabricate bioreplicas. These techniques have been applied to very diverse fields, including forensic science, pest control, and light sources.