Sub-Wavelength Probing and Modification of Complex Photonic Structures

The aim of this thesis consists in the study and modification of complex photonic nano-structures. Nowadays, propagation of light in such materials is a rich and fascinating area of research, both for its fundamental implications and for its practical technological impact. To deeply investigate light propagation inside these structures a high spatial resolution technique is required, especially because intriguing effects often occur on length scales comparable with the diffraction-limit or involve coupling phenomena on this length scale. For this reason in this thesis a Scanning Near-Field Optical Microscope represents one the most straightforward tool both to study and locally modify complex photonic nano-structures from perfect periodic to completely random ones

Block copolymer self-assembly for nanophotonics

The ability to control and modulate the interaction of light with matter is crucial to achieve desired optical properties including reflection, transmission, and selective polarization. Photonic materials rely upon precise control over the composition and morphology to establish periodic interactions with light on the wavelength and sub-wavelength length scales. Supramolecular assembly provides a natural solution allowing the encoding of a desired 3D architecture into the chemical building blocks and assembly conditions. The compatibility with solution processing and low-overhead manufacturing is a significant advantage over more complex approaches such as lithography or colloidal assembly. Here we review recent advances on photonic architectures derived from block copolymers and highlight the influence and complexity of processing pathways. Notable examples that have emerged from this unique synthesis platform include Bragg reflectors, antireflective coatings, and chiral metamaterials. We further predict expanded photonic capabilities and limits of these approaches in light of future developments of the field

Disordered Cellulose-based Nanostructures for Enhanced Light-scattering

Cellulose is the most abundant bio-polymer on earth. Cellulose fibres, such as the one extracted form cotton or woodpulp, have been used by humankind for hundreds of years to make textiles and paper. Here we show how, by engineering light matter-interaction, we can optimise light scattering using exclusively cellulose nanocrystals. The produced material is sustainable, biocompatible and, when compared to ordinary microfibre-based paper, it shows enhanced scattering strength (x4) yielding a transport mean free path as low as 3.5 um in the visible light range. The experimental results are in a good agreement with the theoretical predictions obtained with a diffusive model for light propagation

The influence of pigmentation patterning on bumblebee foraging from flowers of <em>Antirrhinum majus</em>

Patterns of pigmentation overlying the petal vasculature are common in flowering plants and have been postulated to play a role in pollinator attraction. Previous studies report that such venation patterning is significantly more attractive to bee foragers in the field than ivory or white flowers without veins. To dissect the ways in which venation patterning of pigment can influence bumblebee behaviour, we investigated the response of flower-naïve individuals of Bombus terrestris to veined, ivory and red near-isogenic lines of Antirrhinum majus. We find that red venation shifts flower colour slightly, although the ivory background is the dominant colour. Bees were readily able to discriminate between ivory and veined flowers under differential conditioning but showed no innate preference when presented with a free choice of rewarding ivory and veined flowers. In contrast, both ivory and veined flowers were selected significantly more often than were red flowers. We conclude that advantages conferred by venation patterning might stem from bees learning of their use as nectar guides, rather than from any innate preference for striped flowers. © 2013 Springer-Verlag Berlin Heidelberg

The limitations of extending nature’s color palette in correlated, disordered systems

Living organisms have developed a wide range of appearances from iridescent to matte textures. Interestingly, angular independent structural colors, where isotropy in the scattering structure is present, only produce coloration in the blue wavelength region of the visible spectrum. One might, therefore, wonder if such observation is a limitation of the architecture of the palette of materials available in nature. Here, by exploiting numerical modeling, we discuss the origin of isotropic structural colors without restriction to a specific light scattering regime. We show that high color purity and color saturation cannot be reached in isotropic short-range order structures for red hues. This conclusion holds even in the case of advanced scatterer morphologies, such as core-shell particles or inverse photonic glasses — explaining recent experimental findings reporting very poor performances of visual appearance for such systems

Role of Anisotropy and Refractive Index in Scattering and Whiteness Optimization

This is the final version. Available from Wiley via the DOI in this record.The ability to manipulate light–matter interaction to tailor the scattering properties of materials is crucial to many aspects of everyday life, from paints to lighting, and to many fundamental concepts in disordered photonics. Light transport and scattering in a granular disordered medium are dictated by the spatial distribution (structure factor) and the scattering properties (form factor and refractive index) of its building blocks. As yet, however, the importance of anisotropy in such systems has not been considered. Here, a systematic numerical survey that disentangles and quantifies the role of different kinds and degrees of anisotropy in scattering optimization is reported. It is shown that ensembles of uncorrelated, anisotropic particles with nematic ordering enables to increase by 20% the reflectance of low-refractive index media (n = 1.55), using only three-quarters of material compared to their isotropic counterpart. Additionally, these systems exhibit a whiteness comparable to conventionally used high-refractive index media, e.g., TiO2 (n = 2.60). Therefore, the findings not only provide an understanding of the role of anisotropy in scattering optimization, but they also showcase a novel strategy to replace inorganic white enhancers with sustainable and biocompatible products made of biopolymers.Biotechnology and Biological Sciences Research Council (BBSRC)European Research Council (ERC)Leverhulme Trus

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.S.V. thanks the Biotechnology and Biological Sciences Research Council (BBSRC) David Phillips fellowship (BB/K014617/1), the European Research Council (ERC-2014-STG H2020 639088), and the European Commission (Marie Curie Fellowship Looking Through Disorder (LODIS), 701455) for financial support. B.D.W. was financially supported through the National Center of Competence in Research Bio-Inspired Materials and the Ambizione program of the Swiss National Science Foundation (168223)

Protocol for Extraction and Electron Microscopy Visualization of Lipids in Viburnum tinus Fruit Using Cryo-Ultramicrotomy.

We recently reported that Viburnum tinus fruit generates its metallic blue color using globular lipid inclusions embedded in its epicarpal cell walls. This protocol describes steps to visualize the lipidic nature of the nanostructure using cryo-ultramicrotomy, chloroform extraction, and transmission electron microscopy (TEM) imaging. This method is useful to localize and characterize novel lipidic nanostructures embedded in both plant and animal tissues at the TEM resolution. For complete details on the use and execution of this protocol, please refer to Middleton et al. (2020)