Classification of dispersion equations for homogeneous dielectric-magnetic uniaxial materials

The geometric representation at a fixed frequency of the wavevector (or dispersion) surface $\omega(\vec k)$ for lossless, homogeneous dielectric--magnetic uniaxial materials is explored, when the elements of the relative permittivity and permeability tensors of the material can have any sign. Electromagnetic plane waves propagating inside the material can exhibit dispersion surfaces in the form of ellipsoids of revolution, hyperboloids of one sheet, or hyperboloids of two sheets. Furthermore, depending on the relative orientation of the optic axis, the intersections of these surfaces with fixed planes of propagation can be circles, ellipses, hyperbolas, or straight lines. The obtained understanding is used to study the reflection and refraction of electromagnetic plane waves due to a planar interface with an isotropic medium

Color optimization of a core–shell nanoparticles layer using machine learning techniques

Neural networks were recently introduced in the field of nanophotonics as an alternative and powerful way to obtain the non-linear mapping between the geometry and composition of arbitrary nanophotonic structures on one hand, and their associated properties and functions on the other. Taking into account the recent advances in the application of the machine learning concept to the design of nanophotonic devices, we employ this tool for the optimization of photonic materials with specific color properties. We train a deep neural network (DNN) to solve the inverse problem, i.e., to obtain the geometrical parameters of the structure that best produce a desired reflected color. The analyzed system is a single layer of core–shell spheres composed of melanin and silica embedded in air, arranged in a hexagonal matrix. The network is trained using a dataset of the three CIE 1976 (L*a*b*) color coordinates obtained from the simulated reflectance spectra of a large set of structures. The direct problem is solved using the Korringa–Kohn–Rostoker method (KKR), widely applied to calculate the optical properties of sphere composites. The color optimization approach used in this work opens up new alternatives for the design of artificial photonic structures with tunable color effects.Fil: Urquia, Gonzalo Martin. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Grupo de Electromagnetismo Aplicado; ArgentinaFil: Inchaussandague, Marina Elizabeth. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Skigin, Diana Carina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentin

Diffraction gratings of isotropic negative phase-velocity materials

Diffraction of electromagnetic plane waves by the gratings made by periodically corrugating the exposed planar boundaries of homogeneous, isotropic, linear dielectric--magnetic half--spaces is examined. The phase velocity vector in the diffracting material can be either co-parallel or anti-parallel to the time-averaged Poynting vector, thereby allowing for the material to be classified as of either the positive or the negative negative phase-velocity (PPV or NPV) type. Three methods used for analyzing dielectric gratings - the Rayleigh-hypothesis method, a perturbative approach, and the C formalism - are extended here to encompass NPV gratings by a careful consideration of field representation inside the refracting half--space. Corrugations of both symmetric as well as asymmetric shapes are studied, as also the diversity of grating response to the linear polarization states of the incident plane wave. The replacement of PPV grating by its NPV analog affects only nonspecular diffraction efficiencies when the corrugations are shallow, and the effect on specular diffraction efficiencies intensifies as the corrugations deepen. Whether the type of the refracting material is NPV or PPV is shown to affect surface wave propagation as well as resonant excitation of surface waves.Comment: 28 pages, 10 figures in 27 file

Structural color in the Swallow Tanager (Tersina viridis): using the Korringa-Kohn-Rostoker method to simulate disorder in natural photonic crystals

The plumage of birds often exhibits attractive color effects produced by the interaction of light with the photonic microstructure present in the feather barbs and barbules. This microstructure constitutes a natural photonic crystal that rejects radiation of wavelengths contained within the band gap, which significantly alters the observed coloration depending on the incidence conditions. In spite of the high degree of regularity exhibited by the barb's microstructure of many species, the disorder present in these natural photonic crystals might modify the reflected response. In this paper, we address the problem of modeling the electromagnetic response of a quasiordered photonic structure, using an electromagnetic method only suitable for strictly periodic structures. In particular, we simulate the reflected response of the plumage of the Swallow Tanager (Tersina viridis) by two different approaches. On the one hand, we compute the reflected response by averaging reflectance spectra calculated by the Korringa-Kohn-Rostoker method for different geometrical parameters. We also apply the inner extinction approximation, which represents imperfections in the structure by adding a small imaginary part to the dielectric constant of the inclusions. The agreement between the experimental and the simulated results evidences the potential of the proposed methods to reproduce the electromagnetic response of natural photonic structures.Fil: D'ambrosio, Christian Nahuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Grupo de Electromagnetismo Aplicado; ArgentinaFil: Skigin, Diana Carina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Grupo de Electromagnetismo Aplicado; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Inchaussandague, Marina Elizabeth. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Física. Grupo de Electromagnetismo Aplicado; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Barreira, Ana Soledad. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; ArgentinaFil: Tubaro, Pablo Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; Argentin

Analysis of the optical properties of the silvery spots on the wings of the Gulf Fritillary, Dione vanillae

The ventral face of the wings of the butterfly Dione vanillae is covered with bright and shiny silvery spots. These areas contain densely packed ground- and coverscales with a bright metallic appearance reflecting more than 50% of light uniformly over the visible range. Our analysis shows that this optically attractive feature is caused by the inner microstructure of the scales located in these areas. Electron microscopy of cross sections through the scales shows that upper and lower lamina, supporting trabeculae, and topping ridges can be approximated by a ‘circus tent’-like geometry. By simulating its optical properties, we show that a moderate disorder of this geometry is important for the uniform reflection of light resulting in the silvery appearance

Characterization of the iridescence-causing multilayer structure of the Ceroglossus suturalis beetle using bio-inspired optimization strategies

We investigate the iridescence exhibited by Ceroglossus suturalis beetles, which mostly live endemically in the southern end of South America. Two differently colored specimens have been studied. We observed and characterized the samples by different microscopy techniques, which revealed a multilayer structure within their cuticle. Using measured reflectance spectra as input data, we applied heuristic optimization techniques to estimate the refractive index values of the constituent materials, to be introduced within the theoretical model. The color of the samples was calculated for different incidence angles, showing that multilayer interference is the mechanism responsible for the observed iridescence.Centro de Investigaciones Óptica

Structural Color in Marine Algae

Structural colouration is widespread in the marine environment. Within the large variety of marine organisms, macroalgae represent a diverse group of more than 24,700 species. Some macroalgae have developed complex optical responses using different nanostructures and material compositions. In this review, we describe the mechanisms that are employed to produce structural colour in algae and provide a discussion on the functional relevance by analysing the geographical distribution and ecology in detail. In contrast to what is observed in the animal kingdom, we hypothesise that structural colour in algae predominantly functions for a non-communicative purpose, most likely protection from radiation damage, e.g. by harmful UV light. We suggest that the presence of structural colour in algae is likely influenced by local factors such as radiation intensity and turbidity of the water.Biotechnology and Biological Sciences Research Council (Grant ID: BBSRC David Phillips, 13 BB/K014617/1), European Research Council (Grant ID: ERC-2014-STG H2020 639088), Department of Chemistry, Cambridge (Philip and Patricia Brown Next Generation Fellowship), National Centre of Competence in Research “Bio-Inspired Materials”, Adolphe Merkle Foundatio

Diffraction of electromagnetic waves at periodically corrugated anisotropic media

En esta tesis se desarrollan nuevos métodos teóricos rigurosos que permiten resolver el problema de la difracción de luz en una superficie corrugada periódicamente que separa un medio isótropo (dielétrico, metal o conductor perfecto) de un medio anisótropo (cristal uniaxial o biaxial). Los métodos desarrollados se basan en el empleo de transformaciones de coordenadas que convierten la superficie periódica en un plano, simplificando de este mod0 el manejo matemático de las condiciones de contorno en la interfase. Escribiendo las ecuaciones de Maxwell en el espacio transformado, el problema original se reduce a la resolución numérica de sistemas de ecuaciones diferenciales acopladas cuyas características varían de acuerdo con el tipo de transformación utilizada. Se emplean dos tipos de transformaciones: contravariantes y covariantes. El método de transformaciones covariantes permite tratar superficies periódicas iluminadas por una onda plana que incide desde cualquiera de los medios y con vector de onda orientado arbitrariamente con respecto a los surcos de la red. Se estudia la consistencia de los formalismos investigados mediante el análisis de casos límites como el de interfases planas y redes isótropas y se comparan los resultados con los obtenidos utilizando métodos aproximados que emplean en su formulación la hipótesis de Rayleigh. Como ejemplos de aplicación de los métodos presentados en este trabajo, se analiza la conversión entre modos de polarización y la excitación resonante de plasmones superficiales

Dispersion relation and band gaps of 3D photonic crystals made of spheres

In this paper, we introduce a dispersion equation for 3D photonic crystals made of parallel layers of non-overlapping spheres, valid when both wavelength and separation between layers are much larger than the distance between neighbouring spheres. This equation is based on the Korringa-Kohn-Rostoker (KKR) wave calculation method developed by Stefanou et al. and can be used to predict the spectral positions of bandgaps in structures made of dispersive spheres. Perfect agreement between the spectral positions of bandgaps predicted with our simplified equation and those obtained with the numerical code MULTEM2 was observed. We find that this simplified relation allows us to identify two types of bandgaps: those related to the constitutive parameters of the spheres and those related to the three dimensional periodicity (distance between layers). Bandgaps of the first type are independent of the frequency and the distance between layers, while those of the second type depend only on these two quantities. We then analyze the influence of the constitutive parameters of the spheres on the spectral position of bandgaps for spheres immersed in dielectric or magnetic homogeneous media. The number and positions of the bandgaps are affected by the permitivity (permeability) of the host medium if the spheres have dispersive permitivity (permeability).Fil:Inchaussandague, M.E. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Depine, R.A. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina