Search

This paper reveals an enhanced fluorescence in the hindwings of the male Troïdes magellanus, due to the confinement of fluorophores in a three-dimensional photonic structure. It is characterized by a spatial variation of the emission intensity and coloration. It also reveals the role of the structure on the emission and reflection complementary processes. We focus on the experimental analysis of these phenomena by means of a morphological study, a reflection characterization, and an emission characterization. Collecting and analyzing data over every emerging direction was important in this work. A theoretical approach is proposed to explain the experimental observations.

The birdwings butterfly Troides magellanuspossesses interesting properties for light and thermal radiation management. The black wings of the male exhibit strong (98%) absorption of visible light as well as two strong absorption peaks in the infrared (3 m and 6 m) both due to chitin. These peaks are located in the spectral region where the black body emits at 313K. The study of absorption enhancement in this butterfly could be helpful to design highly absorbent biomimetic materials. Observations of the wings using a scanning electron microscope (SEM) reveal that the scales covering the wings are deeply nanostructured. A periodic three-dimensional (3D) model of the scale nanoarchitecture is elaborated and used for numerical transfer-matrix simulations of the absorption spectrum. The complex refractive index of the wing material is approximated by a multi-oscillator Lorentz model, leading to a broad absorption in the visible range as well as two peaks in the infrared. The absorption peak intensities turn out to be dependent on the complexity of the nanostructures. This result clearly demonstrates a structural effect on the absorption. Finally, a comparison with a planar layer of identical refractive index and material volume lead us to conclude that the absorption is enhanced by nanostructures.

Abstract: Chrysidid wasps in the subfamily Chrysidinae are brood parasitoids or cleptoparasites of other insects and famous for their cuticular iridescence. In this study, we examine the dorsal abdominal cuticle of the chrysidid wasp Hedychrum rutilans to identify the underlying color mechanism. Using scanning electron microscopy, reflectance spectral analysis, and theoretical calculations, we demonstrate the presence of an epicuticular multilayer reflector consisting of six lamellae with a thickness of 185 nm each. The lamellae exhibit a rough surface probably functioning as spacers between the individual layers. The reflector has a measured reflectance maximum at λ = 630 nm, i.e., in the red part of the visible spectrum of light at normal incidence and the reflectance maximum shifts to green as the angle of incidence increases. Complementary theoretical modeling corroborates the view that the epicuticular multilayer generates the iridescent color of the chrysidid cuticle.

Members of the order Coleoptera are sometimes referred to as ‘living jewels’, in allusion to the strikingly diverse array of iridescence mechanisms and optical effects that have arisen in beetles. A number of novel and sophisticated reflectance mechanisms have been discovered in recent years, including three-dimensional photonic crystals and quasi-ordered coherent scattering arrays. However, the literature on beetle structural coloration is often redundant and lacks synthesis, with little interchange between the entomological and optical research communities. Here, an overview is provided for all iridescence mechanisms observed in Coleoptera. Types of iridescence are illustrated and classified into three mechanistic groups: multilayer reflectors, three-dimensional photonic crystals and diffraction gratings. Taxonomic and phylogenetic distributions are provided, along with discussion of the putative functions and evolutionary pathways by which iridescence has repeatedly arisen in beetles.

Iridescent butterfly wing colours result from the interaction of light with sub-micrometre structures in the scales. Typically, one scale contains one such photonic structure that produces a single iridescent signal. Here, however, we show how the dorsal wings of male Lamprolenis nitidaemit two independent signals from two separate photonic structures in the same scale. Multiple independent signals from separate photonic structures within the same sub-micrometre device are currently unknown in animals. However, they would serve to increase the complexity and specificity of the optical signature, enhancing the information conveyed. This could be important during intrasexual encounters, in which iridescent male wing colours are employed as threat displays. Blazed diffraction gratings, like those found in L. nitida, are asymmetric photonic structures and drive most of the incident light into one diffraction order. Similar gratings are used in spectrometers, limiting the spectral range over which the spectrometer functions. By incorporating two interchangeable gratings onto a single structure, as they are in L. nitida, the functional range of spectrometers could be extended.

We demonstrate highly tunable one-dimensional photonic band structures under normal incidence. The system consists of a multilayer film that alternates two anisotropic layers. The band-structure characteristics of this multilayer stack, such as location and width of stop bands, can be tailored by altering the relative orientation of the optical axes of the adjacent layers. This structure can be realized by employing liquid crystals and transparent electrodes. In some cases, we observe the ability of this structure to distinguish between different linear polarizations. We propose a depolarizing and filtering device based on this multilayer medium.

The complex photonic crystal type nanoarchitectures found in the wing scales of the male butterfly Cyanophrys remuswere investigated structurally by electron microscopy and optically by reflectance spectroscopy. Both the vivid metallic blue of the dorsal scales and the matt, pea-green coloration of the ventral scales are attributed to photonic crystal type structures composed of chitin and air. The dorsal scales are single crystalline, while the ventral ones contain a large number of randomly oriented micron size single crystalline grains of face centered cubic inverse opal. The remarkable complexity and efficiency of biologic photonic crystals may provide clues in designing artificial structures with similar parameters.

With use of plane waves as a basis for the band-structure calculation of a periodic assembly of highly refringent microspheres, it can be shown that resonance-mode frequencies of isolated dielectric spheres show up in the band structures. The strongly localized bands provided by the photonic-crystal analysis is compared with exact calculations made in spherical symmetry for an isolated microsphere. This comparison sheds some light on the effectiveness of the methods based on the description of mode coupling and, in particular, on the validity of tight-binding approaches of the description of photonic band structures. In addition, examining the effect of modifying the distance separating the spheres in the lattice, makes it easy to visualize the overlap between the modes of individual spheres. Thus quantitative information is provided on the geometry needed to feed energy into low-angular-momentum morphology-dependent resonances from a sharp source of the evanescent field and on the lifetime of these modes, when the resonances are disturbed by the proximity of a dielectric object of similar radius.

Alpha particles accelerated towards a flat Al sample accumulate and precipitate in the form of dense He bubbles. Under fast-electron bombardment, these bubbles have been observed to generate He excimer molecules which decay by emitting vacuum-ultraviolet (VUV) fluorescent radiation. The calculation of the VUV dielectric function and reflectance shows that a thin planar film made of such He–Al composite and driven out of equilibrium can generate amplifying reflection. This phenomenon occurs at specific isolated frequencies and incidence angles when the excimer concentration in the pressurized fluid of the bubbles becomes sufficient. By use of analytical and numerical multiple-scattering simulations, the expected gain of this VUV amplifying mirror is studied, and the possible improvement brought about by shaping the film into a periodic array of adequately adjusted microresonators is demonstrated.

For the purpose of simulating photon-stimulated field emission by taking account of three-dimensional aspects, the transfer matrix and Green's functions formalisms, used in previous works for the simulation of the Fresnel projection microscope, was combined with a Floquet expansion of the wave function in order to study quanta exchanges between the electrons and the external radiation. In this new formulation, the wave function is expanded along basic states that describe its spatial variation for the various discrete energy levels associated with absorption/emission of photons. The transmission of these basic states between the two electrodes providing the field emission voltage is described within the transfer matrix formalism, where specific techniques are used to preserve stability. The transmission of these basic states from the cathode to a distant screen is achieved within the Green's functions formalism. The technique takes advantage of n-fold symmetries and provides control of computation accuracy. The theory is applied to the stimulation of field emission from a 1-nm-long tungsten nanotip. The application specifically focuses on the current increase due to electromagnetic radiation. Total energy distributions (TED) and current-relative-increase curves are provided for radiation wavelengths ranging from 0.1 to 10 μm and power flux densities ranging from 10¹⁰ to 10¹² W/m². The results point out a resonance in the current enhancement at a radiation wavelength of approximately 0.3 μm. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 80: 816–823, 2000

The observation of the motion of an adatom on the reconstructed Ge (111) is a rare event, which will be examined by way of simple adatom–surface interaction models. Estimations of the residence time of adatoms on energetically favorable sites indicate that a thermal excitation can account for casual adatom motion and that a strong tip–surface interaction is not obviously needed to explain the changes found in sequences of scanning tunneling microscopy (STM) images of the same surface areas. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 1093–1097, 1998

We propose a computational technique to evaluate the reflectivity and the transmission of the harmonics generated by a planar-stratified material containing nonlinear layers. Maxwell's equations are solved and the scattering boundary conditions are implemented using a transfer matrix technique. As our goal beyond this study is to model sum-frequency generation spectra, our approach is limited to second-order processes. The great advantage of this algorithm is that owing to a self-consistent procedure the nonlinear scattering problem is reduced to the repeated solving of linear equations. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 763–770, 1998

Light can tunnel through a high-reflectivity dielectric multilayer film when a photonic-crystal-type defect is introduced in the structure, which is useful for optical signal processing. We consider chirped structures with a defect in layer thickness for which high reflectivity is achieved over a broad wavelength range except within a narrow spectral window. The useful transmission window, while it shifts toward shorter wavelengths as the angle of incidence of the light beam is increased, does not, in general, survive; i.e., transmission disappears progressively. We show that wide angular range operation can, however, be achieved by a proper design of the chirped structure. Analytical expressions for the design parameters are derived on the basis of a semi-infinite photonic crystal model. Theoretical reflectance spectra of defect SiO_2/TiO_2 chirped multilayer films are presented and discussed in terms of the dispersion of the electromagnetic radiation modes of the finite photonic crystal. These devices offer a simple way to mechanically tune (through inclination of the film) the wavelength transmitted from a fixed white-light beam.