Cédric Vandenbem

Color-selecting reflectors inspired from biological periodic multilayer structures

We propose a semi-infinite 1-D photonic crystal approach for designing artificial reflectors which aim to reproduce color changes with the angle of incidence found in biological periodic multilayer templates. We show that both the dominant reflected wavelength and the photonic bandgap can be predicted and that these predictions agree with exact calculations of reflectance spectra for a finite multilayer structure. In order to help the designer, the concept of spectral richness of angle-tuned color-selecting reflectors is introduced and color changes with angle are displayed in a chromaticity diagram. The usefulness of the photonic crystal approach is demonstrated by modelling a biological template (found in the cuticle of Chrysochora vittata beetle) and by designing a bio-inspired artificial reflector which reproduces the visual aspect of the template. The bioinspired novel aspect of the design relies on the strong unbalance between the thicknesses of the two layers forming the unit cell.

Structurally tuned iridescent surfaces inspired by nature

Iridescent surfaces exhibit vivid colours which change with the angle of incidence or viewing due to optical wave interference in the multilayer structure present at the wavelength scale underneath the surface. In nature, one can find examples of iridescent Coleoptera for which the hue changes either greatly or slightly with the angle. Because these species typically make these structures from a single biological material (usually chitin) and air or water as the low refractive index component, they have evolved by adjusting the layer thicknesses in order to display quite different iridescent aspects. Taking inspiration from this proven strategy, we have designed and fabricated periodic TiO2/SiO2 multilayer films in order to demonstrate the concept of structurally tuned iridescent surfaces. Titanium or silicon oxide layers were deposited on a glass substrate using dc reactive or RF magnetron sputtering techniques, respectively. Two structures were designed for which the period and the TiO2/SiO2 layer thickness ratio were varied in such a way that the films displayed radically different iridescent aspects: a reddish-to-greenish changing hue and a stable bluish hue. The fabricated samples were characterized through specular reflectance/transmittance measurements. Modelling of transmittance spectra using standard multilayer film theory confirmed the high quality of the twelve-period Bragg reflectors. The chromaticity coordinates, which were calculated from measured reflectance spectra taken at different angles, were in accordance with theoretical predictions.

Electromagnetic surface waves of multilayer stacks : coupling between guided modes and Bloch modes

The study of the dependence of surface mode dispersion on the termination of multilayer stacks reveals interesting features. For stratified media with high-refractive-index contrasts, surface modes can shift across several bandgaps if the thickness of the final layer is changed. The distance to the photonic band edge influences the decay length of the mode inside the multilayer stack. In the middle of the bandgap, the electromagnetic energy is concentrated in the final layer of the crystal, while near bandgap edges the decay length extends over several periods. Additional evidence suggests that surface modes behave like guided modes that can couple with the extended Bloch modes and give rise to evanescent field profiles oscillating along several periods.

Mie resonances of dielectric spheres in face-centered cubic photonic crystals

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.

Tunable band structures in uniaxial multilayer stacks

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.

Bi-functional photonic structure in the Papilio nireus (Papilionidae): modeling by scattering-matrix optical simulations.

Scales of the Papilio nireus combine fluorophores confined in a natural photonic structure. By means of numerical simulations based on the scattering-matrix formalism, we reveal the bi-functional optical role of this peculiar architecture. Two aspects are considered: the absorption of an incident light flux and the emission of another luminous flux. First, results highlight a light trapping effect and a light absorption increase in the ultraviolet, visible and near infrared ranges. Then, results highlight an enhanced fluorescence occurring in the spatial as well as in the frequency domain. This observation could be of great interest to design new optical devices.

Nanoarchitecture in the black wings of Troides magellanus: a natural case of absorption enhancement in photonic materials

The birdwings butterfly Troides magellanus possesses 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 μm and 6 μm) 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.

Method for modeling additive color effect in photonic polycrystals with form anisotropic elements: the case of Entimus imperialis weevil

The calculation of the reflectance of photonic crystals having form-birefringent anisotropic elements in the crystal unit cell, such as cylinders, often turns out to be problematic, especially when the reflectance spectrum has to be computed according to different crystal orientations as in polycrystals for instance. The method we propose here solves this problem in the specific case of photonic crystals whose periodicities are such that there are no diffraction orders except Bragg reflection in the visible range. For a given crystal orientation, the crystal is sliced into layers and the periodic spatial variations of the dielectric function ε are homogenized. Thanks to that homogenization, the calculation can be performed using standard thin film computation codes. In order to demonstrate the usefulness of our method, we applied it to the case of a natural photonic polycrystal found on the cuticle of Entimus imperialis weevil which is a remarkable example of additive color effect. Although each photonic crystal grain of the polycrystal produces a single bright iridescent color, a non-iridescent green matt coloration is perceived by the human eye due to multiscale averaging effects.

Additive photonic colors in the Brazilian diamond weevil, entimus imperialis

Structurally colored nano-architectures found in living organisms are complex optical materials, giving rise to multiscale visual effects. In arthropods, these structures often consist of porous biopolymers and form natural photonic crystals. A signature of the structural origin of coloration in insects is iridescence, i.e., color changes with the viewing angle. In the scales located on the elytra of the Brazilian weevil Entimus imperialis (Curculionidae), three-dimensional photonic crystals are observed. On one hand, each of them interacts independently with light, producing a single color which is observed by optical microscopy and ranges from blue to orange. On the other hand, the color perceived by the naked eye is due to multi-length-scale light effects involving different orientations of a single photonic crystal. This disorder in crystal orientations alters the light propagation in such a way that the crystal iridescence is removed. Entimus imperialis is therefore a remarkable example of additive photonic colors produced by a complex multi-scale organic architecture. In order to study this specific natural photonic structure, electron microscopy is used. The structure turns out to be formed of a single type of photonic crystal with different orientations within each scale on the elytra. Our modeling approach takes into account the disorder in the photonic crystals and explains why the structure displays bright colors at the level of individual scales and a non-iridescent green color in the far-field.

Wide angular range operation of a dielectric multilayer film with a photonic-crystal-type defect

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 SiO2/TiO2 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.