Finite element modeling of the radiative properties of Morpho butterfly wing scales.

With the aim of furthering the explanation of iridescence in Morpho butterflies, we developed an optical model based on the finite-element (FE) method, taking more accurately into account the exact morphology of the wing, origin of iridescence. We modeled the photonic structure of a basal scale of the Morpho rhetenor wing as a three-dimensional object, infinite in one direction, with a shape copied from a TEM image, and made out of a slightly absorbing dielectric material. Periodic boundary conditions were used in the FE method to model the wing periodic structure and perfectly matched layers permitted the free-space scattering computation. Our results are twofold: first, we verified on a simpler structure, that our model yields the same result as the rigorous coupled wave analysis (RCWA), and second, we demonstrated that it is necessary to assume an absorption gradient in the true structure, to account for experimental reflectivity measured on a real wing.

Optical Properties of Ag-TiO(2) Nanocermet Films Prepared by Cosputtering and Multilayer Deposition Techniques.

Ag-TiO(2) nanocermet thin films, deposited for optical filtering applications by two sputtering techniques, codeposition and multilayer deposition, exhibit surface plasmon absorption in the spectral range 450-500 nm. The cosputtering technique induces a columnar growth, whereas multilayer deposition produces a more-random distribution of silver inclusions. Both films have large, flat silver grains at the air-cermet interface. An optical double-heterogeneous layer model based on the experimental morphological parameters of the films accounts well for their experimental transmittance, notably for extra absorption near 700 nm, which is attributed to a surface plasmon in the flat silver grains of the surface.

Optical properties of Ag -TiO 2 nanocermet films prepared by cosputtering and multilayer deposition techniques.

Ag -TiO2 nanocermet thin films, deposited for optical filtering applications by two sputtering techniques, codeposition and multilayer deposition, exhibit surface plasmon absorption in the spectral range 450 -500 nm. The cosputtering technique induces a columnar growth, whereas multilayer deposition produces a more-random distribution of silver inclusions. Both films have large, flat silver grains at the air -cermet interface. An optical double-heterogeneous layer model based on the experimental morphological parameters of the films accounts well for their experimental transmittance, notably for extra absorption near 700 nm, which is attributed to a surface plasmon in the flat silver grains of the surface.

Effective media equivalent to an asymmetric multilayer and to a rough interface.

A computer fitting program is used to show that not only a symmetric but also an asymmetric multilayer, composed of a periodic structure with small thickness irregularity or with slightly rough interfaces, can be equivalent to an anisotropic single layer within a good approximation. It is also shown that an interface layer equivalent to a rough interface does not exist, in spite of the fact that such an assumption has widely been used.

Effective medium equivalent to a symmetric multilayer at oblique incidence.

It is shown that a multilayer with repeated symmetric unit structure is equivalent to a homogeneous and optically biaxial medium for obliquely incident light, the three principal axes being normal to the plane of incidence, normal to the film plane, and normal to the other two. Unit thickness dependence and incident angle dependence of the dielectric tensor and the effective thickness are calculated using exact formulas. It is found that, at small unit structure thickness, the equivalent medium is almost uniaxial, the optical axis being normal to the film, and the effective thickness is very close to the mechanical one. Absorbing films are also envisaged and the domain of validity (in terms of unit structure thickness) of the uniaxial equivalent medium is defined by numerical examples.