Observation of Selényi rings in the diffraction patterns of layered samples with periodic arrays of cylindrical structures.

We present an experimental and numerical study of the optical properties of nanofabricated samples with layered dielectric structures. The samples, which contain periodic arrays of silicon disks over a flat layer of silicon dioxide on a silicon substrate, present diffraction and thin film interference effects. Well-defined circular fringes that modulate the intensity of the diffraction orders are observed in the far-field angular distribution of scattered light. We also find that although the angle of incidence modulates the intensity of the observed circular ring patterns, it has little or no effect on their angular position. The problem is modeled theoretically through numerical calculations based on a Rayleigh method.

Metamodeling of high-contrast-index gratings for color reproduction.

We propose a metamodel-based optimization technique to tailor the chromatic response of high-contrast-index gratings. The algorithm, which couples a population-based metaheuristic with a neural network, is used to retrieve the optimal geometrical parameters of a grating to reproduce a prescribed color. By means of some examples, we assess the possibilities and limitations of our optimization scheme. The numerical evidence found shows that the metamodel approach offers an alternative to traditional metaheuristic techniques that not only provides the best solution for a given geometry and a material but also significantly improves the computing time required for the optimization process.

Optimization of all-dielectric structures for color generation.

In this work, we propose an inversion scheme to tailor the chromatic response of an all-dielectric structure. To this end, we couple, through a previously defined objective functional involving the concept of color difference, a forward solver with an optimization algorithm. The former is based on the differential method, whereas the latter is based on particle swarm optimization. The optimal geometrical parameters of the structure that generates a specific color are obtained through the solution of an approximation problem. We illustrate the performance of our inversion scheme through examples and discuss its limitations and potential applications.

Experimental and numerical studies of the scattering of light from a two-dimensional randomly rough interface in the presence of total internal reflection: optical Yoneda peaks.

The scattering of polarized light from a dielectric film sandwiched between two different semi-infinite dielectric media is studied experimentally and theoretically. The illuminated interface is planar, while the back interface is a two-dimensional randomly rough interface. We consider here only the case in which the medium of incidence is optically more dense than the substrate, in which case effects due to the presence of a critical angle for total internal reflection occur. A reduced Rayleigh equation for the scattering amplitudes is solved by a rigorous, purely numerical, nonperturbative approach. The solutions are used to calculate the reflectivity of the structure and the mean differential reflection coefficient. Optical analogues of Yoneda peaks are present in the results obtained. The computational results are compared with experimental data for the in-plane mean differential reflection coefficient, and good agreement between theory and experiment is found.

Reflection of diffuse light from dielectric one-dimensional rough surfaces.

We study the reflection of diffuse light from 1D randomly rough dielectric interfaces. Results for the reflectance under diffuse illumination are obtained by rigorous numerical simulations and then contrasted with those obtained for flat surfaces. We also explore the possibility of using perturbation theories and conclude that they are limited for this type of study. Numerical techniques based on Kirchhoff approximation and reduced Rayleigh equations yield better results. We find that, depending on the refractive index contrast and nature of the irregularities, the roughness can increase or decrease the diffuse reflectance of the surface.