By using metamaterial, scattering and absorption properties of systems can be engineered to meet certain criteria. Although metamaterials are usually based on the structuralization of matter, we demonstrate that electromagnetic radiation can be manipulated via randomly distributed nanoparticles. On the one hand, scattering in nanoparticle suspensions occurs traditionally whenever the size of the nanoparticles is non-negligible compared to the wavelength, which induces high incoherent radiation. However, high scattering is also obtained with subwavelength nanoparticle clouds in spectral bands where resonant behaviors appear. In such configuration, we demonstrate that scattering and incoherence are not correlated anymore. On the other hand, high absorption is achieved by means of the plasmonic response of the nanoparticles. Because the system is resonant, the required representative volume element to include the numerous interactions becomes very large, rendering three-dimensional computation impractical. Therefore, we have implemented most of our simulations in two-dimensional systems where the computational load is manageable. Nevertheless, 3D computations are still performed on reduced systems. Interestingly, we found that the relation between 2D and 3D representative volume is non-linear. Finally, the control of the mechanisms at stake in polydisperse-disordered media allows us to engineer systems in which the absorption is greatly enhanced by critical coupling effect.
We model a photonic crystal slab as a Fabry-Perot resonator with two propagating Bloch waves in the periodic medium. This provides a semi-analytical recipe for the computation of photonic crystal slab modes' dispersion and quality factors. We apply for the search and study of bound states in the continuum, which exist above the light line, among the leaky modes, but nevertheless are decoupled from the continuum of propagating modes and are confined inside the periodic medium. We identify them as a set of optogeometric parameters for which the quality factor of a given photonic crystal mode goes to infinity. Also, we illustrate a simple example of the vertical symmetry breaking by adding a semi-infinite dielectric substrate, and comment on some other asymmetric configurations.
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