17 Décembre – Thesis defense - Romain Dezert
10 h Amphi - Centre de Recherche Paul Pascal (Pessac)
Theoretical study of isotropic Huygens particles for metasurfaces.
Recent developments in optics at the nanoscale have given rise to a new branch of nano-photonics aimed at manipulating the scattering of nanoparticles, with numerous potential applications in optical communication, nano-antennas, photovoltaics, sensing, etc. The response of nano-scatterers is often characterized in terms of electromagnetic multipoles. Tailoring these multipoles represents an efficient scheme to engineer three-dimensional radiation diagrams. For instance, destructive interferences between multipoles of opposite spatial parity can be exploited to cancel backscattering. This effect, theoretically predicted 30 years ago by Milton Kerker, makes it possible to produce subwavelength particles that scatter light in the forward direction, thus sharing the main features of the fictitious sources used in the Huygens-Fresnel principle. Once assembled in a periodic two-dimensional network, such particles, named ''Huygens sources'', offer unique opportunities for the development of flat and ultrathin optical components called "metasurfaces" that enable the arbitrary control of the phase, amplitude and/or polarization of a beam of light. Over the past few years, Huygens metasurfaces have been widely explored to engineer highly efficient lenses, beam deflectors, vortex beams, holograms or perfect absorbers, that have relied on two-dimensional anisotropic Huygens sources. In contrast to approaches investigated thus far, this thesis focuses on the study of isotropic Huygens sources. We investigate homogeneous, composite and core-shell particles as a solution to reach the Kerker regime. Subsequently, we demonstrate that wave-front shaping is indeed possible by using spherical systems composed of clusters of dielectric inclusions and we present a multipolar formalism that can be used as a guideline to optimize the absorption of Huygens arrays. The structures we study are realistically achievable by bottom-up fabrication and self-assembly, offering an alternative to the classical lithographically fabricated metasurfaces.
