Extremely anisotropic boundary conditions and their optical applications

We discuss here the design of artificial nanoparticles, surfaces and composite materials that may realize extremely anisotropic boundary conditions, with values not easily attainable in natural optical materials. We apply the closed-form analytical solution available in the case of an individual nanosphere composed of two conjoined hemispheres with opposite permittivity in order to show that it may be possible to realize tunable local impedance surfaces with extreme values. In the case of a single nanoparticle, it is shown that a simple mechanical or polarization rotation may tune the effective local impedance from very high to very low values of impedance. Combinations of such engineered nanoparticles forming a metasurface or a metamataerial may provide novel tools for the realization of extreme boundary conditions and effective parameters in the visible. These findings may have a relevant impact in a variety of applications in optics.