Aristi Christofi

Strong circular dichroism of core-shell magnetoplasmonic nanoparticles

Composite magnetoplasmonic nanoparticles with a core-shell morphology exhibit intriguing optical properties and offer impressive opportunities for tailoring in a controllable manner the light-matter interaction at subwavelength dimensions. These properties are usually analyzed in the framework of the quasi-static approximation, which, however, is often inadequate; thus, a full electrodynamic treatment is required. In this respect, we developed a rigorous method for an accurate description of electromagnetic scattering by a gyrotropic sphere coated with a nongyrotropic concentric spherical shell, based on the full multipole expansion of the wave field. The method was applied to specific examples of core-shell cobalt-silver spherical nanoparticles, where the occurrence of strong circular dichroism induced by magnetoplasmonic interaction, which largely exceeds that of homogeneous noble metal nanoparticles in an external magnetic field, was found. Our results were also explained by reference to the quasi-static approximation, which, though it reproduces the main features of the absorption spectra, strongly overestimates circular dichroism in the cases we studied.

Strong magnetochiral dichroism of helical structures of garnet particles

We report on the occurrence of strong nonreciprocal magnetochiral dichroism in helical structures of magnetic garnet spheres, which emerges as a result of the simultaneous lack of time-reversal and space-inversion symmetries, by means of rigorous full-electrodynamic calculations using the layer-multiple-scattering method. It is shown that a strong effect appears in flat band regions associated with enhanced natural and magnetic optical activity.

Nonreciprocal guided modes in photonic crystals of magnetic garnet particles with a planar defect

It is shown that a planar defect in the stacking sequence of an all-dielectric photonic crystal of garnet spheres strongly supports localized optical guided modes, which originate from Mie resonances of the individual spheres. If the defect breaks space-inversion symmetry and the garnet particles are magnetized inplane, nonreciprocal and lossless transport of light on the defect plane, expected on the basis of group theory in the Voigt–Cotto–Mouton configuration, is demonstrated in ultrathin films of the defect crystal by means of full electrodynamic calculations using the layer-multiple-scattering method properly extended to photonic crystals of gyrotropic spheres.

Photonic structures of metal-coated chiral spheres

A detailed analysis of the optical properties of photonic structures of metal-coated chiral spheres, calculated by the full electrodynamic layer-multiple-scattering method, is presented. Easily tunable narrow bands, originating from particle-like plasmon modes of the metallic shells, hybridize with the extended bands of the underlying effective chiral medium and give rise to sizable partial gaps and strong band bending with consequent negative-slope dispersion. The photonic band diagram is discussed in the light of group theory, in conjunction with relevant transmission spectra, and the occurrence of polarization-selective transmission and negative refraction for a short range of angles of incidence is demonstrated.

Design, fabrication and characterization of suspended silicon nitride photonic crystal nanocavities

We report on the design, fabrication and characterization of PhC cavities in Si3N4 suspended nanowires. Electromagnetic simulations indicate that the structures are promising to achieve strong light localization in visible and near infrared region.

Helical assemblies of plasmonic nanorods as chiral metamaterials

We report on the optical properties of a layer-by-layer structure of silver nanorods, with their axes aligned perpendicular to the z direction and mutually twisted through an angle of 60° from layer to layer, by means of rigorous full electrodynamic calculations using the layer-multiple-scattering method, properly extended to describe axis-symmetric particles with arbitrary orientation. We analyze the complex photonic band structure of this crystal in conjunction with relevant polarization-resolved transmission spectra of finite slabs of it and explain the nature of the different eigenmodes of the electromagnetic field in the light of group theory. Our results reveal the existence of sizable polarization gaps and demonstrate the occurrence of strong optical activity and circular dichroism, combined with reduced dissipative losses, which make the proposed architecture potentially useful for practical applications as ultrathin circular polarizers and polarization rotators.