Temperature and magnetic field tunability of composite metamaterials containing spherical semiconductor particles in far-infrared and terahertz regimes

The effect of temperature and dc magnetic field variations on effective electromagnetic parameters of metamaterials (MTMs) containing spherical semiconductor particles is studied theoretically. The effect of temperature is taken into account through its influence on semiconductor carrier density and mobility. The effect of dc magnetic field is included using an extension of the Mie theory, describing the interaction of a plane wave with a gyrotropic sphere. The effective parameters such as relative permittivity and permeability are calculated by proper application of the Maxwell Garnett (MG) theory and its extensions to quasi-static condition and multi-phase structures. Based on these theories, the temperature and dc magnetic field tunability of three different MTM structures is investigated. First a single phase medium is considered which contains spherical semiconductor particles of one kind, randomly dispersed in a dielectric host. Then two multi-phase structures containing (a) two kinds of spherical semiconductor particles or (b) spherical particles with core-shell topology are investigated. The two multi-phase MTM structures can exhibit negative index of refraction in far-infrared spectral region. The measure of the temperature and dc magnetic field tunability of effective parameters such as relative permittivity and refractive index of the structures is evaluated and it is shown specifically that the real part of refractive index can be tuned to get negative, zero or positive values in far-IR or THz regimes, but the imaginary part of the index and the Figure of Merit (FOM) are also quite sensitive to the temperature and magnetic field variations. The tunable MTMs can find new applications in THz devices such as switches, tunable mirrors, isolators, converters, polarizers, filters and phase shifters.     

Electromagnetic transparency and slow light in an isotropic 3D optical metamaterial, due to Fano-like coupling of Mie resonances in excitonic nano-sphere inclusions

A three dimensional isotropic metamaterial is proposed and theoretically studied, which is composed of excitonic spherical nanoparticles in a dielectric host and exhibits electromagnetic transparency and slow light effects in optical regime. The approach is different from the conventional methods of realizing classical Electromagnetically Induced Transparency (EIT) or plasmon induced transparency effects, which are usually based on the interaction of dark and bright states of the medium plasmonic constituents. Instead, it is based on the Fano-like coupling of Mie resonances in the spherical inclusions, resulting from sharp and strong excitonic resonance of the particles. Using the Extended Maxwell Garnett effective medium approximation for calculating the effective electromagnetic parameters of the proposed metamaterial structure, it is shown that EIT-like effects can be produced, such as steep normal dispersion profiles within narrow transparency windows, resulting in high values of group index of refraction on the order of several thousands and figure of merit values around 10, near the excitonic resonance of the nanoparticle inclusions in UV region.     

Effective response and scattering cross section of spherical inclusions in a medium

The Maxwell-Garnett theory for a right-handed homogeneous system is extended in order to investigate the effective response of a medium consisting of low density distributed 3-dimensional inclusions. The polarisability factor is modified to account for inclusions with binary layered volumes and it is shown that such a configuration can yield doubly negative effective permittivity and permeability. Terms representing second-order scattering interactions between binary inclusions in the medium are derived and are used to reformulate conventional effective medium theory. In the appropriate limit, the one-body theory of Maxwell-Garnett is recovered. The scattering cross section of the spherical inclusions is determined and comparison is made to homogeneous dielectric scatterers in the Rayleigh limit. It is found that the scattering resonances can be manipulated using the inclusion parameters. Furthermore, the effect that two-interacting spherical inclusions in a medium have on the scattering cross section is investigated via higher order dipole moments while the issue of reducing the scattering cross section to zero is also examined.