From extraordinary optical transmission to left-handed fishnet metamaterials

Abstract

In this work, I describe the theory of the negative electric response exhibited by metals and negative magnetism in artificially fabricated nano-structures. We show the interconnection of the surface plasmon polaritons and extraordinary optical transmission phenomena with the performance of a left-handed fishnet metamaterial, and we present the results of our experimental and numerical studies on the transmission of light through periodic, quasi-periodic and chirped size apertures as well as studies of the properties of tri-layer fishnet structures. Specifically, we study experimentally the transmission of light through periodic and chirped lattices of nano-holes perforated in an optically-thick gold film. We observe that the periodicity of the structure enhances the light transmission for specific wavelengths, and we analyse this effect theoretically by employing finite-difference time-domain numerical simulations. Furthermore, we demonstrate experimentally the possibilities for manipulation of the spectral transmission in quasi-periodic and chirped lattices consisting of square nano-holes with varying hole size or lattice periodicity. We also study the light transmission through a metal film perforated with an array of square apertures of varying size. We combine near-field optical microscopy with confocal microscopy to identify the surface plasmon polariton modes and the propagating hole modes in the different sized apertures. We study experimentally the transmission properties of tri-layer fishnet metamaterials in the near-infrared spectral range and analyse the change of the transmission resonances at varying angles of incidence and different input polarisations. The results show that the main transmission peak through the fishnet is due to the excitation of hole modes. This high transmission region is significantly influenced by surface plasmon polariton coupling, when the incident electric field has a component normal to the metal plates, while little change with respect to tilt is observed, when the electric field is parallel to the two metal films of the fishnet. We analyse numerically the influence of the dielectric substrate on the optical response and effective macroscopic parameters of fishnet metamaterials. We show that the substrate can suppress the region of the negative refractive index due to the weakening of the gap plasmon polariton coupling associated with the magnetic resonance of the structure. We also demonstrate that the effect of substrate vanishes, when the number of functional layers increases. We analyse numerically the optical response and effective macroscopic parameters of fishnet metamaterials infiltrated with a nematic liquid crystal. We show that even a small amount of liquid crystal can provide tuning of the structures due to the reorientation of the liquid crystal director. This enables switchable optical metamaterials, where the refractive index can be tuned from positive to negative by an external field. This switching is primarily determined by the shift in the cutoff wavelength of the holes, with only a small influence due to the change in the gap plasmon polariton dispersion. This work deepens the understanding of the transmission and negative refraction mechanisms in multilayer fishnet geometries and it should assist in the development of metamaterial research and, in particular, in the design of the tunable left-handed structures.