Negative Refraction And Local Field Enhancement In Sub-wavelength Metal Periodic Structures

Understanding the interaction between light and matter and manipulating light and electromagnetic waves at will have always been the people's pursuit, and also been the essential issues in science and technology. With the advance of technology, micron and even nano-scale metal periodic structures can be fabricated. When electromagnetic waves illuminate these structures with sub-wavelength size, there will appear a series of new phenomena or effects, such as "metamaterials" and surface plasmons. Metamaterials are artificial composite or structured materials constructed by periodically arranging special geometric elements, which exhibit properties not found in naturally occurring materials or compounds. Wave propagation in metamaterials with simultaneously negative dielectric permittivity and magnetic permeability can exhibit exotic properties, for instance backward-wave transmission, negative refraction, evanescent wave amplification etc. The electromagnetic properties of matematerials depend highly on the element's geometry. This flexibility provides a novel concept for us to design new materials with extraordinary electromagnetic properties, and also open a new channel to manipulate and control the light and electromagnetic waves.Metamaterials are usually based on metallic structure units since it is convenient to use metal to realize structure with various shapes or patterns. However, conducting loss is the major drawback limiting their applications. Metamaterials with simple structures for easy preparation and excellent performance (low loss, broadband, solid, uniform isotropic) are desired. In this thesis, several novel structured metamaterials working respectively in microwave, terahertz and infrared frequencies are designed aiming at achieving negative refraction with lower losses and clarifying related mechanisms therein. Moreover, metallic structures based on metamaterials with tunable, stable and large local field enhancement are explored for possible applications as substrates of surface-enhanced spectroscopy, aiming at breaking the bottleneck hindering wide applications of surface-enhanced spectroscopy, namely, the poor reproducibility of signals with traditional substrates due to the lack of control in metal particles produced by chemical process.The main progresses and innovation of this work are listed as follows: (1) We investigated the electromagnetic responses and mechanisms of double split-and closed-ring resonators at normal-to-plane incidence. It is found that the double ring metamaterials exhibit respectively antisymmetric and symmetric modes in lower and higher frequencies due to the resonance coupling of the inner and outer rings. By combining the cut-wire type electric resonance in both outer and inner rings excited by external electric field and the negative magnetic response associated with antisymmetric mode, negative refraction can be achieved in the double-ring metamaterials at normal-to-plane incidence.(2) We designed an isotropic-like fishnet metamaterial in terahertz frequencies. The influences of its main geometry parameters on both the electromagnetic response and the left-handed property are investigated. An effective LC circuit description for magnetic resonance is employed to explain the dependence of left-handed frequency band on geometry parameters. Compared to the current "metal/dielectric/metal" fishnet metamaterials with rectangular holes, this structure is insensitive to polarization configurations, exhibits a large figure of merit (FOM>4) and has a high transmission (T>80%).(3) The physical mechanism that the left-handed behavior occurring together with the extraordinary optical transmission (EOT) in fishnet metamaterials is explored. It is found the left-handed band is adjacent to the EOT peak that is associated with waveguide resonant mode, and the EOT effect can be used to reduce the losses and improve the figure of merit in metamaterials. The influence of metal-layer thickness on magnetic resonance and left-handed performance in fishnet metamaterials with rectangular-hole or cross-hole originate mainly from the dependence of waveguide resonant mode on the hole's size and depth.(4) A metal-dielectric-metal sandwich structure perforated by an array of asymmetric cross holes (i.e. both arms in cross hole have different length or width) is proposed and a novel planar metamaterial with dual left-handed bands is demonstrated. The tunability of dual left-handed bands by changing the arms' length or width is investigated, and the mechanism is elucidated with a theoretical model. Furthermore, by investigating the corresponding electric field distributions, the maximum field enhancement factor of 12 is achieved within the dielectric layer between two metal plates, meaning a Raman signal enhancement of～104 for probe molecules located at these positions.(5) We investigated the plasmon resonance properties at normal incidence and the large field enhancement effects from different plasmon excitations in asymmetric double split rings (ADSRs) arrays. The ADSRs exhibit two intense excitation peaks in the near-infrared and visible regions, corresponding to first-and second-order resonance modes. By exciting different resonances, the electric field can be effectively localized and enhanced in the same area, i.e. the gap regions, in both the infrared and the visible for ADSRs with different asymmetries. The field "hotspots" are demonstrated to be at the tips or the edges with the maximum field enhancement close to or over 103, corresponding to a infrared absorption and Raman enhancement factor of about～105 and 1012～1013 respectively. The local field enhancement can be further enhanced by decreasing the gap's width and optimizing the tips'ends of split rings.