Research Of Spherical Negative-index Metamaterials At Optical Frequencies

Negative-Index Metamaterials (NIMs) are new man-made electromagnetic materials with a negative index of refraction (NIR).Since these metamaterials enable a variety of novel applications such as superlens,optical nanocircuits and cloaking, marvelous progresses on the NIMs have been made in recent years.Along with the rapid development of NIMs,it has been a new task to research three-dimensional NIMs at optical frequencies.Motivated by this background,we analyse and study all dielectric nanospheres and nanoshells.It has been demonstrated theoretically that all dielectric nanoshells would exhibit negative index of refraction at optical frequencies.Firstly, fundamentals and applications of negative-index metamaterials have been presented.Based on the detailed understanding on the progress of NIMs at home and abroad, the research target and content are proposed.Secondly, methods on construction of three-dimensional negative-index metamaterials at optical frequencies studied by other researchers are summarized.For three-dimensional geometric construction method, a structure constructed by split-ring resonators and metal wires has been investigated using full wave simulation to verify its feasibility at microwave frequencies.After comparison of each method, Mie-exptended effective theory is selected for our research theoretical basis.Thirdly, a new method to achieve the magnetic and electric responses at optical frequencies is proposed.It is modeled by all dielectric nanospheres,on the foundation of the Mie-extended effective medium theory.For dielectric nanospheres,a negative effective permeability originates from the strong magnetic dipole resonance.And an electric dipole resonance leads to the negative effective permittivity.On the condition of different radius,the number density, the permittivity of the nanospheres as well as the permittivity of the host medium,magnetic/electric responses have been obtained through numerical simulations.Meanwhile,the impact trend of these parameters on the resonant frequencies and effective permeability has been described qualitatively.Finally, characteristics of all dielectric nanoshells have been researched.The theoretical result shows that a material composed of all dielectric nanoshells would exhibit equivalent negative refraction.According to the law summarized in the above, compromise between the material parameters and the magnetic/electric responses has been made in order to couple these two resonances within overlapped frequencies. The dielectric core results in the magnetic response, and the corresponding electric response is induced by the shell with higher dielectric constant. Numerical simulations are used to achieve a negative index of refraction of-1.2 at 265THz, which verify that three-dimensional NIMs can be realized with all dielectric nanospheres. This provides a practicable way for the construction of three-dimensional NIMs at optical frequencies.