The Study On Optical Properties Based On The Magnetic Responses From Asymmetric Metamaterials

Metamaterials are artificially ordering composite structures or composite materials,they can exhibit many extraordinary physical phenomena which natural materials cannot do.The word of "metamaterials" was first proposed by Prof.Rodger M.Walser at the Texas State University in 1999.In recent decade,metamaterials became the interational front area of physics,electromagnetic field,and materials due to their unique characteristics.Several research progresses have been regarded as one of the ten most significant discoveries in science community in 2003 and nature community in 2006,respectively.Due to metamaterials can break through some of the traditional limitations of well-known physical laws and thus is promising for potential applications in the communication engineering,biological engineering,medical imaging,biological sensing,etc.Metamaterials are generally made of metal.It is well known that metal can reflect and absorb most of the electromagnetic wave when the electromagnetic wave is irradiated on the metal.However,some different electromagnetic responses will be excited in the metallic metamaterials due to its unique sub-wavelength structure when the electromagnetic wave is irradiated on the metamaterials.Because of near-field effect,the coupling will occur between these different electromagnetic responses which will lead to electromagnetic wave propagation and excite several novel optical phenomena in the metamaterials such as electromagnetically induced transparency(EIT)and circular dichroism,etc.These novel physical phenomena are very useful for slow-light,filtering,and sensing potential applications.In this thesis,first of all,we elaborate the effective medium theory,development and prospect of metamaterials.Secondly,we introduce the concepts,features and applications of some typical electromagnetic responses in metamaterials,such as surface plasmon polariton(SPP),Fano resonance,bright and dark modes.Finally,based on these electromagnetic responses in metamaterials,we design metamaterials with different structures and explore the unusual characteristics of the electromagnetic coupling which is excited due to the asymmetry of the unit cell in metamaterials.In diffident frequency bands,we give a deep exploration on generation mechanism,characteristics and applications of some specific optical phenomena in different asymmetric metamaterials.The research work is mainly for full-wave numerical simulations by commercial packages based on the finite-element method(FEM).The main contents of this thesis are summarized as follows:1.Designed a kind of U-shaped metal split ring resonator(SRR)structure,observing the electromagnetic response by changing the direction of incident electromagnetic wave.The analogue of electromagnetically induced transparency(EIT)phenomenon will excite by the suitable geometric parameters in this U-shaped metamaterial,and we will explain the physical mechanism of this EIT-like phenomenon in detail.2.The central symmetry of SRR is broken through introducing rotational asymmetry in the U-shaped metamaterial.The dark mode will be excited with the symmetry breaking,and the EIT-like phenomenon will occur at the microwave frequency range.The intensity of the EIT-like effect will be tuned by adjusting the asymmetry of the U-shaped metamaterial.This result was demonstrated by microwave experiments.3.A kind of metamaterials by asymmetric ring-disk structure was designed.The toroidal dipole will be excited in this metamaterial,whose localized magnetic field distribution has a loop characteristic.We will explore the physical mechanism of the EIT-like effect by coupling between the toroidal dipole(bright mode)and the traditional magnetic response(dark mode)in the near-infrared region.4.Exploring the polarization manipulation in the U-shaped metamaterials that can excite the EIT-like phenomenon.The linearly polarized incident light and the circularly polarized one can realize the polarization conversion between them,by adjusting the geometric parameters and the asymmetry of the metamaterial in the near-infrared region.5.We propose a dual-chiral metamaterial composed of bilayer asymmetric split ring resonators(ASRR)with a mirror-symmetry shape.It is demonstrated that the circular dichroism(CD)can be excited and get an enhancement in the terahertz regime.Moreover,the CD can be further improved by modulating the asymmetry of ASRR.