Optical Metamaterials: From Hermiticity To Non-Hermiticity

In recent years,metamaterials,i.e.,artificial composite materials,by virtue of their extraordinary physical properties,have caused extensive research upsurge in the field of wave dynamics(particularly in optics).As the artificial composite structures can be arbitrarily designed,metamaterials can greatly break through the parameter limitations of the existing media in nature,which can make people manipulate the propagation of electromagnetic(EM)waves more freely.Generally speaking,the manners of manipulating EM waves based on metamaterials can be divided into two categories: one is relying on the real parts of metamaterials(no gain and loss),belong to Hermitian scope;the other one is relying on the imaginary parts of metamaterials(with gain and loss),belong to Non-Hermitian scope.In this thesis,we will discuss the Herimitian and Non-Hermitian metamaterials,and concentrate on gradient index metamaterials(GIMs)and zero index metamaterials(ZIMs)under Herimiticity,and PT symmetric metamaterials and conjugate metamaterials(CMs)under Non-Hermiticity,as shown in the following.1.The investigation of gradient index metamaterials under HermiticityFirstly,this thesis studies the composite structure of a metallic grating with the introduced GIMs,and find that the diffraction behavior of EM waves in such a structure follows a more generalized law: the modified generalized Snell law.Secondly,we introduce GIMs into a metallic parallel plate waveguide and design several waveguide devices using the guided mode conversion feature.Lastly,this thesis studies a mirrored Luneburg lens and find its function of retroreflector.Meanwhile,we employ the acoustic GIMs to design and fabricate such a device and test its functionality in acoustic experiments.Therefore,this design greatly enriches the applications of Luneburg lens.2.The investigation of zero index metamaterials under HermiticityFirstly,we reveal the unidirectional transmission in an array of prisms made of ZIMs and practically implement such a device using Dirac-cone like photonic crystals,where unidirectional transmission can function in a broadband of frequencies.Secondly,we focus on the defect properties inserted in a ZIM waveguide,and find that not only monopole modes are inside the defects but also the additional modes can exist in the defects.Lastly,we explore the inhomogeneous field in cavities of ZIMs,and find that such cavities can be used to manipulate the radiation of EM waves.3.The investigation of PT symmetric metamaterials under Non-HermiticityFirstly,we apply the concept of PT symmetry into ZIMs,and focus on the scattering properties of a ZIM waveguide with PT symmetry.We find that such a PT symmetric system can possess two exceptional points and support coherent perfect absorber(CPA)-laser modes and bidirectional total transmission.Secondly,we insert the dielectric defects with gain/loss into epsilon near zero metamaterials and utilize such a composite structure to design ZIMs with PT symmetry.Lastly,we combine PT symmetric metamaterials and ZIMs in a three-port waveguide system,and find that such a three-port waveguide system can support several asymmetric phenomena.4.The investigation of conjugate metamaterials under Non-HermiticityFirstly,we explore a purely imaginary metamaterial(PIM)(a special kind of conjugate metamaterials,CMs)slab and find that such a slab can support CPA modes,laser modes(even their coexistence)and perfect absorber modes.Secondly,we focus on a pair of PIM slabs and find that bidirectional negative refraction,planar focusing and unidirectional excitation can be achieved in such structures.Lastly,we introduce CMs into two dimensional cylindrical structures,and find that CPA modes,laser modes and their coexistence can be obtained for incident waves with angular momenta.