Metamaterials Design Based On Classic Concepts In Materials Science

Metamaterials are the artificial structures composed of sub-wavelength unit cells.The electromagnetic properties of metamaterials are mainly determined by unit cells instead of material attributes.By designing the structure of unit cell,we can get arbitrary electric or magnetic response and exotic phenomena and functionalities beyond natural materials,such as invisible cloak and perfect absorber,which will have great values in both military and civil applications.Macroscopically,treating metamaterials as an effective medium has been accepted widely.However,the relationship between sub-wavelength unit cell(meta-atom)and real atom is still ambiguous.It is of important significance to establish the microscopic physical pictures of metamaterials.In this paper,through theoretical calculations,numerical simulations and experimental measurements,we explore the systematic rule of metamaterials’ equivalent energy level.Furthermore,we will design and optimize a series of high performance metamaterials,such as multilayer metasurfaces,chiral metamaterials and dielectric-based metasurfaces,based on the classic concepts in materials science.The conclusions are listed below:(1)In view of equivalent energy levels,the characteristic frequency of meta-atoms can be equivalent to the energy level of real atoms so that we can establish the microphysical relationship between meta-atoms and real atoms.The characteristic frequency of meta-atoms has similar principles to the real atoms’energy level,such as energy level splitting,quasi-continuous energy band formation,convergence and overlap of energy bands.Hence,we get the intrinsic relationship between photonic transport properties in metamaterials and electric transport properties in real materials.(2)Based on the equivalent energy levels,we design a four-layer metasurface array which can convert the circularly polarized wave with high efficiency in a broad bandwidth.The simulated and measured results show the transmission coefficient of cross-polarization(opposite handedness of incident wave)is above 0.7,while co-polarization(same as the incident wave)is always below 0.2 within 9.8～13.5 GHz.Furthermore,we utilize the Pancharatnam-Berry(P-B)phase to reshape wavefront.A gradient phase metasurface array or polarization beam splitter is designed to demonstrate the abnormal transmission for different polarized waves.A broadband meta-lens is also developed to converge and diverge plane wave efficiently.With more flexible control of P-B phase,two more metasurfaces,a spiral phase plate and an axicon,are designed for high efficient beam transformation,from plane wave to optical vortex beam and Bessel beam,respectively.The simulated and measured results show all of the above applications are well inherited of high efficiency and broadband features.(3)Based on the concept of symmetry breaking in unit cell,firstly we design a planar chiral metamaterial consisting of double-layer SRRs to demonstrate asymmetric electromagnetic wave transmission of linear polarization.The simulated result shows the asymmetric transmission parameter is exceeding 0.47 from 8.0 to 9.0 GHz.Secondly,a quarter-wave plate is developed to convert linearly polarized wave into circularly polarized wave.The simulated result shows the amplitude of transmitted wave is above 0.8 within 10.6～13.2 GHz and the polarization ellipticity of transmitted wave is exceeding 40°,implying an approximate circularly polarized wave.Finally,based on the proposed quarter-wave plate,we demonstrate a multi-function chiral metamaterial which behaves as a quarter-wave plate or polarization converter for different incident direction of linearly polarized wave.The simulated and experimental results show the chiral metamaterial behaves as a quarter-wave plate(left-handed and right-handed circularly polarized wave,absolute value of ellipticity is above 38.5°)for propagation of linearly polarized wave(x-or y-polarization)in forward direction.Besides,the chiral metamaterial possesses high transmittance(>0.70)and broadband(10.7～13.2 GHz)features in the X-band.On the contrary,the chiral metamaterial behaves as a polarization converter for propagation of linearly polarized wave(x-or y-polarization)in backward direction.The polarization of transmitted wave with high transmittance(>0.88)will rotate 45° clockwise in a broad bandwidth(10.7～13.2 GHz).(4)Based on the concept of symmetry breaking in periodicity,we designed one-dimensional asymmetric dielectric gratings which can greatly improve the quality factor of resonance and enhance the localized field,facilitated to the interaction between light and matter.The one-dimensional asymmetric dielectric gratings can be severed as a gas sensor with a high sensitivity of S = 1157 nm/RIU and a high figure of merit(FOM)of 287.Furthermore,we designed a hybrid graphene/one-dimensional asymmetric dielectric gratings with reflectance modulation depth of 80%and 70%for TE and TM polarization,respectively,when the graphene Fermi level is changed from 0 eV to 0.8 eV.Finally,a hybrid graphene/two-dimensional dielectric metasurface with symmetry breaking is designed and demonstrated a left-handed passband modulation depth of 85%when the graphene Fermi level is changed from 0 eV to 0.8 eV.