| Metamaterials are artificial structured materials composed by artificially designed subwavelength unit cells,which can be regarded as artificial atoms.Through controlling the electromagnetic response of subwavelength unit cells,people can arbitrary control the macroscopic electromagnetic properties of bulk metamaterials.Therefore,people can control the propagation direction,amplitude,phase,polarization and other degrees of freedom of electromagnetic wave on demand.However,the narrow and fixed working frequency of metamaterials greatly limits the development of metamaterials in practical applications.Among them,the electromagnetic response of the metamaterial depends on the structural parameters of the metamaterial unit cell and the composed material properties.Studying the relationship between the electromagnetic response and the parameters of the metamaterial structural unit cell is crucial to realize the tunability of the metamaterial properties.The tunable metamaterial can greatly expand the application of metamaterial in various fields,such as micro-nano source,lens,sensor and other devices.In this doctoral thesis,we study the electromagnetic response of the artificial sub-wavelength structure mainly through numerical simulation and theoretical derivation,and the physical mechanisms of the response are also analyzed which enables to design the smart metamaterials with tunable response.The work of this paper mainly includes the following aspects:1.Study on fluid soft metamaterials with electric field induced structure tunability.1)We have investigated the metamaterials based on fluid system composed by gold nanoparticle suspension.Based on this system,we have designed a soft infrared metamaterial absorber composed by gold nanorods dispersed in liquid crystal(LC).Through simulation and effective medium theory,we theoretically investigate its total absorption character.As the results show,the nanorods align with the LC molecule and the gold nanorods/LC hybrid exhibits different permittivity as a function of tilt angle of LC.At a certain tilt angle,the absorber shows an omnidirectional total absorption effect.2)We have also proposed an electrically controllable soft optical cloak based on a fluid system containing gold nanorods,which can be transformed from isotropic to anisotropic epsilon-near-zero(ENZ)state at a certain incident optical frequency due to the orientation of gold nanorods under an external electric field stimulus.Both effective medium theory and 3D finite element simulations demonstrate that,at the ENZ point,the scattering from arbitrary-shaped objects can be nearly perfect suppressed.The loss and aspect ratio of gold nanorods have an effect on the ENZ point and scattering suppression behavior.The flexible,controllable,and multi-frequency responsive characteristics make the fluid metamaterials possess potential use in soft smart metamaterial devices.2.Study on electrically tunable metasurface based on graphene structure.1)We have investigated the graphene based tunable metasurfaces.We have designed a graphenebased terahertz metasurface with tunable spectrum splitting function.This metasurface consists of two different trapezoid graphene ribbons patterned in opposite direction which can cover nearly 2π phase region for reflected wave with high efficiency in different spectral region.So the metasurface can reflect different frequency waves to totally different directions.By changing the Fermi level of graphene ribbons,the response frequency of the proposed metasurface can be adjusted,as a result,tunable spectrum splitting can be realized.2)We also investigate the interaction of graphene metasuface and charged particles and design a graphene metasurface to full control Smith-Purcell radiation.Not only can we strongly enhance the efficiency of Smith-Purcell radiation,but also the amplitude,phase,and polarization state of the radiated wave can be fully manipulated through proposed graphene metasurface.By tuning the Fermi level of the graphene metasurface,we can tune the intensity of the radiated wave.Our findings provide a new way to design electron-beam-induced light sources as well as particle detectors with high efficiency and compact footprint.3.Study on the electrically tunable metamaterial composed by resonant unit cells.1)We have investigated the relation between the resonance behavior of metamaterial unit cell and its structure and composed material properties.We have designed a metasurface based on electrically tunable Mie-type resonators.With the metal film and ion gel film as electrodes,the permittivity of disk-like ferroelectric resonators can be adjusted by applied external electric field,as a result,we can tune the Mie resonance of the resonators.Theoretical research demonstrated that the reflection phase of the resonators can be dynamically tuned in a wide range.By programmable controlling the electric field strength applied on resonators of metasurface,a 3600 phase ramp can be realized and thus the arbitrary reflection behavior of incident waves with varied frequency is obtained.Because of the excellent tunability,this metasurface can also be used to design adaptive metasurface lens and carpet cloak with tunability for incident waves.2)We proposed a novel strategy to design deep sub-wavelength thick metasurface with full 2πtransmission phase modulation and high transmission efficiency by applying resonators with interfering Fano resonance.We have obtained the analytical expression of the relation between the transmission coefficient of the unit cell and the structure and material parameters.We can continuously adjust transmission phase from 0 to 2π through changing the geometric parameters of resonators and construct a deep sub-wavelength metasurface with the resonators to manipulate transmission wave with high efficiency.The proposed strategy may provide an effective guidance to design a deep sub-wavelength metasurface for controlling transmitted wave with high efficiency... |
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