| Metamaterials with artificial subwavelength elements can achieve unusual electromagnetic properties that are inaccessible in naturally occurring materials.In recent years,manipulating polarization of electromagnetic waves has attracted considerable attention based on metamaterials.Chiral and anisotroptic metamaterials can realize cross-polarization conversion due to the cross coupling between the magnetic and electric responses,which is of great significance to the integration development of polarization devices.On the other hand,the study of tunable properties also possesses considerable flexibility in terms of structure design in metamaterial,which promises important applications such as optical switches,modulators and filters.In this thesis,we mainly investigate cross-polarization conversion and tunable properties in metamaterial theoretically and experimentally.Several different metamaterial structures have been constructed and these structures’ eletromagnetic properties and physical mechanism have been discussed systematically utilizing the full-wave electromagnetic simulations.The main contents and acquired innovative achievements are as follows:(1)We have proposed a split-square-wire mematerial structure.The simulation results indicate that the cross-polarization conversion can be achieved in the proposed structure for both x and y polarized wave,and it is valid for reflected and transmitted waves simultaneously.The designed single-layer metamaterial structure has many advantages including ultra broadband,simple structure and low fabrication cost.(2)We have designed a bilayered chiral metamaterial consisting of asymmetric split ring apertures and analyzed in details the cross-polarization conversion properties and asymmetric transmission phenomenon.The experiment and simulated results are in a good agreement.The results show that cross-polarization conversion coefficient is more than 90%and relative bandwidth of asymmetric transmission is larger than 50%.Further,we investigate the dependence of the cross-polarization conversion properties on the structure’s asymmetry and interlayer coupling.As a result,metamaterials with reasonable structural parameters can realize a high asymmetric transmission.(3)We have demonstrated a kind of optically controlled terahertz switching based on the cross-polarization conversion principle.The simulation results manifest that the hybridized chiral metamaterial allows us to effectively tune the cross-polarization transmission by controlling external optical stimuli,and the tunability of the resonant frequencies between resonances Ⅰ and Ⅱ is about 11%.The photoexcited mode-switching effect can be well understood by surface current distributions at resonant frequencies corresponding to the cross-polarization transmission peaks.The optical control cross-polarization efficiency can be engineered by modifying interlayer coupling effect,and the appropriate dielectric layer thickness can achieve an optimal switch effect.(4)We have presented a temperature controlled terahertz switch based on an insulator-to-metal transition in a vanadium dioxide metamaterial.The simulation results reveal that the switch structure with incorporated VO2 can hold cross-polarization transmission of 50%and the modulation depth of up to 96.2%.The dependence of the temperature controlled switching property on structural geometric parameters is analyzed in details,and the optimal switching performance can be also achieved by reasonable parameters matching. |
