Research On Terahertz Phase Modulation Devices Based On Liquid Crystals

In recent years,with the development of terahertz(THz,1 THz=10¹² Hz)generation and detection technology,THz technology shows the great advantages and application prospects in sensing,imaging,spectrum,and communication.For THz applications and developments,the research on THz functional devices is critical,including THz phase modulation devices such as polarizers,phase shifters,and wave plates.In order to satisfy the requirements of THz applications,developing the high-performance and controllable THz phase modulation devices are highly in demand.Recently,the metasurfaces,which are composed of subwavelength metal or dielectric units periodically or quasi periodically arranged on the thin chip,provide new approaches for THz phase modulation devices.In this dissertation,on the basis of the summary on the progress of THz phase modulation devices both at home and abroad,a few of THz phase shifter and THz wave plate based on metamaterials have been theoretically and experimentally investigated.The properties of materials in the THz frequency range have been studied,and the simulation,designing,fabrication and experimental test of devices have been studied comprehensively.This paper includes the following several part:1.A dielectric metasurface with line-square compound lattice structure was designed and fabricated.A electromagnetically induced transparency?EIT?effect and a large artificial birefringence effect can be achieved in this compound metasurface on experiment.When the frequency is larger than 0.8THz,the artificial birefringence of this metasurface is over 0.6.The simulated results show that the mechanism of this effect is due to the mode coupling and interference between the strong resonance mode?called as bright mode?in the square subunit and the weak resonance mode?called as dark mode?in the line subunit.2.On this basis,we prepared an electrically tunable THz LC phase shifter based on dielectric metasurface with compound lattice.The experimental results show that its tunable phase shift under the biased electric field reaches 0.332?,1.8 times higher than the bare silicon at 0.7THz,which confirms the enhancement role of THz microstructure on the LC phase shift in the THz regime.3.Two kinds of switching quarter-wave plate based on double layer graphene gratings was designed:equal-periodic grating and periodic gradient grating,which can covert linear polarization state to left-circular polarization state at the central operating frequency 1.4THz with its effective bandwidth of 0.25 and the central operating frequency 1.0THz with its effective bandwidth of 0.38,respectively.It is investigated that the additional phase distribution of the gradient structures can significantly enhance the phase difference between two orthogonally polarized components and reduce the central operating frequency.Moreover,the performance of the devices,such as co-polarized and cross-polarized amplitude transmissions,phase differences between orthogonal components,and the corresponding ellipticities,operating in both ON and OFF states was analyzed.4.A broadband tunable quarter-wave plate based on electrically controlled the graphene gratings with LCs was proposed.By electrically controlling the molecular director of the LCs and the Fermi level of the graphene grating,the switching characteristics and tuning characteristics of this broadband tunable quarter-wave plate can be obtained in a wide frequency range of 0.7-1.6THz.