Active Terahertz Wavefront Modulations Based On Liquid Crystals And Metasurfaces

In recent years,terahertz(THz)technology has received increasing attention due to its potential applications in high-speed wireless communications,security inspections and biomedical testing,etc.Compared with the well-researched THz emitters and detectors,THz modulators,especially tunable devices,are not yet mature.The THz band exceeds the cutoff frequency of many semiconductor amplifiers and mixers,making existing RF devices no longer compatible in this band.Therefore,to develop photonic-based THz wavefront modulators is urgent.Initial researches reported the refractive or diffractive THz elements made of polymers or crystals.Unfortunately,their bulky architectures and single function distinctly restrict the miniaturization,integration and multi-functionalization of THz systems.Liquid crystals(LCs)possess broadband birefringence and excellent external field induced tunability.LC-based THz filters,phase shifters and waveplates can realize the dynamic control of THz intensity,phase and polarization.Their drawbacks lie in the slow response,large insertion loss and small modulation depth.Recently,metasurfaces,which are subwavelength artificial electromagnetic microstructures,have gradually become a hot spot in the study of THz wavefront modulation.Such devices possess the characteristics of small size,flexible and diversified functional design,but lack tunability.This thesis,focused on the active THz wavefront modulation,presents an in-depth study on the pure LC-based and LC integrated metadevices.By fully exploring the electric field tunability of LCs,the diversity of LC domain structure orientations and the design flexibility of metasurface,some key issues of existing THz wavefront modulators are expected to be addressed.The main research results are as follows:(1)LC and liquid crystal polymer(LCP)based THz modulatorsVia introducing the concept of geometric phase and designing a checkerboardshape spatial multiplexing lens phase mask,we propose a flat THz lens that can achieve the spin-selected focusing property in a broadband.The lens is fabricated by the photopatterning to control the local LC orientation.The switchability of focusing is demonstrated with an external bias applied on the transparent graphene electrodes.Due to the inevitable substrate loss,slow response and high operating voltage of LC devices,we further propose the LCP-based planar THz photonic elements.Through encoding the geometric phase into the LC orientation and polymerizing it under ultraviolet light,LCP modulators with specific functions are obtained.Their advantages include nonsubstrate self-supporting,dynamic functions via mechanical deformation and distinguished stability.Polarization control,beam deflection,tunable focusing,vortex and Bessel beam generations are experimentally demonstrated.(2)LC integrated metallic metadevices for THz filteringBy integrating LCs with metasurface,the tunability of LCs and flexible design of metasurface can be fully explored.Firstly,we design a Fano-resonance metasurface which is sensitive to the orthogonal linear polarizations.Then,cascaded with an electrically tuned LC polarization converter,we achieve a large modulation depth over 50% within a broadband of 660 GHz in both the FDTD simulation and experiment using the THz time-domain spectroscopy(THz-TDS).It is promising in ultra-sensitive biosensing due to the high quality factor of Fano resonance.Secondly,in order to avoid the issues caused by the excessive LC thickness up to hundreds of microns,we propose a design that integrates 5-?m-thick LC layer with the metasurface and sub-wavelength wire grid electrode,where the LC serves as the environmental medium with tunable refractive index.The wire grid electrode functions as a polarizer to select the incident polarization.We realize the electromagnetically induced transparency analog and an absorber in the transmission and reflection mode separately,dependent on the incident polarization direction.Upon biasing,the modulation depth in the transmission and reflection mode reaches 37% and 15%,respectively.A fast response of 2.4 ms when switching the bias is achieved.(3)LC integrated dielectric metadevices for THz lensThe inherent ohmic loss of metallic metasurface limits the modulation efficiency.We further introduce the dielectric metasurface,which is combined with LCs,to achieve dynamic and efficient THz metalenses.Firstly,a polarization-dependent dielectric metalens is designed using the FDTD simulation.Its focal lengths are different with incident x-and y-polarizations.Combined with the electrically tuned LC waveplate,we realize the tuning of the focal length.Secondly,cascading the photopatterned LC polarization grating with the metalens,the dynamic addressing of the focal spot is realized according to the incident polarization and applied bias.In addition,via integrating the switchable geometric phase of LCs with the resonant phase of metasurface,we realize the active tuning of chromatic dispersion for the first time.When the bias is OFF,a broadband achromatic focusing from 0.9 to 1.4 THz is achieved.With a 75 V square wave signal,the focal length decreases as the frequency increases.The average efficiency is 30% within the broadband.The THz imaging with this lens verifies its active chromatic dispersion.Further,we numerically exhibit a THz beam deflector with tunable dispersion using the same design,which proves its versatility.