Metasurface-based Dynamic-controlling And High-efficient Wavefront-shaping Terahertz Functional Devices

Recently,researchers are working on speeding the practical application of terahertz.To achieve this,high-performance terahertz functional devices,especially tunable and highefficient terahertz functional devices are indispensable.Metamaterials offers a powerful solution.However,compared with optical and infrared regimes,high-performance terahertz functional devices,especially tunable and high-efficient terahertz functional devices are in high demand.In this thesis,we mainly focus on metasurface-based dynamic-controlling and high-efficient wavefront-shaping terahertz functional devices,as listed below:1.We home-setup and developed all-fiber terahertz time domain spectroscopy(AFTDS),including the single-mode-fiber(SM)and polarization-maintaining-fiber(PM)versions.Besides,we also developed photoconductive-probe-based high-resolution fiber near-/farfield scanning imaging system.These systems provide experimental platforms for my later research work.2.We experimentally investigated the mode transition effect based on capacitive and conductive coupling in ring dimers.Such effect is proved to be general.Based on this mode transition effect,we experimentally realized active modulation of the transmission spectrum by using cryogenic superconductor niobium nitride(Nb N)as the junction between the dimers.These results are useful in designing plasmonic sensor and terahertz optical switch.3.We designed a Fano-resonance-based coherent perfect absorber(CPA).This CPA can serve as a dual-band CPA under TE oblique-symmetric incidence.Besides,the output amplitude can be effectively tuned from 0 to almost 1 by only varying the relative phase difference between the two coherent inputs.The designed CPA could find potential application in terahertz imaging and terahertz optical switch.4.We proposed a coherent Pancharatnam-Berry phase optical element in the terahertz regime,which can simultaneously support nearly 100% conversion efficiency and unitary output.We then experimentally verified the high-efficient and dynamic control over the deflection efficiency using a spin-dependent linear-phase-gradient metasurface.Our method provides a potential route to high-efficient and active-controlled optical spin-orbital interaction(SOI),and has potential applications in generating structured wave states.5.Polarization-independent and polarization-multiplexing all-silicon terahertz wavefront controlling metasurfaces were proposed and experimentally demonstrated.The efficiency of the structural layers of the polarization-independent metasurfaces are high up to82.5%.It is worth noting that these polarization-insensitive metasurfaces can be easily combined to form composite devices with extended functionalities.The efficiency of the structural layers of the polarization-multiplexing metasurfaces are high up to 65%.Furthermore,to suppress the reflection loss of silicon substrate,a high-performance polarization-independent anti-reflection layer consisting of silicon posts was also proposed.Such all-silicon dielectric metasurfaces can be very promising in developing next-generation high-efficient and ultra-compact terahertz functional devices.