Theoretical Research On Phase Modulation Of Terahertz Wave Based On The Surface Of Medium Metastructure

Terahertz(THz)wave,which is a kind of electromagnetic radiation with special frequency region,and the wavelength range lies between the microwave and infrared bands.Hence,the research of THz radiation is at the intersection of electronics and photonics,and a series of special properties have attracted wide attention by researchers.In recent decades,the developments of ultra-fast laser and micro-nano scale semiconductor technology have provided advanced solutions for the research of THz wave sources and detectors,and THz spectrum and imaging technology has been applied in many fields.For instance,chemical analysis,environmental testing,security check,non-destructive testing,military communication and biomedicine.At present,the research and application of THz wave emitting and receiving modules are relatively mature.However,for some traditional THz devices in the middle of the THz system,the complexity of structure brings many disadvantages,such as bulk and poor stability.Therefore,searching a simple and feasible THz device solution has become the focus for current research.Recent years,with the continuous development of nano-processing technology,some emerging micro-nano photonics research fields have produced.Specially,the proposal and development of metasurfaces provide novel design ideas for the realization of light-thin and stable electromagnetic devices.Optical metasurface is a two-dimensional artificial material that arranged in a planar space by subwavelength scale resonators.Based on the resonator elements can be analogized to "meta-atom",which realizes the modulation of electromagnetic wave characteristics due to the dipole resonance with incident electromagnetic waves.To date,researchers have employed metasurfaces to many photonic devices,such as ultrathin lenses,optical vortex converters,optical wave plates,beam deflectors,and polarization beam splitters.Furthermore,they have also been employed to realize some optical processes,such as meta-hologram,the generation of photon spin Hall effect,the excitation of surface plasmon polaritons.Beyond above,researchers also apply them to THz devices.Nevertheless,these designs are mainly based on metal-type resonance elements,which inevitably brings huge ohmic loss,and greatly reduces the working efficiency of the devices.In addition,the incomplete symmetry of resonance elements leads to polarization conversion to THz devices,which reduces the device performance.Therefore,it is particularly vital to design the metasurface composed of highly efficient and polarization-independent resonator elements to implement some THz devicesWe first proposed a transmissive THz metasurface,consisting of periodically arranged subwavelength silicon(n=3.4)cross resonators.Based on the theory of electromagnetic dipole resonance,via optimizing the structural parameters and directions of the cross resonator,almost full phase 2? control at target THz frequency is achieved,and the transmission reaches over 90%.Moreover,based on the above work,we proposed a transmissive THz metasurface composed of subwavelength silicon post resonant elements with full-phase shift.Owing to the dipole resonance between the elements and incident THz waves,the phase gradient of the transmitted waves is obtained,and a broadband THz beam deflector with bandwidth of 0.3 THz is realized.Simultaneously,the transmission of resonant element modulation is above 94%.Finally,we arranged 16 silicon post resonant elements with high transmission(?94%)and full-phase 2? modulation at the incident wave frequency of 1.0 THz,and they are arranged with wavefront phase along the lens radial gradient to obtain a thin THz flat lens with a hyperbolic phase profile.For the above two devices,the cylindrical structure of post elements has completely symmetrical characteristic,resulting in the polarization states of the deflected and focused beams remain basically unchanged,thereby achieving high-quality device performance.All above designs and results were verified via numerical simulations based on the finite element method(FEM).These feasible solutions also provide the new ways for the improvement of THz system.