A Research Of Terahertz Beam Controller Based On Metasurfaces

Terahertz waves are located in a special position between microwave and infrared bands on the electromagnetic spectrum,and have unique advantages that electromagnetic waves in other frequency bands do not have.Its applications are mainly summarized in three aspects:communication,imaging and spectrum.It is used in military,security,astronomy,Biomedicine and other fields play an important role,so it is highly valued by countries around the world.Due to the lack of materials that can produce a strong electromagnetic response to terahertz waves in nature,the regulation of terahertz waves is still a major difficulty.This article focuses on this problem and focuses on the terahertz beam control device based on metasurfaces.The main research contents are as follows:(1)A perfect reflection metasurface of terahertz is designed,the working frequency is 0.34 THz,and the reflection efficiency can reach 90.3%when the reflection angle is66°.The metasurface is arranged by 3-bit coding units,and the coding sequence is designed according to the phase distribution required for impedance matching.In addition,the factors that affect the reflection efficiency are studied,including the initial phase,coding sequence and array size,and the design process of the perfect reflection metasurface is optimized.(2)A tunable reflection-type terahertz wave focusing device based on vanadium dioxide metasurface is designed.Utilizing the phase transition of vanadium dioxide,the phase response of the unit differs by 180°before and after the phase change under the condition that the size of the structure remains unchanged,forming a 1-bit metasurface.Then arrange the elements according to the phase distribution required for beam focusing,and use the Fresnel diffraction integral formula to calculate the electric field distribution of the focal plane.The focal positions(0,0,950),(500,2400,1900)and(-300,1200,1100)(unit:μm)are taken as examples to verify the correctness of the design method.(3)Preliminary exploration of the realization of terahertz reconfigurable metasurface based on vanadium dioxide is carried out.Aiming at the problem of achieving independent controllability of each unit,the reliability of the light control method is studied.Experimental results show that vanadium dioxide can undergo phase transition after being irradiated with a continuous laser source with a power density of 4.5 W/cm~2for several seconds.Only by using a high-power laser source for short-time irradiation can the effect of thermal diffusion be reduced.Finally,according to the experimental results,a rewritable metasurface with memory effect is proposed and the working principle is explained in detail.Additionally,an electrically actuated,broadband THz switch is presented,which transitions from a transparent state with low reflectivity,to an absorptive state for which both the reflectivity and transmission are strongly suppressed.For a modulation depth of 0.65,the effective bandwidth is 0.13 THz centered at 0.63 THz.