The Design And Application Of Metasurfaces

Controlling the electromagnetic(EM)wave propagation and scattering is essential for many real-world applications,such as communication,signal processing,EM imaging,radar technique and deep space exploration.Metasurfaces,which show the major advantage of desirable EM parameters over the naturally occurring materials,recently have endowed people with great power to manipulate the EM wave.Metasurface comprising a monolayer of engineered subwavelength meta-particles can provide phase discontinuities across the surface,thus the EM wavefront can be completely manipulated at will.The ability to tailor the electromagnetic EM wave has been significantly improved with the help of the emerging concept of metasurface,making the EM devices with compact packaging,planar version,conformal configuration and diverse functionalities.Metasurfaces play an important role on pushing the development of EM science and technology,and have been a hot spot of research.With the rapid development of metasurface,the study of metasurface have gradually changed from initial theoretical exploration to the current state of both theory and application reserach,aiming to accelerate their applications for real-world devices.Researchers are now exploring the design of multifunctional,reconfigurable,and tunable metasurfaces,which could be applicable for complex EM environments and varying demands of EM wave control.Now,the reconfigurable metasurfaces are being a hot spot of research.On this regard,this thesis mainly focuses on the design principle of tunable,reconfigurable and active metasurface,and their applications to focusing lens,polarizer and asymmetric transmission,etc.Meanwhile,we have also proposed some passive metasurfaces aiming to the realization of broadband low-scattering coding surface,highly directive antennas,one-way focusing lens and bi-functional surface,etc.The main content of this thesis are as following:(1)We proposed an actively tunable Huygens' metalens by employing voltage-controlled varactors into the resonating meta-atoms,whose individual phase responses can be dynamically and continuously manipulated at will.In particular,we demonstrate that multiple focal spots can be realized simultaneously at distinct spatial positions,and that can be moved along pre-designed trajectories,with high efficiency and fast response time,by simply changing the spatial distribution of control voltage applied to each varactor.(2)We proposed a new strategy to realize dynamic control of the asynmetric transmission for certain linearly polarized EM waves at microwave frequencies.Through elaborately designing a chiral meta-atom incorporating active component of PIN diode,without much extra complexity in the structural design and optimization,we are able to dynamically control over the transmission property.Benefit from the transient state of the PIN diode as the bias voltage gradually changes,the asymmetric transmission can be continuously and flexibly manipulated at will in real-time according to the external voltage stimuli.In addition,by introducing the grating structures into the element design,the performance of the active chiral metasurface have been obviously improved.(3)We try to employ the geometric phase concept to the digital metasurface design,which only contains a single structured meta-particle to achieve either 1-bit or multi-bits coding elements that has distinct scattering behaviors for different circularly polarized EM waves.Such coding metasurface can be either used for directive radiation with regular coding sequences or broadband diffuse-like backward scattering with randomized coding sequences.We have also reported the design of a diffusion-like coding metasurface based on highly optically transparent and electrically conductive material(indium tin oxide,ITO)deposited on transparent and flexible back-grounded substrate that shows broadband reduction of backward scattering in microwave frequencies,which can be an improvement or additional functions to conventional coding metasurfaces.(4)We have proposed passive metasurface for the improvement of antenna directivity.By using Huygens' phase compensation metasurface,the directivity and the impedance bandwidth has been simultaneously improved.Besides,the directive radiation and diffusion-like backward scattering have been integrated into one metasurface design by using bi-isotropic metasurface elements.(5)We have introduced the phase gradient into the conventional chiral metasurface design,and thereby proposed the concept of gradient chiral metasurface.Based on this kind of metasurface,we have realized several useful functionalities,such as one-way abnormal refraction,one-way focusing lens,etc...