Generation Technology And Applications Of Quasi-nondiffracting Beam Based On Metasurface

All the existing electromagnetic waves has diffraction phenomenon when propagating in space.However,the degree of diffraction of electromagnetic waves is different under different conditions.Diffraction causes the electromagnetic waves to be more dispersed and the energy density is reduced when propagating,but there is theoretically spatial propagation.The presence of electromagnetic waves that do not diffract,also known as non-diffracting beams.There are many types of non-diffracting beams in theory,including Bessel beam(BB),X beam,Mathieu beam(MB)and so on.The main research object of this paper is Bessel beam,and a kind of theoretical solution is solved from the wave equation.Because it contains Bessel function,it is called Bessel beam.Since Duinin proposed and experimentally verified the existence of Bessel beam in 1987,it has become a research hotspot in the field of optics and electromagnetism.From the Bessel beam electric field expression,the ideal Bessel beam can carry infinite energy,so to produce an ideal Bessel beam also needs a source with infinite space size,which is unsatisfactory in practice.Therefore,in fact,we can only use a finite-size radiation source to produce a Bessel beam that does not diffract over a distance.This beam is also called a quasi-nondiffracting beam.According to the form of Bessel beam solution,it can be divided into two categories: the first type of zero-order Bessel beam;the second type is high-order Bessel beam.These two types of beams have different application requirements due to their different spatial field distributions.According to its different application requirements,we conducted specific research.The specific research contents of this thesis include the following aspects:1.Research on efficient generation of nondiffracting vortex beam.The high-order Bessel beam is generated by the electromagnetic metasurfaces to improve the diffusion characteristics of the orbital angular momentum vortex beam.The vortex electromagnetic wave carrying the orbital angular momentum can be used as a multiplexing technique that may be used for communication in the future because it can improve spectrum utilization and increase communication capacity.However,the existing vortex electromagnetic wave has obvious diffraction effect during the propagation process,and serious diffusion phenomenon occurs.Therefore,we want to overcome the general vortex wave diffusion characteristics by using the high-order Bessel vortex wave without diffraction.Combining electromagnetic metasurface energy to achieve precise regulation of electromagnetic waves,we propose a new method for efficiently generating high-order Bessel beams with reflective electromagnetic metasurfaces.Using the newly designed miniaturized metasurface structure,a second-order Bessel beam operating at 10 GHz was simulated and fabricated,and its working efficiency was as high as 57%.Compared with the previous research results,its efficiency was greatly improved.The simulation and test results verify the correctness of the proposed method and can generate high-order Bessel vortex beams that do not spread over a range of distances.2.Research on the generation of zero-order nondiffracting beam in any direction.Zeroorder Bessel beam applied to wireless energy transmission.The zero-order Bessel beam has its electric field strength that remains maximal on the transmission axis and theoretically remains constant during transmission.In practice,we can only use a source of finite size to produce a Bessel beam that does not diffract within a finite distance range.We use the zeroorder Bessel beam whose amplitude remains constant over a range of distances.Combined with the electromagnetic metasurface,it can realize high-efficiency,precise and controllable propagation direction in any direction.The nondiffracting beam generated by this method can achieve efficient transmission of wireless energy from a source to an arbitrary distance.The simulation verifies the correctness of the method.3.Research on the generation technology of nondiffracting multi-beam.The nondiffracting beam in any direction can only achieve one-to-one wireless energy transmission from source to load,and the generation of nondiffracting multi-beam can realize one-to-many wireless energy transmission from source to multiple loads.The principle is to use the nondiffracting beam generated in any direction,combined with the principle of superposition of electromagnetic waves,we can design an electromagnetic metasurface for generating nondiffracting multi-beam.We simulated and processed a reflective electromagnetic metasurface working at 10 GHz to produce a nondiffracting dual beam with beam directions of(30,0)and(30,180),respectively.Simulation and test results show that electromagnetic energy can be efficiently and accurately radiation in the set direction.This approach provides a new solution for one-to-many wireless energy transfer.4.Generation of non-diffractive hollow beam and nondiffracting localized hollow beam generation.For the energy stability of the nondiffracting beam over a range of distances,energy stable transmission in the range of no diffraction distance can be realized,but the local hollow beam can realize the control of the beam energy distribution in the range of no diffraction distance.The set area achieves a very small or even zero energy density.This feature has great practical significance for the application of wireless energy transmission.The high-order Bessel beam is also a kind of local hollow beam.We use the zero-order non-diffracting beam generated in any direction previously,combined with coordinate transformation technology,to generate high-order Bessel beams radiating in any direction.The first introduction to the generation of high-order Bessel beams in any direction.