Reconfigurable Metasurfaces For Wavefronts Control-application To The Generation Of Complex Beams

In the field of electromagnetics,how to flexibly control the characteristics of electromagnetic waves(EM)to serve the communication and electronics industries has been a very challenging research topic.The introduction of the concept of artificial metasurface provides a new idea to tailor electromagnetic waves.Metasurface,a two-dimensional artificial structure composed of subwavelength elements,can achieve the flexible manipulation of phase,magnitude,and polarization of EM waves.Nevertheless,the functionality of generally designed passive metasurfaces is limited due to the fact that the resonance of the constituents are fixed once the passive metasurface is fabricated.The narrow frequency band of operation also limits the application of passive metasurfaces.However,reconfigurable metasurfaces,where the resonance of the constituents can be adjusted by changing the external stimuli,overcome these limitations,thus laying the foundation for the application of metasurfaces in engineering such as wireless communication and wireless energy transfer.In this dissertation,two kinds of reflective reconfigurable metasurfaces,a one-dimensional(1D)one and a two-dimensional(2D)one,are designed in the microwave domain by using electrical mechanism through the incorporation of varactor diodes.These two metasurfaces allow a modulation of the reflection phase,only along one axis in the case of the 1D reconfigurable metasurface and along two axes in the case of the 2D reconfigurable metasurface.While the unit cells on the1 D metasurface are controlled in a row by row manner,those of the 2D metasurface can be individually addressed.Five different applications of complex beams,including directional beaming of diffracted electromagnetic waves from a subwavelength aperture,Airy beam,zeroth-order Bessel beam,vortex beam,and holographic imaging,are explored based on these two metasurfaces.The details of the five tested applications are summarized as:1.The directional beaming of electromagnetic waves diffracted through a subwavelength aperture is proposed.This work constitutes the first one to realize the dynamic directional beaming from a subwavelength aperture using a reconfigurable metasurface,especially in the microwave domain.The on-axis beaming of electromagnetic waves from the subwavelength aperture is realized by applying symmetric phase profiles to the two parts of the 1D reconfigurable MS surrounding the subwavelength aperture.The onaxis beaming is also performed at different working frequencies to validate the frequency agility property of the metasurface.In addition,the off-axis beaming in different directions is realized by applying asymmetric phase profiles on the two sides of the subwavelength aperture.2.The generation and manipulation of the diffraction-free Airy beam by applying the1-bit coding scheme on the reconfigurable metasurfaces are studied.Indeed,the reconfigurable metasurface can be used as a 1-bit coding metasurface by appropriately adjusting the voltage applied to the metasurface.Both the one-dimensional and twodimensional Airy beams are dynamically generated and manipulated.Firstly,the influence of Airy function parameter on the 1D Airy beams generation is explored before validating the generation over a wide frequency band.The diffraction-free,self-bending,and selfhealing properties of Airy beams are also investigated.The steering of Airy beams is studied to further show the flexible regulation ability of the designed 1D reconfigurable metasurface.Secondly,the 2D Airy beams generation is also studied and analyzed using the 2D reconfigurable metasurface.The 2D Airy beams are also generated in a wide frequency band.3.The flexible generation and control of zeroth-order Bessel beams based on reconfigurable metasurface are studied.Two far-field radiation modes of the zeroth-order Bessel beams are reported,which are the single beam mode for the small phase tilt angle scenario,and the dual-beam mode for the large phase tilt angle scenario.Both the 1D Bessellike and 2D Bessel beams are explored.The arbitrary control of the diffraction-free zone is studied by using different phase configurations on the 1D metasurface.The far-field patterns of different phase configurations are explored and the reason for the difference among them is clearly explained.The diffraction-free and self-healing properties of 1D Bessel-like beams are also analyzed.Furthermore,the beam steering and flexibly control of 1D Bessel-like beams are realized.The dynamic control of diffraction-free areas of 2D Bessel beams is also explored.4.The dynamic generation and manipulation of vortex beams carrying orbital angular momentum(OAM)are explored.Three different types of single vortex beams radiating in the direction normal to the metasurface,including Laguerre-Gaussian beam,focusing vortex beam,and non-diffracting vortex beam,are explored.By comparing the three types of vortex beams with the same mode,the diffraction-free vortex beam is shown to have the smallest divergence angle.Then,the generation of non-diffracting vortex beam over a wide frequency band is achieved.Besides,the simultaneous generation of the quad-mode vortex beams in different directions is demonstrated.5.The digital holographic imaging generation and multibit spatial energy modulation are studied using the 2D reconfigurable metasurface.The dynamic generation of holographic imaging using the modified weighted Gerchberg-Saxton(GSW)algorithm is validated.Three different types of holographic imaging,including focusing spots,Arabic numbers,and Alphabet letters,are explored at different detecting planes and operating frequencies.Besides,the multibit spatial energy modulation in different channels is proposed and validated.It is the first time that multibit spatial energy modulation in different channels is achieved.