Study On Phase Manipulation Of Huygens-Geometric Metasurface And Its Applications

Metasurfaces composed of sub-wavelength scale unit cells show unique superiority in wavefront manipulation,in which phase manipulation is the core of wavefront manipulation.The all-dielectric Huygens metasurface can achieve phase manipulation of 0 to 2πby changing structural parameters while maintaining high transmittance.However,accurate phase manipulation depends on machining accuracy.The phase manipulation of geometric metasurface is achieved by changing the rotation angle of unit cells.It is easier to control the azimuth angle than adjust the structure size during manufacturing,while the optical polarization conversion efficiency of geometric metasurface is relatively low.Aiming at the accuracy of phase modulation and the polarization conversion efficiency at optical wavelengths,this thesis mainly focuses on all-dielectric Huygens-geometric metasurfaces,by arranging the well-designed anisotropic Huygens meta-atoms corresponding to the target phase profile,efficient phase manipulation can be realized.Its applications in beam deflector,metalens,optical vortex generation,and metasurface hologram are also discussed.The research contents of this thesis are as follows:（1）A Huygens-geometric metasurface based on two magnetic dipoles is designed,which can achieve delicate phase modulation accuracy while maintaining high transmission efficiency.A theoretical model supporting two magnetic dipoles is developed to analyze the transmission property of the designed Huygens-geometric metasurface,and the required conditions for zero-backward scattering are analytically derived.The feasibility of the theoretical model is verified by exploring the electromagnetic response characteristics of the composite structural units using the design of an anisotropic Si-Si O₂-Si meta-atom.The simulation shows that the polarization conversion efficiency of the meta-atom reaches 98.8%and can support the excitation of two magnetic dipoles.By changing the rotation angle of meta-atoms,a geometric phase manipulation of 0 to 2πcan be obtained,which can be applied to beam steering devices such as beam deflectors and metalenses.The influence of phase modulation accuracy on the performance of meta-devices is discussed,and a beam deflector with deflection efficiency of 92.2%is achieved.This provides a new idea for the design of high-performance meta-devices.（2）An efficient focused optical vortex generator and perfect optical vortex generator are designed based on the all-dielectric Huygens-geometrical metasurface.Anisotropic Huygens meta-atoms can excite resonance modes of polarization-dependent parity,achieving polarization conversion of incident circularly polarized light at visible wavelength（633 nm）with an efficiency of 93%.The spatial arrangement of meta-atoms according to a specific phase profile enables the reconstruction of the optical wavefront,including the generation of high-efficiency focused optical vortex and perfect optical vortex whose ring radius is independent of the topological charges.Using spatial degrees of freedom,a polarization-controlled perfect optical vortex generator is designed by the superposition of two sets of meta-atoms.High-efficiency and tunable optical vortex generators are promising for promoting the construction of integrated and multi-functional optical systems,and have great application potential in the field of optical communication and nanoparticle manipulation.（3）A transmissive broadband achromatic metalens with large numerical aperture is designed.This metalens adapts Si-Si O₂-Si three-layer structure,whose geometric phase and transmission phase can be manipulated by rotating and changing the structural parameters of the well-designed meta-atoms.The design of the composite structure can provide a sufficiently large of phase dispersion control range for eliminating chromatic aberration,thereby enabling achromatic focusing with a large numerical aperture.The achromatic metalens with a numerical aperture of 0.68 in the visible region（420 to 700 nm）is simulated.The elimination of chromatic aberration and diffraction-limited focusing performance are achieved while a large numerical aperture is obtained.By encoding the helical phase of a n optical vortex into the phase profile of an achromatic metalens,the generation of a broadband achromatic focusing optical vortex is realized.The designed broadband achromatic metalens with a large numerical aperture have significant application value in imaging and display.（4）A design method of metalens for generating polarization-wavelength multiplexing holographic image is proposed.The focal points of the metalens are regarded as the pixels of a holographic image.By using the quasi-continuous contour of the focal points,the patterned holographic image with sub-wavelength resolution can be reconstructed.Switchable holographic images under different incident polarization are realized by utilizing the ability of polarization-dependent phase manipulation brought about by anisotropic meta-atoms;different responses to incident wavelengths and color hologram are obtained by utilizing the resonance frequency changes caused by different structural parameters of meta-atoms;a six-channel metasurface color hologram is achieved by combining the three degrees of freedom of phase,polarization,and wavelength.This function of storing different information at different polarizations and wavelengths has multiplied the information storage capacity,and has potential application prospects in fields such as information engineering,optical encryption,and color display.