Study On The Structure Design And Tunable Mechanism Of Metasurface

Through the artificial design of the geometric structure and size of electromagnetic metamaterials,new material properties will be completely different from the constituent materials,which can achieve arbitrary permittivities and permeabilities and become one of the current research hotspots.Metasurfaces,as its two-dimensional representation,allow electromagnetic waves to propagate in a desired state and along a prescribed path at subwavelength thicknesses,with easier fabrication and lower loss.Compared with traditional optical devices,miniaturized and integrated devices can realize more powerful functions such as ultra-high-speed and large-capacity transmission on a smaller platform,while reducing power consumption and saving energy,which has broad application prospects in the age of intelligence and information.At present,there are the following problems in the metasurfaces structural design and device application:First,there is a complex mapping relationship between the periodic element structure,constituent materials and device functions of metasurfaces,that requires a large number of model design,repeated iterations and numerical simulations,means high time cost and design complexity.Second,limited to the properties of the materials,the efficiency of tunable metasurfaces is generally low,and the lack of sufficient flexibility restricts the development of reconfigurable devices and multifunctional devices.In response to the above problems,we carried out research on the structural design and tunable mechanism of metasurfaces.The specific contents and innovations are as follows:(1)Design of quarter-wave plate structure based on electric dipole resonance.Most of the metal transmission quarter-wave plates in the previous literature are explained by the principle of plasmon resonance,which need to consider the influence of each structure on electromagnetic waves and the interaction between the structures.We use Mie resonance to explain the mechanism,the periodic structure is regarded as an integral unit,and the scattering of the wave plate unit is solved.This scheme does not need to consider the interaction of each antenna structure in the unit,and the parameters that have a decisive influence on the phase saltation can be determined through numerical analysis.Based on electric dipole resonance,the 2×2 holes array structure with a thickness less than λ/50 was designed,and realized a phase difference of 90° in the orthogonal direction with a bandwidth of more than 500nm centered at 1550nm;at 1550nm,the linear polarization-circular polarization conversion efficiency is close to 100%,and the transmission efficiency is 0.44.The thickness of the optical device is effectively reduced,widen the working bandwidth of the waveplate,and improve the robustness of the structure to the aperture size,effectively alleviate the performance degradation caused by the fabrication error.(2)Universal design rules for phase metasurfaces based on Fabry-Perot resonance,and efficient deflectors and beam splitters.The root cause of a large number of iterations and repeated calculations in numerical simulation is that the ability of metasurface periodic antenna patterns to manipulate electromagnetic waves is not clear and the mapping is complex.Starting from the working mechanism of the resonant phase metasurface,we propose a universal design rule:according to the design logic,by synergistically considering the relationship between the parameters of the constituent materials,the geometrical parameters and the performance of the metasurface,the "effect" of various parameters on the beam manipulation is presented in multidimension,which greatly reduces the time cost of device design and improves device performances.Based on this rule,the number of undetermined calculation parameters of the meta-unit is reduced by more than 50%,and the effect of the rule is verified through device designing:1.Visible waveband deflector,the+1st-order diffraction efficiency exceeds 0.5,and the deflection angle ranges from 37° to 61°;2.Near-infrared broadband deflector,centered at 1550 nm,the bandwidth is greater than 700 nm,and the deflection angle at 1550 nm is 57°,the reflectivity is 0.98;3.The center-symmetric trapezoidal beam splitter,it has a maximum conversion efficiency of over 95%in the near-infrared.The design idea of this rule is clear,and the designed deflector and beam splitter have excellent performances.At shorter wavelengths where there exists 2nd-order diffraction,metasurface has the problem of low 1st-order diffraction order conversion(strong 0-order,2nd-order diffraction),and the universal rule achieve excellent performance by avoid this waveband.The structure is improved based on Fabry-Perot resonance,which can improve the performance at shorter wavelengths,realize the polarization-dependent beam manipulation function,and design a beam splitter with variable output power ratio.The improved structure changes the wavelength and width of resonance peak,so that the phase distribution in the period at shorter wavelengths will be more ideal,the 1st-order deflection efficiency is significantly improved.That is,under the x-polarization,the output light is mainly deflected along the+1st order of y-direction,while under the y-polarization,the output light along the-1st order of y-direction mainly.In most areas of the 1000-2500nm,an equal-power beam splitter can be realized by rotating the polarization direction of the incident light;and when the wavelength is fixed,an adjustable beam splitter with a wide range of output power ratios can be realized by changing the polarization angle.(3)Through the independent control of reflection amplitude and phase,the simulation realizes the band-tunable phase gradient metasurface and amplitude control device:1.The material response model assists the device structure design and performance prediction:The Drude model and the carriers drift-diffusion equation are used to calculate the mapping relationship between different initial doping concentrations of indium tin oxide,applied voltage,incident wavelength,and material permittivities.Giving the realization conditions of three optical properties of indium tin oxide:dielectric-like,epsilon-near-zero region,and metal-like.Explaining the phase tunable mechanism of indium tin oxide through equivalent optical circuit elements,and using the RLC oscillator circuit model to calculate the response of the material frequency.2."Binary grating-like" metasurface based on plasmon resonance:The wavelength of the plasmon resonance peak has a great correlation with the material parameters and the size of the nano-antenna.We use semi-infinite phase gradient metasurface with different strip widths.The simulation achieves a phase distribution of 0-2πin the period.,as a deflector at 1550nm,the reflectivity exceeds 0.6,the+1 st order diffraction conversion rate is close to 100%,and the deflection angle is 60°.3.Simulation verification of indium tin oxide electronically controlled phase gradient metasurface:the problem of narrow working waveband and wavefront distortion of "binary grating-like" metasurface can be compensated by electrical control,and the working bandwidth can be widened to the original two times,the range of reflection amplitude modulation nearly 50%at 1420nm.