Research On Metasurface With Long Depth Focus For Passive Millimeter Wave Imaging

In this paper,we study the long depth focus of metesurface for passive millimeter wave imaging,focusing on how to elongth the depth of focus and maintain high lateral resolution.The research is carried out from two directions.Firstly,the holographic algorithm is used to shape the focal spot shape to achieve the focal spot with the characteristics of long focal depth.The second is to realize polarization multiplexing.The metasurface is composed of birefringence elements,which can control the electromagnetic wave of orthogonal polarization independently.These two methods are analyzed from different aspects and do not affect each other.For meeting different application requirements,this paper choosed different forms of unit structure to achieve reflective and tranmissive metalens.We compared the effect of GS,GSW and improved GSW algorithm in optimizing the holographic phase.In order to achieve greater adjustment ability of electromagnetic wave field distribution in longitudinal direction,the improved GSW algorithm is used to optimize.The reflected metasurface adopts the "Y" shape element and adjusts the size.When one linear polarization realizes one phase,its orthogonal polarization can cover 330 degrees phase ranges.Then,we analyze the focusing effect of different feeding methods.For the 0 degree incident,the asymmetry of the supersurface electric field is caused by the asymmetry of the upper and lower parts of the y-shaped element.By changing the arrangement of the element,the whole supersurface array achieves the symmetrical effect,and the focal spot shape is more round.For oblique feeding,the direction of the incident wave is 45 degrees above the metasurface,and the focal spot formed is on the axis directly in front of the metasurface.The numerical aperture corresponding to the front and rear nodes of the 3dB focal depth is 0.6 and 0.17,respectively.In order to improve the efficiency of transmittance metasurface,the multi-layer structure can increase the phase coverage under the condition of the same amplitude,and the 3-layer unit structure can achieve the 360-degree phase coverage under the condition of-3db amplitude.diH unit structure,two orthogonal work,corresponding to two polarized work mode· equation,changing the two horizontal lines of a "H" can realize the independent control of the polarization phase of a line,without affecting the transmission coefficient of orthogonal polarization.The focal depth with a numerical aperture range of 0.14 to 0.75 is realized in the 10 GHz transmission imaging table,and the focal spot size 300 mm away from the metasurface is 0.71 times the wavelength,and the focal spot size at 1200 mm is 2.74.Both reflective metasurface and transmittance te were simulated by theoretical calculation and CST STUDIO SUITE simulation.The transmittance metasurface was also verified by a fabricated model and tests.The near-field distribution of electric modulated by the metasurface was tested,and the corresponding numerical aperture range before and after the focal depth was 0.56 to 0.17,efficiency of x,y oriented polarization in 10 GHz is 48% and 51%,respectively.The test results are consistent with the theoretical calculation and simulation results,which proves the feasibility of the design method.