Study On Terahertz Sub-diffraction Focusing Planar Lens Based On Optical Super-oscillation

THz imaging technology plays more and more crucial role in biomedicine,nondestructive-detection,industrial inspection,public security and so on.However,restricted by the diffraction limit of traditional optical elements,the existing terahertz imaging system cannot realize high resolution which lead to many limitations in the application.Although,relatively high resolution terahertz image can be obtained through near-field scanning and digital image processing,there are inevitably problems such as short working distance and complex subsequent processing.Therefore,carrying out the study of terahertz sub-diffraction focusing planar lens with property of lightweight and long working distance,in order to break through the diffraction limit and solve the problem of low imaging resolution from the aspect of optical devices,is becoming the research hotpots in recent years.Based on optical super-oscillation technology,here we demonstrate a binary phase terahertz sub-diffraction focusing planar lens with long focal length optimized by particle swarm optimization algorithm under the phase modulation pattern and the vector angle spectrum theory.The planar lens is designed for circularly polarized terahertz waves,where the wavelength ? is 118.8?m,the lens radius is 160?,the focal length is 420?,and the numerical aperture is 0.356.The focusing performances of the lens in the focal plane are obtained by theoretical calculation,results show that the full width at half maximum of focal spot is 1.179? which is less than the diffraction limit(DL)of 1.404?.In addition,the planar lens generates a sub-diffraction optical needle with length of 21.7? in the optical field along the propagation direction.Through studying the micro-fabrication technology,the binary phase modulation terahertz sub-diffraction focusing planar lens with long focal length was fabricated by inductive coupled plasma etching process.Finally,a system was built up for testing the focusing performance of the lens,and the results showed that such a planar lens produced an optical needle with depth of focal length(DOF)of 33? and the sub-diffraction length is 19.7?.The minimum focal spot with the full width at half maximum of 1.212? was found at the focal length of 444.36?,breaking through the diffraction limit of 1.476?,and which verified the far-field sub-diffraction focusing feature of this planar lens experimentally.The main works of this paper are as follows:(1)Investigated the research status of terahertz super-resolution technology,and analyzed the existing key problems.Proposed the research purposes and contents of this paper.(2)Researched the theory of sub-diffraction and optical super-oscillation,and sub-diffraction focusing realizing methods based on optical super-oscillation technology.Introduced the angular spectrum theory and particle swarm optimization algorithm method.(3)The lens structure was designed through optimizing the transmission function of binary phase sub-diffraction focusing planar lens,based on the angular spectrum theory and particle swarm optimization method.And analyzed the focusing performance theoretically.(4)The planar lens was fabricated by inductive coupled plasma etching micro-maching process.Built the optical systems and completed the testing and analyzing of the lens' focusing performance.