Study On Terahertz Longitudinally Polarized Super Resolution Focusing Device

The terahertz wave,due to its unique low photon energy that presents high penetrability on organism and various non-metallic materials without causing damage,has broad application prospect in nondestructive testing,material identification,imaging and other fields.However,traditional terahertz focusing lens such as parabolic mirror and spherical lens are mostly heavy and restricted by diffraction limit.The meta-surface structure can flexibly modulate the phase,amplitude and polarization of electromagnetic waves.The meta-surface lens designed based on this advantage and combined with the principle of optical super oscillation is not only light and thin,but also can achieve superresolution focusing.Despite this,most of the existing terahertz meta-surface focusing lenses have low apertures with the focal spot size close to the diffraction limit and difficult to compress.Radially polarized light has remarkable tight focusing characteristics after being focused by a high numerical aperture lens.Its strong longitudinal electric field component is easier to compress the focal spot size,which is significant to improve the resolution of terahertz microscopy.Therefore,for the terahertz wave with a wavelength of 79.36 ?m,this thesis proposes a terahertz radially polarized light super-resolution focusing device that integrates polarization conversion and super-resolution focusing functions.The main work of this thesis is as follows:(1) Introducing the application background and current research result of radial polarized light focusing devices and terahertz focusing devices.Proposing the research of this thesis based on the deficiencies of existing devices.(2) Introducing relevant theories and optimizing methods of half-wave plates and terahertz radial polarized light focusing devices.(3) Optimizing eight sets of superatoms by using the particle swarm optimization algorithm and the electromagnetic wave Finite Time Domain difference algorithm.The relative initial phases uniformly cover the range of 0-2?.According to the amplitude and phase of the eight groups of superatoms,the design of device employs the particle swarm optimization algorithm and the vector angular spectrum diffraction calculation method.The device radius R=390 ?,the focal length f=130.1 ?,and the numerical aperture NA=0.95.The design results show that the lens converts linearly polarized light into radially polarized light and focuses at z=130.1 ?.The focal spot has a FWHM of 0.467 ?which is below the diffraction limit of 0.526?.The sidelobe ratio is 14%,making the device achieving super-resolution focusing.The result of simulation verification completed by COMSOL Mutiphysics shows that at z=130.1 ?,the FWHM of the focal spot is 0.45 ? and the sidelobe ratio is 16.8%,proving that the device achieves superresolution focusing.When blocking the light at the center of lens and the light-blocking radius is 130 ?,the simulation results of COMSOL Mutiphysics show that at z=130.1 ?,the FWHM of the focal spot is 0.432 ? and the sidelobe ratio is 15.6%.The focal spot size is further reduced in this case.Analyzing the effect of processing error on the focusing performance of the device,the results show that this can be ignored when the lateral machining error is ±1 ?m.(4) Developing the processing plan according to the feature of device and characterizing the processed device.The results show collapse in the device superatoms with collapsing area accounting for 1/4 of the total area.When performing focus testing on the device by the established experimental system,results show that two focal spots are formed in the x direction at z=130.1 ?.In this case the FWHM is 1.3 ? and the sidelobe ratio is 14%,indicating a big difference between experimental results and theoretical simulation results.Combined with the result of device characterization,the vector angular spectrum diffraction calculation method is used to calculate the focused light field after the random collapse by 1/4.The calculation shows that two focal spots are formed in the x direction at z=130.1 ?.The FWHM is 1.07 ? and the sidelobe ratio is 32%.This result is consistent with the experimental results,which explains the cause of gap between experimental results and design results.