| The cylindrical vector beam is a vector beam whose polarization state is distributed in a cylindrical symmetry.This unique polarization characteristic makes its characteristics and application value in sub-wavelength focusing favored by the majority of researchers.According to the spatial distribution characteristics of the electric field component of the cylindrical vector beam,it can be divided into radial polarized light,rotationally polarized light and generalized cylindrical vector beam.Up to now,scientific researchers have proposed many different methods for adjusting the focal field of cylindrical vector beam,But there are certain limitations.For example,focusing through a high numerical aperture requires a larger numerical aperture,and the conversion efficiency of the beam exiting from the middle of the aperture at the focal point is low;when focusing by a parabolic mirror,it is necessary to achieve focusing under paraxial conditions,and the incident light and The reflected light is on the same side of the lens and cannot achieve different measurement focusing;the plasmon lens has a better focus on the radial polarized beam due to the polarization dependence of its plasmon excitation,which forms the polarization state of the focused beam Dependence.How to further achieve the tight focusing of the cylindrical vector beam,break through the diffraction limit,and break through the limitations of the polarization state,realize the focusing structure and means are effective for both radial and rotational polarized light,and at the same time reflect the advantages of simple structural design and lower process cost,It is the main research goal of this thesis work.Firstly,using the equivalent negative refraction effect of the first order diffraction of the alldielectric grating,inspired by the design of the photonic crystal negative refraction plano-concave mirror structure,a cylindrical symmetric negative refractive index focusing lens structure is designed to achieve the focusing of the cylindrical vector beam,And embodies a good focusing effect.Secondly,the all-dielectric grating plano-concave mirror based on the-1st order diffraction wave is analyzed and optimized.The equivalent negative refractive index of the-1st order diffracted wave is calculated,and the exit profile curve of the grating is derived using the principle of equal optical path,and the relationship between the horizontal and vertical coordinates of the grating step is determined.The plano-concave mirror structure with refraction effect achieves good focusing on radial polarized light and rotationally polarized light.By adjusting the polarization component of the incident beam,combining different preset focal length structures,adjusting the degree of polarization of the outgoing light and adding a blaze structure to the exit surface,the adjustment of the focus focal field morphology is achieved.The focus is focused at the same time,increasing the longitudinal depth of focus and improving the energy utilization efficiency of the focal field.Thirdly,the influence of the material and geometric parameters of the grating on the focusing effect is analyzed in depth,and the parameters are optimized to improve the focusing effect.A comprehensive calculation and analysis of the influence of different materials refractive index,different equivalent negative refractive index,and different preset focal lengths on the actual focusing effect.The results show that the refractive index of the material strongly affects the peak electric field intensity of the focal spot.As the refractive index of the material increases,the peak of the electric field intensity of the focal spot increases;as the equivalent negative refractive index value changes,The size decreases first and then increases,and when the equivalent negative refractive index is-1,the focus size is the smallest;with the change of the preset focal length,the focus size is constantly changing.The smaller the preset focal length,the smaller the focus size.Finally,the effects of blazed grating structure,higher order diffracted wave and total reflection effect on focusing effect under different refractive index conditions are studied,and the planoconcave mirror structure is optimized on the basis of it.The grating lens after adding the blaze step has little effect on the shape of the focusing field,but it can greatly improve the energy efficiency of the focal field.The maximum change in the peak value of the electric field intensity can reach 1.76 times without the blaze step.High secondary diffracted waves are the main cause of secondary focus.The structural details of this part can be modified and optimized to effectively suppress the secondary focus.The secondary focal spot after the structure optimization disappears,the focal field distribution becomes smoother,and the focal field The field strength has also been further improved,the maximum change in the peak value of its electric field strength can reach 1.41 times.Theoretically deduce and verify the possibility of total reflection,and provide quantitative parameter reference for the design of grating topography.The research results in this paper show that the negative refraction effect is the prerequisite for achieving tight focusing.The negative-refractive grating plano-concave mirror can break through the limitation of the polarization state and achieve the simultaneous focusing of radial and rotationally polarized light.The structure design and the actual preparation process are introduced.The optimization analysis of structural parameters,the addition of blazed structure,and the analysis of advanced order diffraction provide a way and method for further optimization of the structure and improvement of the focusing effect.The work in this paper will provide a reference for the design of related micro-nano structures,provide a reference for the vector beam focusing control method,and also have potential application value in light field control,micro-nano processing,sensing and other application fields. |
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