Study On Design Method And Fabrication Technique Of Subwavelength Azimuthally Polarized Metallic Gratings

Azimuthally polarized light is a kind of cylindrical vector light.On the beam cross section,the polarization at each point is along the tangential direction.Azimuthally polarized light has important applications in laser welding,laser drilling,atomic guidance,optical tweezers and super resolution imaging.Using inner cavity method to produce azimuthally polarized light has advantages that the structure is simple and utilization of light energy is high.Grating mode selecting is one kind of inner cavity method.An azimuthally polarized grating is used as the rear mirror of the laser cavity.The grating can directly selects the azimuthally polarized light to form the oscillation when the laser is generated,and restraining the radial polarized light at the same time.Subwavelength azimuthally polarized metal grating can be used in industrial 10.6?m carbon dioxide laser as the rear mirror of resonant cavity.By selecting the polarization,the laser can output azimuthally polarized light.Compared with circularly polarized light,the laser processing efficiency of azimuthally polarized light can be improved by 1.5-4 times.Subwavelength azimuthally polarized metal gratings should be designed to have high polarization selection characteristics,and the working band should cover the excitation waveband of carbon dioxide laser.At the same time,it is necessary to study the fabrication technique and quality control of the grating to ensure that the grating can meet the design objective.However,there are still some problems such as poor polarization selection,narrow band width and poor thermal conductivity,which restrict the further application of azimuthally polarized gratings.In view of this,this paper,by combining theory with experiment,systematically studied the parameter design,structure design,processing tolerance and fabrication techniques of the azimuthally polarized grating used in 10.6?m wavelength high-power carbon dioxide lasers.The main work is as follows:Firstly,the subwavelength azimuthally polarized metal grating is designed by particle swarm optimization.Rigorous coupled wave analysis is used to calculate the diffraction efficiency in particle swarm optimization algorithm.The grating material and structures are determined.The designed grating has a high polarization selection and good thermal conductivity,and the band width meets the requirements,the thermal deformation is small.The influence of the grating structure on polarization performance is analyzed,and the processing tolerance is calculated.Secondly,the design method of subwavelength azimuthally polarized metal grating is analyzed.The influence of inertia weight factor in the operation of particle swarm optimization algorithm is analyzed.Aiming at the spectral response characteristics of subwavelength azimuthally polarized metal gratings,a step by step PSO algorithm based on adjusting inertia weight by particle position is proposed,which improves the efficiency and accuracy of grating design.Thirdly,an online monitoring and control system for metal grating electrodeposition is established.The deposition height of metal grating can be determined by real-time monitoring of diffraction efficiency curve during the process of grating electrodeposition.On the one hand,the grating structure parameters which need to be strictly controlled can be directly controlled by online monitoring in the fabrication process of the grating;on the other hand,the preliminary experiments and tests to determine the deposition rate can be avoided.Fourthly,factors influencing metal grating electrodeposit quality are analyzed,and the method improving electrodeposition quality is studied.Ultrasonic oscillation is introduced in deposition.The electrodeposition rate is improved,and ions diffuse more evenly on reaction surface,which reduces the surface roughness.Finally,The construction of metal grating electrodeposition system and the fabrication of subwavelength azimuthally polarized metal grating were carried out.The parameters of fabricating photoresist mask are determined by orthogonal experiment.The parameters of fabricating grating in high quality are determined by experiments,and the subwavelength azimuthally polarized metal grating is fabricated.