Research On The Structure And Characteristic Of Mid-infrared Metallic Wire-grid Polarizer

Polarizer is one of the important optical elements in optical systems.By obtaining the polarization information of the target,the information dimension of detection target is enriched,and the ability of target recognitionis enhanced in the infrared polarization imaging technology.Compared with traditional polarizers,metallic wire-grid polarizer has distinct advantages of small volume,light weight,flexible design,easy to be integrated and so on.In this dissertation,metallic Al wire-grid polarizer with a MgF₂ dielectric layer was designed and fabricated in the mid-infrared wavelength range of 3.0⁵.0μm.The influence of structure parameters of the single-layer metal Al wire grids on polarization performance was systematically optimized and analyzed via finite-difference time-domain（FDTD）method.The effects of the refractive indices,thicknesses and structure forms of the dielectric layer on the polarization behavior of the Al wires/dielectric layer/Al₂O₃ structure was studied;Influence of the orders of the three wire grids on polarization of the Al/MgF₂/Al/Al₂O₃ structure was both optimized and analyzed.The simulation results provided effective reference for the following experimental fabrication.Polarization testing system was built,which was capable of characterizing metallic wire-grid polarizers.The designed metal wire-grid polarizers were fabricated on Al₂O₃ substrate by electron beam lithography（EBL）and thin film deposition technology.The micromorphology was characterized and the polarization properties were tested and analyzed.The influence of structural parameters such as period,duty cycle,the height of Al wire grids,substrate materials and metal wire grids materials on the polarization characteristics of single layer Al wire-grid polarizer was optimized and analyzed using FDTD method.The results showed that period is the main affecting factor on the polarization performance.When the duty cycle and the height of Al wire grids were controlled within a range of change,TM transmission and extinction ratio could be both high enough.Metallic wire grids showed higher TM transmittance when the refractive index of substrate materials was much lower.Metal materials with larger imaginary parts of the dielectric constants were selected for better polarization characterization.Considering the actual process of preparation conditions,the structural parameters were determined as follows:the substrate is Al₂O₃,the metal material is Al,period is 400 nm,the height of Al wire grids is 100 nm and the duty cycle is 0.5.MgF₂ dielectric layers were inserted between metal Al wire grids and Al₂O₃substrates in the form of continuous thin film and grating to construct type-I and type-II sub-wavelength wire-grid polarizers,respectively.The structures of type-I and type-II polarizers were optimized in terms of the refractive index and thickness of dielectric layer.The results showed that surface plasmon modes at the interface between Al wire grids and Al₂O₃ substrate are weakened owing to the introduction of MgF₂ dielectric layer.In addition,equivalent refractive index of the MgF₂ dielectric grating is lower than the refractive index of ideal anti-reflection layer.The distribution of electric field intensity indicated the existence of acavity mode and stronger electric field intensity existed in the metal wire grids grooves of type-II.Higher TM transmittance and extinction ratio were achieved for type-II,which were obtained to be 98%,29 dB and 99%,30 dB at the wavelength of 4.0μm for type-I and type-II,respectively.A MDM structure of polarizer was proposed.The influence of sequence of the three grating layers on polarization characteristics was optimized and compared with single-layer Al wire-grid polarizers.The results showed that the average of TM transmission of the MDM structure was intermediate,TE transmission was the lowest and the extinction ratio was the highest in the 3.0⁵.0μm wavelength range.The influence of refractive indices of the dielectric gratings,period and duty cycle on the TM transmssion of was analyzed.By analyzing the the equivalent LC circuit and the distribution of the electromagnetic field intensity,it is showed that when the incident light is matched with the impedance of the periodic wire grids,magnetic polaritons is excited,stronger magnetic field intensity is located between the upper and lower metal wire grids,and the peak of the TM transmittance curve is existed.However,the magnetic field has no such effect at the non-magnetic polaritons,the magnetic field distributes uniformly between metal wire grids,and the TM transmittance has no peak value.A polarimetric testing system was built based on a FTIR spectrometer to measure the polarimetric characteristrics of the metallic wire-grid polarizer.The principle and method of measurement were explained,the error sources of the system were analyzed.Al wires/Al₂O₃ and Al wires/dielectric layer/Al₂O₃ structure structures were fabricated by EBL technology,polarization characteristrics were measured,and the microstructure was characterized by SEM（scanning electron microscopy,SEM）.The polarization test results showed that the experimental results were consistent with the theoretical calculation results.Because of the introduction of MgF₂ dielectric layer,the transmittance of TM and the extinction ratio increased.The measured TM transmission and TE transmission were lower and higher than the theoretical values respectively.When the thickness of the MgF₂ layer was 500 nm,the period was 400nm and the duty ratio was 0.5,the measured TM transmission of the Al wires/MgF₂film/Al₂O₃ structure was close to 83.3%,and TE transmission was 8.2%at 4μm,respectively.