Design And Optimization Of Subwavelength Metallic Grating Filter

The subwavelength metal grating can realize the filtering effect in different wave bands by designing the grating structure and changing the grating size.It has the advantages of high transmittance,small size,good polarization performance and easy integration,and can achieve mass production by using micro-nano manufacturing technologies such as nano-imprinting.It has huge application prospects in the fields of optical display,optical sensors,and optical detection,and has received more and more attention in recent years.The paper first introduces the basic analysis theory applicable to sub-wavelength gratings.The finite-difference time-domain algorithm theory and the rigorous coupled wave analysis theory are vector diffraction theories,which can perform accurate numerical calculations in the process of grating simulation.The effective medium theory is an approximation method,which can replace the periodic structure of sub-wavelength gratings with a uniform medium.And the effective medium can explain the polarization property of grating.Secondly,a double layer subwavelength metal grating filter is designed,the influence of grating structure parameters(grating period,duty cycle,metal grating height,dielectric grating height,dielectric grating refractive index)on polarization and transmission performance is analyzed,and the abnormal phenomenon in transmission curve is explained.The structure is used to realize the high transmission and narrow bandwidth polarization filtering function of blue,green and red colors in the visible light band.The FWHM(full width at half maximum)is 35 nm,37nm and 38 nm respectively,and the peak transmission reaches 77%,75% and 73% respectively.The extinction ratio corresponding to the center wavelength is 20 dB,25dB and 30 dB respectively.In addition,a new type of stepped multilayer metal grating structure is also designed.The intermediate layer is a dielectric and the two sides are metal.By adjusting the refractive index of the intermediate dielectric layer,the filtering function of cyan,magenta and yellow is realized in the visible wave band.The trough transmittance is well suppressed and the lateral transmittance is high.The structure has a good effect of color subtractive filtering.At the end of the paper,a new combinatorial optimization algorithm is proposed,which combines the genetic algorithm with the gradient descent method,and simultaneously realizes the large-range rough search and the small-range fine search.This algorithm is used to optimize the structure of the designed grating and further improve the filtering effect of the grating.This paper analyzes the optimization performance of the algorithm,optimizes various targets with the combination algorithm and the traditional genetic algorithms respectively,and compares the convergence speed of different algorithms.In addition,the influence of population size on the accuracy of the algorithm is discussed.The results show that under the selection of appropriate parameters,this method has good convergence speed and accuracy,and can effectively optimize the design of sub-wavelength grating structures.