Enhancement Of Photonic Band Gap In Moderate Refractive Index Ratio Two-dimensional Photonic Crystals

Because the photonic band gap can prohibit the transmission of light within the band gap frequency range,it has a widely applications in the field of optical communication and optical devices.Compared with one-dimensional and three-dimensional photonic crystals,the two-dimensional photonic crystal is not only easy to calculate and analyze,but also easy to manufacture.At the same time,it has a good control effect on light,so the photonic band gap of two-dimensional photonic crystal becomes the most concerned research object.Usually,it is easier to obtain two-dimensional photonic band gap in high refractive index ratio.The study of two-dimensional photonic band gap enhancement is mostly limited to the high refractive index ratio.However,this limits the material range and wavelength range of photonic band gap applications.For example,Materials such as silicon nitride and optical wavelength range such as visible band cannot be used.Therefore,it is necessary to study the enhancement of two-dimensional photonic band gap in the material system with low refractive index ratio,such as in moderate refractive index ratio.The enhancement of two-dimensional photonic band gap in moderate refractive index ratio is mainly studied in this dissertation and some meaningful results have been achieved.(1)A method of realizing large photonic band gap in moderate refractive index ratio is proposed.In this method,firstly,the distribution of gray pixels in the simplest Brillouin region of the photonic crystal is iterated to obtain suitable new photonic crystal structures.Secondly,in the process of analysis and calculation,the number of calculated forbidden bands should be increased.Not only the low-order band but also the high-order band should be calculated.Both low-order band and high-order band can obtain large band gap at high refractive index ratio.However,with the decrease of refractive index ratio,the band gap of low-order band decreases faster than that of high-order band.In the range of moderate refractive index ratio,the band gap of low-order band decreases rapidly until it disappears,while the band gap of high-order band is larger than that of low-order band with a certain value.In this case,the enhancement of photonic band gap will be achieved in moderate refractive index ratio.(2)Based on the above implementation method,the enhancement of photonic band gap in moderate refractive index ratio square lattice square ring cross connecting rod structure is studied.The influence of the parameters of the inner and outer edge length on the TM,TE and complete photonic band gap in the Square ring cross connecting rod structure is analyzed in detail.It is found that TM band gap decreases with the increase of inner and outer edge length,TE band gap and complete photonic band gap first increase and then decrease with the increase of inner and outer edge length.The TM band gap of 6% to 10%,TE band gap of 6% to 15% and complete photonic band gap of 2% to 9% are obtained in the range of refractive index ratio from 2.5 to 3.0,so the enhancement of the photonic band gap in the moderate refractive index ratio two-dimensional square lattice is realized.(3)Based on the above implementation method,the enhancement of photonic band gap in moderate refractive index ratio triangular lattice ring hole peripheral connecting rod structure is studied.A new type of photonic crystal structure of the ring hole peripheral connecting rod is proposed.The influence of the inner and outer radius of the ring hole and the width of the connecting rod on the complete photonic band gap is analyzed in detail.It is found that the complete photonic band gap first increases and then decreases with the increase of the radius of the inner and outer circles.Finally,in the range of refractive index ratio from 2.5 to 3.0,more than 10% or even close to 20% of the complete photonic band gap is obtained,so the enhancement of the complete photonic band gap in the moderate refractive index ratio two-dimensional triangular lattice is realized.At the same time,the photonic crystal of the cylindrical connecting rod structure is analyzed in detail.It solves the problem that the complete photonic band gap cannot be obtained when the refractive index ratio is less than 2.5 in this structure,and about 3% of the complete photonic band gap is obtained in the range of refractive index ratio from 2.3 to 2.5.(4)The implementation method mentioned above involves many works including algorithm improvement,structure calculation,data processing and analysis,which need to be carried out in strict accordance with the steps.Further,the application of deep learning in the field of photonic crystal is studied tentatively in this dissertation.First,the traditional numerical algorithm is used to obtain the data set of photonic crystal structure and band gap,and the convolution neural network is used to predict and classify the TM band gap and TE band gap.It is found that the prediction effect of 4 layers in the full connection layer is better than that of 3 and 5 layers.At last,80% of TM band gap and 75% of TE band gap prediction and recognition accuracy are achieved on the test set.It provides a reference for the further study of deep learning in the field of photonic crystals.The method of realizing large photonic band gap in moderate refractive index ratio can be applied to band gap enhancement of two-dimensional square lattice and triangular lattice.The method can not only provide reference for the design,calculation and analysis of two-dimensional photonic crystal,but also easy to obtain large photonic band gap under high refractive index ratio.And more importantly,the enhancement of two-dimensional photonic band gap including TM band gap,TE band gap and complete photonic band gap is realized in the range of moderate refractive index ratio.