Research On Topological Photonic Phase Transition Mechanism And Application

Photonic crystals,as a kind of artificial crystal material with a band gap structure,are of great value in optical waveguides and optical communications because of the advantages of fast response speed,small size,high integration,and low price.However,it is easily affected by fabricating defects,resulting in strong backscattering of light and relatively large losses.In recent years,research on the transmission characteristics and application in topological photonic crystals has attracted the attention of a large number of scholars.This is because of the existence of edge states,which are protected by topology,have the characteristic of defect immunity and can suppress backscattering.Therefore,it has great potential and broad prospects in the development of lossless propagation optical systems and efficient optical devices.It means it is possible to overcome the limits of large-scale integration since the performance of conventional optical devices is affected by back reflection and bending loss.Based on the photon quantum valley Hall effect,this paper proposed a kind of Valley photonic crystals（VPC）by breaking the spatial symmetry to achieve the topological phase transition.Based on this,two kinds of optical devices,Mach-Zehnder interferometer（MZI）and refractive index sensor（RIS）are proposed,and the devices were designed,simulated and studied.The main research contents are as follows:（1）By breaking the spatial inversion symmetry of the system,a specific implementation method of VPC is proposed,and the mechanism and process of achieving the topological phase transition are explained in detail.Different from the traditional topological photonic crystal based on the quantum Hall effect,this VPC is realized by using common silicon materials under the time inversion symmetry.This reduces the difficulty of the related processing technology.At the same time,by comparing the transmission characteristics in a VPC-based topological protected waveguide（TPWG）and a traditional photonic crystal-based waveguide（PCWG）,the characteristics of backscattered immune and unidirectional transmission of the edge states in VPC are highlighted.（2）A topologically protected Mach-Zehnder interferometer（TPMZI）is designed and simulated with the VPC mentioned above.The results of optical calculation show that the TPMZI can maintain high efficiency and stability even under complex situations.The main characteristics are:Broad band transmittance of light,in the range of 1500-1600nm,the light transmittance at the output end is over 95%;High sensitivity,when the refractive index in the detection arm changes from 1-1.15,the sensitivity can over662%/RIU;High extinction ratio and modulation depth,the extinction ratio can reach25.3dB,and the modulation depth can reach 99.3%;Defect immunity,when there are non-valley mixed defects in the system,such as obstruction and disorder,the waves can still propagate without backscattering,ensuring the output efficiency and the stability of interferometer function.（3）Based on VPC,a topological-protected refractive index sensor（TPRIS）is proposed.Two different structures based on waveguides and resonator-coupled waveguides are designed,and they are compared with topological trivial RIS based on traditional photonic crystals under the same conditions.The simulation results show that the waveguide-based TPRIS can achieve an average fitting degree of 99.91%within a refractive index detection range from 1 to 2,showing a wide detection range.Its detection sensitivity is 278.8nm/RIU,which is three times than that of trivial RIS under the same conditions.The sensitivity of TPRIS based on resonator-coupled waveguides has been significantly improved to 438nm/RIU,which still has advantages over trivial RIS with291 nm/RIU.Assume that the resolution of the spectrometer is 0.01 nm,its minimum resolution is 2x10^-5 RIU,which is sufficient to detect slight refractive index changes.The sensitivity to the change of rod radius was also studied.Among them,the sensitivity of TPRIS is 3.2 and the sensitivity of trivial RIS is 6.7.The results show that TPRIS has advantages in terms of sensitivity and resistance to processing errors,and is superior to traditional photonic crystal-based RIS.