Slow Light Rainbow Trapping Based On Topological Photonic Crystals And Their Applications

Photonic crystals,which are periodic optical structures with the advantages of good light confinement and multiple adjusting degrees freedom,provide a ideal platform to control the flow of light.However,traditional photonic crystal waveguides are sensitive to fabrication errors such as defects or disorders,which reduces the transmission efficiency.Inspired by the development of the topological insulators,the concept of topology has been extended from condensed matter physics to photonic systems.The introduction of topology endows traditional photonic systems with brand new properties,such as the one-way propagating edge states,robust transmission against impurities and defects,which meet the rapidly growing demands for information processing.Valley photonic crystals,as one kind of topological photonic systems,not only support topologically protected surface modes,but also are friendly to fabrication.These advantages show that it has broad prospects in constructing highperformance photonic devices and photonic integrated circuits.In this thesis,valley photonic crystal waveguides are constructed,its topological properties are proved,and related theoretical and simulation are carried out.On this basis,the phenomenon of rainbow trapping is realized by introducing a structural parameter into the waveguide structure.The specific work of this thesis is as follows:(1)The robust transmission properties of valley photonic crystals are investigated based on valley pseudospin.Firstly,two photonic crystals with different topological properties are constructed by breaking the inversion symmetry in the honeycomb lattice,and the effects of lattice constant,dielectric rods size and height on band structure are investigated.The existence of topological edge states is proved by combining two valley photonic crystals with different topological properties.Secondly,the introduction of different structural defects in the waveguide structure,such as cavity and disorders,indicates that the edge states are immune to impurities or defects.In addition,Z-shaped and Ω-shaped waveguide interfaces are constructed.The simulation results show that light wave can propagate smoothly through sharp bends without backscattering.These properties prove that the edge states support topologically protected robust transmission.(2)Based on valley photonic crystal waveguide structure,rainbow trapping is realized by continuous adjustment of the structural parameter.Firstly,the physical mechanism of rainbow trapping is introduced through theoretical analysis.Secondly,a gradual valley photonic crystal waveguide interface is constructed by continuous adjustment of the structural parameter,and the phenomenon of slow light rainbow trapping is realized.The edge waves of different frequencies are spatially separated and trapped at different positions to form topological rainbow trapping.The robustness of topological rainbow trapping is demonstrated by introducing disorders and obstacles into the valley photonic crystal waveguide.In addition,the system is composed of electro-optic materials,and the refractive index of the dielectric rods can be changed by applying external voltages.Therefore,by tunning the applied external voltages,the switchable between the slow light states with extremely low group velocity and the transport states with large group velocity is realized.The position where the wave stops propagating is given by theoretical analysis and numerical simulation.In conclusion,based on the valley photonic crystal waveguides,the waves of different frequencies are spatially separated and trapped at different positions by changing the structural parameter.Compared with other structures that achieve rainbow trapping,this structure is friendly to fabrication.These results offer a novel scheme for realizing multi-frequency routing.Such a structure could find applications prospects in optical buffers,optical storage,and other optical communication devices.