Investigation On Transmission Characteristics And Application For Helical Edges States In Topological Photonic Crystals

Topological insulators emulating the quantum Hall effect,a fascinating topological material in condensed matter physics,have attracted many attentions in the world.Especially the unique behaviors of topologically protected edge states and defect modes in all-dielectric photonic crystals,have became hotspots in recent years.Here we mainly studied the physical explanation and transmission characteristics of unidirectional edge states simulating quantum spin hall effect in topological photonic crystals.By applying the spin-dependent one-way properties of electromagnetic wave,we proposed a coupling mode of edge states,which can be manipulated in optical communication.We also conducted the mechanism for guiding pseudo-spins in topological photonic crystals made of elliptic cylinders.Detailed study contents as the following:?1?With the preserved time-reversal symmetry in the system,the doubly degenerate Dirac cones can open a band gap by compressing and expanding the honeycomb lattice,which ensures the inversion mechanism of band and parity.The mode hybridization between Bloch states may lead to a pair of unidirectional edge states with opposite pseudo-spins.Based on the backscattering suppression and robustness against defects of edge states,as well as the mode coupling theory,we proposed a trivial-nontrivial-trivial photonic crystal.there appear two homodromous helical edge states excited by the source at each boundary,the coupling of these edge states leads to the behaviors of tough light-protection and strong light-confinement.We calculated the dispersion relations of proposed structure and verified the robustness against disorders,defects and sharp bends.?2?We proposed topological photonic crystals based on elliptic cylinders,where a photonic large band gap can be found in Dirac cones by calculating the band structure.A spin-guiding path along the zig-zag interface is observed,resulting in the light localization.We established the mathematical model between basic parameters of elliptical rods and the operation bandwidth of helical edge sates.The mechanism for guiding pseudo spin states was demonstrated,which leads to the quantum spin hall effect system with large operation bandwidth.Those proposals might provide an alternative practical way for helical edge states with larger operation bandwidth.?3?We studied the performance of helical edge states in topological photonic crystals with different line defects,and discussed the nontrivial topological phases with the breaking ofC₆ symmetry along the interface.The impact of line defects on the transmission characteristics of the pseudospin modes was determined.Besides,we proposed a photonic crystal structure constructed by deformed honeycomb lattices and reconfigurable cylinders,which exhibited defect modes without without the presence of both two distinct topology.Their properties are similar to helical edge states with suppressed backscattering.The transmission direction of defect modes are locking with pseudo-spins,and those modes can easily pass through structure defects.Those researches could be applied in designing topologically protected waveguides with excellent performances.