Transmissions Of Photonic Topological Modes Through Channels With Different Shapes

With the development of topological photonics,there have been many significant advances in the study of systems that imitate the quantum spin Hall effect in photonics.One type of these systems realizes the photonic simulation of the quantum spin Hall effect by constructing the pseudo-spin degree of freedom and the corresponding timereversal symmetry in the photonic crystal structure.The photonic topological interface modes existing in this type of photonic crystal structure is also generally considered to have robust unidirectional transmission characteristic,which can be immune to the interference of possible defects or dislocations in the channel,and achieve lossless transmission without backscattering.In this context,we intend to further study the unidirectional properties of the topological modes in this photonic crystal structure to verify the applicable conditions of its robustness and the factors that may affect it.In the first chapter,we discuss the latest developments in topological photonics and an important branch of it,the development of two-dimensional topological photonic systems related to the optical quantum spin Hall effect,and introduce three main methods of realizing optical quantum spin Hall effect.In addition,we briefly introduce the concept of photonic crystal,the topological interface modes based on pseudo-spin degree of freedom in two-dimensional photonic crystal,and the operation process of COMSOL Multiphysics,the software required to study the above-mentioned system,and briefly describe the application principles of the finite element method in the software.In the second chapter,we use COMSOL Multiphysics software to study the transmission behavior of photonic topological modes which propagate through topological microwave channels with three different geometric shapes on a twodimensional photonic crystal platform,and set up a four-point source array to excite photonic topological modes.We find that the topological modes with positive and negative pseudo-spins can both be excited in a channel,even though they have different intensities.Different channels can provide different transmission ratios between two opposite directions(left and right).These results indicate that the topological modes may undergo a spin flip process at the sharp bend or the exit end face of the topological channels,that is,it will be scattered by the channel structure.During the simulation process,we also find that the change of the source array position has a great influence on the excitation of the photonic modes,which is also discussed.In order to find out the cause of the scattering,in the third chapter,we carry out a series of modifications to the exit end faces of the three topological microwave channels.After comparison and discussion,we find that the existence of bending and the changes of the exit end faces have influence on the light flow,which means that both the bending and the exit end face contribute to the scattering of the photonic topological modes in the channel.Combined with the research on the optimization of the source array position,in order to achieve the stable unidirectional transmission of the photonic topological modes in this topological photonic system,we have to analyze the overall structure of the channel,optimize the source array position and search for the characteristic frequency corresponding to the strong excitation.In other words,the conditions required for stable unidirectionality may be very stringent.The above results have important reference value for the practical application of photonic topological modes and channel design.Finally,we summarize the work of this thesis,and look forward to the future development and potential applications of photonic topological interface modes and related systems.