Study On Topological Photonic States And Negative Refraction Transport In Photonic Crystals

The colorful wings of butterfly,the dazzling tail of peacock,and the deep blue shell of beetle,all of these natural mysteries hidden behind these magical creature colors originate from the unique physical properties of photonic crystals(PCs).PCs are artificial microstructures that are periodically arranged by materials of different refractive indices,which provides a powerful means to mold the flow of light and manipulate light-matter interaction at sub wavelength scale.PCs have many intriguing physical effects such as photonic bandgap,photonic localization,self-collimation,superprism,negative refraction,slow light,and topological photonic states.The concepts and research methods of PCs have been used to develop p the micro-and nano-photonic devices possessing miniaturization,high performance,and highly integration.PCs will be applied as novel optical materials and to design photonic devices for next-generation photonics technologies,and ultimately achieving large-scale,high-profile density integration of photonic deviceIn this thesis,we mainly study topological photonic states(TPSs)in magneto-optical PC structures and negative refraction transport in liquid crystal PCs.We study the interaction between two counter propagating TPSs in a line-defect waveguide,and obtained unique dispersionless slow light states,which can be continuously tuned in a wide frequency range Besides,we study the tunable effects of external saturated electric field to a liquid crystal PC,and the sensitive change of band structures and equal frequency curves(EFCs),which enable an electrically controlled high quality beam scanning device with a wide refractedangle range The specific research contents are as follows1.Study on unique slow light states based on the coupling effect of two counter propagating TPSs.We investigate the coupling effect of TPSs in a two-way waveguide constructed by bringing close two identical magneto-optical photonic crystals(MOPCs)which each supports a counter-propagating one-way TPSs.We find that different band shapes,including concave,flat,and convex can be obtained by appropriately changing the waveguide width,and the physical reason responsible for the deformation of band structure is analyzed By deeply analyzing the eigenfield distribution of the flat band,we reveal the physical mechanism of slow light states which originates from the strong coupling effect of two counter propagating TPSs.By tuning the intensity of external magnetic field,we explore the variation properties of the flat band,further calculate the group velocity and group velocity dispersion of the flat band in different magnetic fields respectively,and analyze the characteristics of the slow light states.Moreover,we conclude that when two MOPCs are close to each other,the coupling phenomenon occurs between two counter propagating TPSs As the waveguide width decreases,the coupling strength gradually increases.Especially,when the waveguide width reaches a half of lattice constant,complete exchange and transfer of energy flow between two TPSs occurs,which induces a vortex-like close loop of energy transport channel around the source,leads to complete destructive interference in the far field,and finally results in a flat band with a very slow propagation velocity of electromagnetic waves.Such slow light states exhibit very small group velocity {ng=5292),zero group-velocity dispersion,distortionless pulse transport,and ease to be magnetically tuned within a broad frequency range[0.465?0.567(2?c/a)],simultaneously.These properties are very beneficial to solve the contradiction between slow-light and large group velocity dispersion,and contradiction between slow light and narrow-bandwidth2.Study on wide-angle continuously scanning device based on liquid crystal PCs.We study the electrically controlled beam steering behaviors in a 2D square photonic crystal composed of Si cylinders surrounded by nematic liquid crystal.The effects of the in-plane saturated electric field on the band structure and EFCs are investigated and they are compared with those without an applied external electric field.The relationships between the direction of the in-plane saturated electric field and the deformation of the EFCs are studied and utilized to achieve tunable negative refraction.Finally,a continuously tunable light beam deflector with a wide angle range is designed.Details are as follows:when applying an in-plane saturated electric field of 450V/m,the liquid crystal becomes anisotropic,which causes the degeneracy of the second and third bands at the high symmetry point M in the first Brillouin zone.What's more,the second and third bands are separated upwards and downwards,so that a single mode band[0.3105?0.3245(2?c/a)]along the ?M direction appears.Besides,the generation of the anisotropic dielectric tensor further breaks down the symmetry of EFCs with respect to the symmetry center of the Brillouin zone and leads to negative refraction transport mode being excited in the liquid crystal PCs within 0.3105?0.3245(2?c/a).As the direction of the saturated electric field rotates in the plane,the shape of EFCs are also continuously rotated and deformed.Especially,the curvature change of the EFCs in the vicinity of the ?M direction is very large,which can be used to achieve significant negative refraction transport.Based on the these properties,we demonstrate an electronically controlled high quality beam continuously scanning within the angle range from -38°to +38°.