Design And Microwave Verification Of Diffraction-free Optical Waveguide Based On Bound States In The Continuum

Diffraction is a universal property of light,which imposes certain restrictions on free-space optical communication.Therefore,it is of great significance to realize diffraction-free beam guiding.Diffraction-free beam refers to the beam that can maintain the beam width constant in the transmission.It could contribute to the realization of dense integration of optical circuits.The conventional mechanisms for realizing diffraction-free beam include optical nonlinearity and self-collimation effects.The use of nonlinear effect relies on high-intensity lasers,while the self-collimation effect can suppress in-plane diffraction without any in-plane physical boundaries.However,the conventional self-collimation phenomenon has relied on the effect of total internal reflection to confine the light in the optical waveguide plane,where the corresponding diffraction-free mode is located below the light cone of the background material.According to the optical waveguide theory,the guided wave mode that relies on total internal reflection has limitations such as cut-off frequency and critical coupling angle.Therefore,in order to overcome these limitations,this thesis studies a photonic crystal slab that can realize diffraction-free beam transmission beyond the total internal reflection essentially beyond the light cone.The research results of this thesis are expected to be applied in the fields of mode sorting,beam routing,sensing and lasing,and provide new ideas for exploring guided-wave physics beyond the light cone.This thesis focuses on the design of diffraction-free optical waveguide structure to carry out the following main work:(1)The proposed diffraction-free beam transmission mechanism is based on the bound states in the continuum:Bound states in the continuum(BIC)are located beyond the light cone with unique physical properties.The BIC with specific spatial dispersion can eliminate the diffraction phenomenon in the plane of the optical waveguide,whose sufficiently high Q factors can suppress the out-of-plane loss of the optical waveguide.Therefore,we can use the physical properties of the BIC to realize the transmission of the diffraction-free beam.(2)Design and numerical simulation of diffraction-free optical waveguide structure:In this thesis,the finite element method(FEM)is used to analyze the eigenmodes of the photonic crystal slab.Under the control of the structural parameters,it is found that there are flat equifrequency contours of the photonic crystal slab,and the quality factors are much larger than 10~6,which are beneficial to reduce the out-of-plane radiation loss in a finite structure.We also simulated the excitation scenarios of Gaussian beam and dipole source for the photonic crystal slabs,analyzed the near-field characteristics of the excited diffraction-free beam,and confirmed that the diffraction-free modes are located beyond the light cone.(3)Microwave experimental verification of diffraction-free optical waveguide structure:Based on the numerical simulation results,we built a microwave experimental platform and carried out the principle verification.Experiments results showed that the transmission of diffraction-free beam can be realized in the photonic crystal slabs under both Gaussian beam and dipole excitations.The transmission characteristics were consistent with the results of numerical simulations,which further verifies the universality of the diffraction-free beam-guiding mechanism.