BIC-driven Light Manipulation Based On Subwavelength Dielectric Multilayer

A bound state in the continuum(BIC)is a localized electromagnetic eigenstate embedded within the continuous spectrum of extended states.BIC possesses a high quality-factor and offers a unique advantage for enhancing light-matter interaction.BIC has emerged as a frontier topic in nano-photonics that has attracted wide attention in recent years.In addition,researches have also revealed that BIC is the polarization singularity in momentum space of the Bloch mode in a photonic crystal slab,which enables BIC to integrate with singularity optics,chiral optics,topological optics and other cutting-edge fields to generate many novel optical phenomena and new photonic devices.A subwavelength dielectric multilayer(one-dimensional photonic crystal)not only possesses a flexible and tunable band structure,but also supports various surface electromagnetic modes.It can serve as a novel optical substrate to construct functional and integrated micro-nano composite structures for investigating mode coupling,manipulating light-matter interaction,and controlling beams.This thesis studies the BIC manipulation,topological polarization singularities and on-chip integrated photonic devices based on hybrid systems composed of dielectric multilayer and various micro-nano structures via numerical simulation.A mechanism to suppress the propagation loss of Bloch surface wave(BSW)is proposed based on the BIC principle,and low-loss waveguide and resonator are designed.The dynamics of various polarization singularities in momentum space are investigated based on the far-field interference mechanism.New phenomena and physical effects are revealed,such as the chiroptical state with an ultranarrow linewidth and the topologically enabled resonance with a high quality-factor.The main research works are as follows:1.The loss mechanism of BSW waveguide is analyzed.It is revealed that the leakage loss due to strong coupling between the surface wave and internal extended states in multilayer is the key factor limiting the propagation distance of the guided BSW.The BIC principle is employed to construct the low-loss guided BSW.We also demonstrate the versatility of this method by extending it to the cylindrical coordinate system and designing a high quality-factor BSW micro-disk resonator.2.A far-field interference mechanism is proposed that enables the generation and manipulation of diverse polarization singularities in momentum space,which is verified in a photonic crystal slab-multilayer hybrid system.Distinct from the BIC splitting in symmetry-broken structures,BICs here are topologically protected due to the preserved symmetries.We also find a series of novel dynamic processes of polarization singularity,in which the intrinsic BIC and the circularly polarized state(CPS)can coexist and be manipulated independently.3.A polarization singularity configuration with multiple CPSs surrounding the merging BIC is proposed.A chiroptical state with an ultranarrow linewidth is realized at the CPS point by taking advantage of the high quality-factor of merging BIC and the chiral selectivity of CPS.Selective perfect absorption of the circularly polarized light with different handedness is also achieved under the critical coupling condition.4.A strategy for designing complex BIC configurations in momentum space is proposed and the merging of diverse BICs is realized.The intrinsic and extrinsic BICs can be manipulated independently,which greatly increases the freedom to design BIC configurations for more applications.Compared to merging two-types BICs,merging diverse BICs further enhances the quality-factor of the resonance mode and is also robust to structural imperfections.Highlights of the thesis are as following:1.The loss mechanism of guided BSW is revealed,and the BIC principle is utilized to suppress this kind of energy leakage.The low-loss BSW waveguide and resonator are designed.The feasibility of BSW as a platform for large-scale photonic integration is verified theoretically.2.A far-field interference mechanism for generating and manipulating diverse polarization singularities in momentum space is proposed based on a hybrid photonic system,which breaks through the limitation that BIC and CPS cannot be manipulated simultaneously in symmetry-broken structures.3.Based on the special polarization singularity configuration where the merging BIC and CPS coexist in momentum space,the quality-factor of CPS is enhanced to 106 and the chiroptical state with an ultranarrow linewidth is realized.This chiroptical state can be utilized for chiral sensing and circularly polarized source.4.A method is proposed by introducing extrinsic BIC to construct complex BIC configurations,which improves the scaling rule of quality-factor from up to Q∝k-14 compared with the merging of two-types BICs and enhances the qualityfactor of resonator with structural imperfections over an order of magnitude.