Research On Physical Mechanism Of Bound States In The Continuum In Optical Systems

Localization of light can trap photons within a certain time,enhance light-matter interaction,and realize slow light effect,all-optical storage,optical sensing,etc.,which makes the ability to confine light extremely important in modern optical science.It is well known that an electromagnetic wave of a specific frequency can be trapped by metallic mirrors,photonic band-gap materials,or total internal reflection,in which outgoing waves are completely forbidden owing to symmetry incompatibility or momentum mismatch.Conversely,a state in any open system can been coupled to external radiation waves and inevitably loses its energy.However,a bound state in the continuum(BIC)constitutes a counterintuitive exception: it remains perfectly confined without any radiation in an open system whose frequency resides within the radiation continuum spectrum.Historically,the concept of BICs was first proposed by von Neumann and Wigner in quantum mechanics.In recent years,with the advancement of nanofabrication technology,photonic structures have proven to be the particularly attractive platform to experimentally observe and utilize BICs.In practice,the remarkable features of BIC-inspired modes(e.g.,quasi-BICs)with ultra-high quality factor and strong local fields have been used for many applications,including laser,sensor and nonlinear optics.Although great achievements have been made in the field of optics in recent years,there are still many gaps to be explored.In this thesis,we develop a theory to study the physical mechanism and near-field properties of BIC in photonic crystal slabs.In addition,we also reveal the effects of PT-symmetric perturbation on BICs.These studies could deepen our understanding of BIC from a new perspective,and pave a way for future theoretical research and practical application as well.The main contents of this thesis are as follows:In the first chapter,we give a brief introduction on the background knowledge involved in this thesis,including the research history of photonic crystals,and the basic concepts of BICs and PT-symmetric systems and their recent research progress in optics.The second chapter mainly introduces the relevant theoretical methods,which would be useful in the study of BIC in photonic crystal slabs.In the third chapter,we develop a theory from a new perspective to explain the physical mechanism of BIC in photonic crystal slabs.The concept of coherent perfect reflection is first proposed for designing the mirror beyond the total internal reflection and band gaps.It arises from the coherence of multiple modes and can be determined by the topological vortex of a transmission coefficient in parameter space.Based on the coherent perfect reflection,a generalized waveguide condition is derived analytically.Finally,we verify in simulation that BIC can be naturally obtained as a result of the generalized waveguide condition.This study offers a new scheme to design the mirror and is of both theoretical and practical interest in guided-wave optics.In the fourth chapter,we further analyze the near-field properties of BICs in the Photonic crystal slab by decomposing the BICs into bulk Bloch modes of Photonic crystal.Based on generalized waveguide condition,the spatial field profile of BICs in detail in the near field can be obtained.We show that the BICs can be characterized by the indices(or number of nodes)of their constituent Bloch modes and specially the number of mode indices is more than one.Moreover,we find that all the guided resonances in addition to BICs can also be labelled by these mode indices.Our results can have potential applications in guided-wave optics,enhanced light-matter interaction,and high-Q cavity sensing.In the fifth chapter,we investigate the effects of additional perturbations from material gain/loss on BICs.It is found that each of these BICs always splits into a pair of BIC and lasing threshold mode when a PT-symmetric perturbation is introduced to the periodic struture.The lasing threshold mode accompanies with the BIC in pair at the same frequency and opposite wave vector.We can reveal the connections between BIC and lasing threshold mode based on the temporal coupled mode theory.The novel phenomena discovered here manifest the new physics arising from the interplay between PT-symmetric perturbation and BICs.In the sixth chapter,all the work of the thesis is summarized and prospected.