Bound State In The Continuum Caused By Anisotropic Material Defects In One-dimensional Photonic Crystals

In recent years,Bound states in the continuum(BICs)have achieved significant progress from theoretical validation to experimental implementation in the field of photonics,where photonic crystals and anisotropic crystals are two important structures for generating BICs.One-dimensional photonic crystals are composed of materials with periodic variations,exhibiting one-dimensional periodic structures and periodic diffraction.Therefore,different polarized light incident into the photonic crystal at the same frequency will result in different transmittance.Compared with isotropic media,anisotropic crystals have more complex optical properties.Therefore,by combining the characteristics of the two materials,this paper further expands the methods and applications of generating BICs.In the waveguide,by combining onedimensional photonic crystals with anisotropic crystals and adjusting the structural parameters,more complex optical control and modulation can be achieved.The main research content of this paper is to embed anisotropic crystals into onedimensional photonic crystals as defect layers to achieve symmetry-protected Bound states in the continuum(SP-BICs)and Friedrich-Wintgen Bound states in the continuum(FW-BICs).Firstly,two kinds of one-dimensional photonic crystals are designed using four types of materials,and the band structure of the one-dimensional photonic crystal is analyzed by Maxwell’s equations and the field continuity conditions at the boundary,that is,the transmittance of different frequency and polarized light incident into the photonic crystal is calculated and analyzed.Then,anisotropic crystals are introduced into one-dimensional photonic crystals as defect layers.When the anisotropic crystal optic axis is rotated,the reflection spectrum of the system exhibits BICs.The manifestation is a linear collapse point in the reflection spectrum,and the frequency position of these points are in the range of single polarized light transmission of the photonic crystal band when the incident light frequency is in this range.In addition,this paper also theoretically analyzes and verifies the quality factor of the two categories BICs as well as analyzes their generation conditions and the rotation angle of the anisotropic optic axis.Secondly,this paper studies the generation of BICs in asymmetric structures composed of one-dimensional photonic crystals and anisotropic crystals.By analyzing the band characteristics of one-dimensional photonic crystals at the Brewster’s angle and combining the role of one-dimensional photonic crystals in symmetric structures,SP-BICs and FW-BICs are achieved through different mechanisms by designing asymmetric structures.By rotating the anisotropic crystal optic axis,the above two kinds of BICs are observed in the system reflection spectrum.In addition,this paper also studies and verifies that the two diffierent categories BICs can still be generated when the incident light deviates from the Brewster’s angle but is still in the range of single polarized light transmission of the photonic crystal band,and analyzes the corresponding conditions.This paper provides ideas for the active modulation of BICs and asymmetric structures provide convenience for the casting process.