In the field of optics, bound state in the continuum (BIC) as a special state to be researched in many photonic crystals and periodic structures, which can produce strong resonance and an ultrahigh Q factor. Some designs of narrowband transmission filters, lasers, and sensors are proposed based on excellent optical properties of quasi-BIC. In this paper, we consider symmetrical rectangular grating structure firstly. Then cut off a corner of rectangular gratings, the Fano line of quasi-BIC can be observed in the spectrum. After that, we change the oblique parameter of the other rectangular grating, which can further decrease the Fano line width. In the momentum space, the change of structure means topological charges split from q=1 into half charges q=1/2. We analyze guided mode resonance (GMR) excitation of the grating structure, and discuss the dispersion relations in the waveguide layer with the position of BIC in energy bands. In addition, the spectrum exhibits asymmetric line-shapes with different values of the asymmetry parameters, M1 and M2. BIC is transformed into quasi-BIC as the symmetry of the structure broken. Thus, a large Goos-Hänchen shift can be achieved as a result of ultrahigh Q factor of quasi-BIC. This work demonstrates a double trapezoid structure with strong resonance properties, which has significant implications for exploring the phenomenon of BIC.

Optical bound states in the continuum (BICs) exist in many photonic crystals and periodic structures with a strong resonance and ultrahigh Q factor. Such phenomena can be used in the designs of narrowband transmission filters, lasers, and sensors. In this paper, we consider the energy bands of a complex structure consisting of a grating and a multilayer substructure to obtain the position of the BIC in the structure. Hence, the higher Q factor can be obtained in the grating-multilayer structure than can be realized in the simple grating geometry. We analyze the wave propagation process in the complex structure and the change in the Q value via the use of transmission matrix theory. In addition, the reflectance spectrum is found to exhibit a series of asymmetric line-shapes with different values of the asymmetry parameter, δ, due to the interference between the two channels. One of these channels is the broadband channel, induced by the Fabry–Perot resonance, and the other channel is the narrowband channel, induced by guided mode resonance. Quasi-BICs are seen to transform into BICs as the value of δ is decreased gradually to zero. Therefore, a large Goos–Hänchen shift can be achieved as a result of the high Q factor and quasi-BIC. This work designs a complex structure with ultrahigh Q factor and strong resonance properties, which has significant implications for exploring the phenomenon of BICs.

No abstract

No abstract