Adjacent Asymmetric Tilt Grating Structure With Strong Resonance Assisted by Quasi-Bound States in the Continuum

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,
M
₁
and
M
₂
. 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.