Flat-top filter using slanted guided-mode resonance gratings with bound states in the continuum

The attribute of bound states in the continuum (BICs) for highly efficient flat-top filter by using a slanted guided-mode resonance (GMR) grating is developed in this work. By introducing a slanted angle θ, the fully symmetric grating is broken to transform the dark BICs into bright quasi-BICs mode. Theoretical analysis based on the finite-different time-domain method is applied to evaluate the filtering performances. It is found that the GMR and quasi-BICs peaks combine at θ=31.4°, and the flat-top filtering response is realized at the central wavelength of 1550 nm under normal incidence. Moreover, the linewidth-tunable and central wavelength-tunable spectral response can be varied according to the change in the coupling strength of the GMR grating by changing the structural parameters such as period, fill factor and refractive index of the grating. It is the first report of a flat-top filter designed by combining GMR and quasi-BICs. The proposed structure shows promise for application in high-performance optical communication systems.