Tunable flat-top filter with cascaded compound gratings based on lithium niobate assisted by quasi-bound states in the continuum

We propose a tunable filter with cascaded compound gratings based on lithium niobate (LN). Herein, the reflectivity spectrum with a narrow-band and flat-top line shape can be obtained by using the quasi-bound states in the continuum (quasi-BICs) and selecting the appropriate grating offset. Simultaneously, we apply a voltage on LN to modulate the refractive index to achieve wavelength tunability. The demonstrated structure is sensitive to the incident angle, which can be used for angle sensing. Interestingly, an accidental BIC occurs by varying the length of the Fabry-Perot cavity to achieve phase shift conditions between the two grating layers. The presented tunable flat-top filter structure has promising applications in integrated sensing and communication.