Quasi-bound states in the continuum in finite waveguide grating couplers.

Finite-size effects occurring for quasi-bound states in the continuum (qBICs) formed in symmetric and asymmetric all-dielectric waveguide grating couplers are investigated using numerical simulations for different configuration parameters. We find that the beam size plays a crucial role in the formation of additional qBIC resonances originating in forbidden for plane-wave incidence BIC resonances, whose manifestation is strongly influenced by the beam divergence. Another parameter is the strength of Bragg reflection, determining the spatial extension of a distributed Bragg resonator (DBR): in configurations with strong Bragg reflection, the excited and Bragg reflected counterpropagating waveguide modes are confined within the footprint of the incident beam. Conversely, the DBR formed in configurations with weak Bragg reflection extends well beyond the incident beam footprint, resulting in high sensitivity of the system response to the grating width and incident beam position. We believe that our findings provide valuable insights for optimizing photonic devices that leverage qBIC resonances in finite-sized structures under realistic excitation conditions.