Free Merging BICs

Our new research on merging bound states in the continuum (BICs) has been published in Physical Review Letters.

BICs are localized resonances embedded in the continuum spectrum by eliminating radiation loss. They have unique advantages in trapping light. Merging BICs can suppress the scattering loss induced by the fabrication imperfections but they are only limited to Γ point before our work.

In this work, we propose a new scheme to construct merging BICs at almost an arbitrary point in the reciprocal space. Instead of merging only accidental BICs at Γ point in the previous work, we show the concurrence of an accidental BIC and a Friedrich-Wintgen BIC (FW-BIC) and then tune them to merge at an off-Γ point. We design a PCS made of Si3N4 and immersed in a liquid (common in lab) to demonstrate our idea and such a proposal can be readily verified experimentally. By breaking the in-plane mirror symmetry, the merging BIC can be tuned to almost an arbitrary position in the reciprocal space. Since FW-BICs and accidental BICs are quite common on the band structure, our proposal provides a general approach to realize off-Γ merging BICs.

Our work enriches the explorations of topological photonics and can boost applications desiring strong light-matter interactions, including nonlinear effects, sensors, lasing and optoelectronic devices. 

Fig. 1. Merging the FW BIC and accidental BIC with opposite topological charge

For full text:

https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.117402

March 2021