Radiation-Free In-Gap Finite Barrier Bound States in a Lithium Niobate-Based Honeycomb Lattice.

Finite barrier bound state (FBBS) represents a unique class of boundary modes characterized by negligible inter-waveguide tunneling, even at subwavelength separations. Although such states hold great promise for suppressing optical crosstalk in integrated photonics, existing FBBS implementations suffer from significant radiation losses because of their eigenfrequencies lying within the bulk continuum. In this work, we introduce a double-channel waveguide system based on honeycomb photonic lattices made of lithium niobate, which supports FBBSs with eigenfrequencies fully residing within the photonic band gap. This strategic positioning effectively suppresses radiation into bulk modes, enabling low-loss photon propagation. Moreover, by leveraging the strong electro-optic response of lithium niobate, we demonstrate dynamic and reversible tuning of the inter-waveguide tunneling probability via an external voltage. Our platform not only overcomes the key limitations of prior FBBS designs but also offers a robust, tunable architecture for high-performance on-chip photonic devices such as lossless modulators, reconfigurable routers, and topological circuits.