Tunable hexagonal boron nitride topological optical delay line in the visible region

Tunability, ultracompact design, high group index, low loss, and broad bandwidth are desired properties for integrated optical delay lines (ODLs). However, those properties are challenging to achieve simultaneously in the visible region. This paper proposes a tunable hexagonal boron nitride topological optical delay line (ODL) in the visible region based on valley photonic crystals. The topological edge state from the beard-type boundary allows the achievement of an ultralow group velocity close to zero, which results in a large group index of 629 at 645 nm. Moreover, we demonstrate tuning of the slow-light wavelength and optical delay times with electrically tunable liquid crystals by applying external voltage. The device has an ultracompact size of 5 µm × 2.7 µm with an optical delay distance of 25a (a is the lattice constant) and a delay time of 12 ps. Our design can provide a new possibility for designing ODLs working in the visible region for optical communication and quantum computing systems.