Continuously tunable topological negative refraction via a tailorable Bloch wavevector in momentum space

Topological photonics provides a strategy that makes light transmission immune to structural-defects-induced backward scattering. Leveraging this, topological negative refraction enables robust, reflectionless light deflection, but directly controlling the refraction direction remains challenging. We demonstrate continuously tunable topological negative refraction at the interface between a one-way waveguide state and a free-space beam, overcoming the limitations of fixed refraction angles in conventional systems. The key insight is the ability to adjust the wavevector of the incident one-way waveguide state. Through manipulating the Bloch wavevector of the waveguide states in momentum space, we achieve a transition from negative to positive refraction. The unidirectional nature of these states prevents backscattering from defects, ensuring immunity to imperfections. As a prototypical demonstration, we achieve dynamic steering of refraction beams from −38° to +12° through active magnetic bias control. Our findings provide an exotic pathway for photon manipulation and a promising route toward topological photonics applications.