Broadband Continuous Frequency Tuning in Non-Hermitian Laser Arrays Enabled by Mode-Switching Boundary Topology

Abstract

Broadband and continuous frequency tuning are central to the versatility of semiconductor lasers, yet existing approaches typically rely on external moving components, limiting scalability and integration. Here, we demonstrate broadband continuous tuning in a coupled non-Hermitian laser array without any moving components. We show theoretically that in a binary laser array with frequency detuning (Δω) and relative loss (Δα), a mode-switching boundary emerges in the (Δω, Δα) parameter space, shaping the frequency landscape of the lower-loss supermode. The topology of this boundary comprises pseudosymmetric (PS) and pseudosymmetry-broken (PSB) branches connected at an exceptional point (EP). When tuning trajectories cross the PS branch, frequency tuning is discontinuous, whereas trajectories that cross the PSB branch enable continuous tuning; trajectories through the EP yield the maximum continuous tuning range. Experiments on two-element terahertz quantum cascade laser arrays validate the theory, demonstrating continuous tuning over 10 GHz via flexible pump-current combinations. Extending the concept to a 22-element array expands the continuous tuning range to 163 GHz, the largest reported for moving-part-free semiconductor THz lasers. These results establish a scalable route to broadband continuous frequency tuning and highlight the potential of dynamic eigenvalue engineering in non-Hermitian photonics and beyond.