Broadband Continuous Frequency Tuning in Non-Hermitian Laser Arrays Enabled by Mode-Switching Boundary Topology
Broadband and continuous frequency tuning is 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 frequency tuning in a non-Hermitian laser array achieved solely by controlling the pump currents. We show that in two coupled sub-lasers with frequency detuning ($\Delta\omega$) and relative loss ($\Delta\alpha$), a mode-switching boundary emerges in the ($\Delta\omega$, $\Delta\alpha$) parameter space, shaping the frequency landscape of the lower-loss supermode. The topology of this boundary comprises pseudo-symmetric (PS) and pseudo-symmetry-broken (PSB) branches connected at an exceptional point (EP). When tuning trajectories cross the PS branch, frequency tuning is discontinuous, whereas trajectories crossing the PSB branch enable continuous tuning; trajectories through the EP yield the maximum continuous tuning range. Experiments using two coupled terahertz quantum cascade lasers demonstrate continuous tuning over 10 GHz, enabled by arbitrarily many combinations of pump currents. Extending this approach to a multi-element array further expands the continuous tuning range to 163 GHz. These results establish a general route to broadband continuous tuning in moving-part-free semiconductor lasers and highlight the potential for dynamic eigenvalue engineering in non-Hermitian photonics and beyond.