Wave morphing towards the flat-top envelope in photonics systems driven by artificial gauge fields.

In quantum physics, classical optics, and many other wave systems, wave confinement in a finite domain leads to discrete eigenstates that typically exhibit spatially nonuniform, oscillatory profiles. A long-standing question is whether a confined system can counterintuitively support an eigenstate with a uniform, nonzero envelope, offering new opportunities for quantum emitters, optical antennas, and lasers. Here, we show that such a state can be realized through spatial phase engineering that acts as an artificial gauge potential. By continuously tuning the accumulated phase, the eigenvalue spectra undergo a spectral flow that reshapes the profiles of the eigenstates, enabling the formation of a flat-top state with a uniform yet nontrivial envelope. We implement this concept in a photonic crystal slab, where a central bulk region is surrounded by heterogeneous band gaps that tailor reflection phases to serve as an artificial local gauge field. By inducing single-mode lasing, we probe the morphing of mode envelope profiles, demonstrating a continuous transition from conventional oscillatory states to a flat-top state via near- and far-field measurements.