Ultrafast vector laser processing of GaN in both linear and nonlinear regimes.

Femtosecond laser ablation is a powerful technique for the high-precision processing of wide-bandgap semiconductors like gallium nitride (GaN). However, conventional Gaussian beams concentrate energy at the optical axis, inducing steep thermal gradients, local overheating, and restricted processing efficiency. While Laguerre-Gaussian (LG) beams offer a spatial alternative through annular energy distribution, their nonlinear, spatiotemporal thermal dynamics during ultrafast irradiation remain largely unquantified. In this study, a unified multiphysics model is developed to comparatively analyze the dynamic thermal responses of GaN under tightly focused Gaussian and LG beam irradiation. The model couples vectorial diffraction theory with a nonlinear two-temperature model, dynamically incorporating multiphoton absorption, impact ionization, and Drude-based surface reflectivity corrections throughout the femtosecond laser pulse. Simulations reveal that the LG beam's hollow structure redistributes optical energy, reducing the maximum radial lattice-temperature gradient compared to the Gaussian beam. Also, under the present modeling conditions, the annular redistribution expands the effective molten coverage, which is favorable for improving the theoretical geometric scanning efficiency in both line-cutting and area-cutting scenarios.