Stagewise evolution of nanovoids into nanotrenches on 4H-SiC under ultrafast laser irradiation

Ultrafast laser extreme structuring strategies provide an important capability of advancing nanoscale surface patterning by progressively exploiting near fields. These fields can be shaped in turn by the evolving surface structures, adapting to a dynamic topography. However, tracking the transition from an initial localized nanovoid to oriented nanotrenches with increasing pulse accumulation is essential to understanding the underlying physical processes. Here, we observe and clarify the stagewise evolution of nanopatterns on 4H-SiC by independently varying single-pulse energy and pulse number. Experiments varying the polarization of subsequent laser pulses show that initially isotropic nanovoids evolve into anisotropic nanotrenches elongated perpendicular to the subsequent pulse polarization. 3D finite-difference time-domain (FDTD) simulations based on representative experimental morphologies show that preformed structures redistribute the local electric field under subsequent irradiation. The simulated field distributions are consistent with the morphological features observed at later stages, supporting a stagewise structural feedback pathway.