Femtosecond-laser-induced folding dynamics in multilayer hexagonal boron nitride revealed by time-resolved microscopy

Hexagonal boron nitride (hBN) holds promise for nano-optoelectronics, yet the ultrafast dynamics governing its laser-induced structural transformations remain elusive. Here, we employ time-resolved microscopy to unveil the spatiotemporal evolution of femtosecond-laser-triggered folding in multilayer hBN on SiO2/Si. A critical pulse energy threshold (>100 nJ, >6.4 J/cm2) is identified to drive a topological transition, manifesting as peripheral edge folding—phenomenologically analogous to graphene. Real-time imaging reveals a four-stage mechanism: (1) initial melting (0–40 ps), (2) molten pool radial expansion and SiO2 ablation (70–130 ps), (3) outward edge folding via thermal stress (260–760 ps), and (4) resolidification (960–3760 ps). Atomic force microscopy confirms folded nanostructures with ∼100 nm protrusions, contrasting with the pristine 50 nm film thickness. Crucially, the ablation threshold is quantified at 30 nJ (single-shot), while folding requires energies ≥100 nJ. This work establishes a foundation for laser-driven topology engineering in 2D materials, enabling precision fabrication of hBN origami nanostructures.