Large area materials processing via unfocused beams of high-energy femtosecond lasers

Petawatt-class laser systems can generate intensities sufficient for material processing even without beam focusing. However, their inhomogeneous energy distribution renders them unsuitable for producing macroscopically uniform patterns. In this study, this limitation is turned into an advantage: upon irradiation with the entire beam cross-section, the surface encodes the spatial energy distribution as a material response library, enabling systematic analysis based on the robust dataset obtained. This framework provides a single-step methodology for determining optical and morphological thresholds, offering an alternative to conventional Laser-Induced Damage Threshold (LIDT) protocols.
As a proof of concept, copper was irradiated by a stationary beam from a diode-pumped ytterbium femtosecond laser (500 mJ and 500 fs at 1030 nm/50 Hz). A reflectance map obtained using a He–Ne probe was correlated with the energy distribution of the processing beam. The morphological processes behind the energy-dependent darkening were examined through constructing a surface morphology map from scanning electron microscopy (SEM) images taken at 5,000–50,000× magnifications. The reflectance-based darkening threshold energy, marking the first visible modification, was 1.46 µJ (≈0.023 J·cm⁻² in terms of fluence), while the morphological threshold appeared at 1.2 µJ, indicating the onset of nanostructure formation. Maximum darkening occurred between 2.4–2.6 µJ, associated with a hierarchical morphology of cauliflower-like microtexture and a highly homogeneous nanostructure, followed by reflectance recovery above 2.8 µJ due to melting and coalescence. Gray-Level Co-Occurrence Matrix (GLCM) texture analysis revealed that homogeneity sensitively indicated the onset of ablation, whereas contrast and entropy reflected changes in granularity and structural order.