Thermal and mechanical limitations to processing resolution in volume non-diffractive ultrafast laser structuring

The minimum feature size and spatial resolution are key factors for ultrafast laser structuring, defining the resulting function of the structured material. With the aim to improve the processing resolution achievable for non-diffractive beams, we analyze in volume the hydrodynamic and thermomechanical material responses during laser structuring defining the affected zone. The extent of laser induced cavitation on a 100 nm scale, accompanied by local annealing, and internal fracture under stress on µm scale are revealed using a combination ion beam milling, chemical etching and electron microscopy. Melting and mechanical stress acting at different planes are proved to be primary factors for restricting the structuring resolution to a critical distance where cooperative effects appear. Their extent is controllable via the local intensity and, hence, the pulse duration. A parametric control on the accumulated energy density limiting the thermomechanical action range and maximizing resolution is then reported.