Ultrafast Electron Dynamics and Optical Interference Tomography of Laser Excited Steel

Femtosecond laser machining of materials has transformed into a mature high-precision technology, giving access to a wide range of applications. Yet, for a complete exploitation of its full potential, a detailed knowledge of the complex laser-matter interaction processes is required. For the case of the application-relevant material steel, a complete study of the electron dynamics and material transformation upon single laser pulse irradiation (800 nm, 120 fs) is reported. Detailed analysis of the obtained time- and fluence-dependent reflectivity map reveals that ultrafast electron heating during the pulse is directly followed by energy transfer to the lattice within a few picoseconds, reaching fluence-dependent peak temperatures from below melting threshold to above the boiling point. Moreover, an innovative approach to obtain the ultrafast-evolving 3D structure of a femtosecond laser excited material is reported, unraveling the dynamics of complex processes as melting, ablation, and solidification. Combined with modeling, the evolving optical properties, multilayer structure, and expansion velocities can be precisely determined. The information obtained from this study will contribute to further increase the achievable precision and wealth of structures that can be produced by designing efficient pulsed energy deposition schemes for selective re-excitation of the material.