Time-resolved probing and modeling of optical signatures of ultrashort-pulse laser spallation and phase explosion in iron-nickel targets

Time-resolved microscopy is an established technique for probing the dynamics of laser ablation, thus enabling the exploration of material behavior under extreme conditions produced by laser excitation. Decoding the time-resolved data on the rapid variation of optical properties of a material undergoing nonequilibrium phase decomposition and ejection, however, presents a significant challenge. In this paper, a closely integrated computational and experimental study of laser ablation of FeNi targets is used to establish direct links between the dynamics of laser ablation and the evolution of optical signal in pump-probe experimental measurements. The experiments and large-scale atomistic simulations are performed for a range of fluences covering the onset of material ejection at the threshold for photomechanical spallation and the transition to the phase explosion regime of laser ablation. The connections between the simulations and experiments are established through numerical modeling of the interaction of an electromagnetic wave representing the experimental probe laser pulse with transient states of the ablation plume predicted in the atomistic simulations. The combined modeling and experiments have revealed a complex interplay of processes defining the transient optical properties of the emerging ablation plume, including the oscillations of reflectance due to the interference of parts of the probe pulse reflected from the spalled layer and the newly formed surface of the target in the spallation regime, the disappearance of the interference pattern upon the transition to the regime of phase explosion, nonmonotonous variation of the refractive index of a transient spongy structure of interconnected liquid regions, and the formation of nanoscale hot spots within the expanding spongy structure due to the near-field concentration of electromagnetic field. The results of this study not only provide reliable guidance for the interpretation of optical signals measured in pump-probe experiments but also suggest new ideas for manipulating the ablation plume dynamics to achieve higher efficiency and precision in laser synthesis and processing of materials in the double- or multipulse irradiation regimes.