Influence of single and two-color double-pulse femtosecond laser on melting dynamics and ablation threshold of tungsten: A TTM-based analysis

Extensive research on ultrashort laser-ablation and surface nanostructuring of refractory metal tungsten (W) is available. However, investigations of cumulative heating by double-pulse (DP) excitation, particularly with tailored spectral arrangements, that enable new possibilities for micromachining of metal surface, remain unexplored. The current paper examines ultrafast heat transport dynamics due to two-color DP femtosecond lasers. This is accomplished by developing two-dimensional axisymmetric two-temperature model codes and validating it against experimental single-pulse ablation threshold of W. A study of four different wavelength-combination DP lasers reveals that both wavelength selection and pulse sequencing play crucial roles in controlling melt-zone dimensions. Dichromatic excitations exhibit minimum enhancement in melt-front radius between pulse-pairs when short wavelength precedes long one and maximum enhancement in the reverse order. Further, short-wavelength initiation yields extensive but shallow surface melting, whereas long-wavelength initiation results in confined surface modification with deeper melt-zones. Interestingly, although short-wavelength single-pulse irradiation produces the highest melt-volume, mixed-wavelength DP excitation enables independent tuning of axial and radial extents of melt-zone dimensions for controlled surface modification. Furthermore, it is shown that, across all inter-pulse delays, short-wavelength pulses require the lowest fluence to induce ablation, whereas long-wavelength pulses require the highest fluence, with mixed-wavelength pulse combinations exhibiting intermediate behavior. A key achievement of this study is the development of a phenomenological model that explains the universal, delay-dependent evolution of DP ablation thresholds across all wavelength blends. Thus, the study offers new insights into delay-controlled DP interactions in W and sets guidelines for optimizing laser parameters to tune surface morphology and ablation thresholds.