The role of laser fluence and ambient environments on femtosecond laser induced breakdown spectroscopy and on surface morphology of Mg and Zr

The effects of laser fluence and ambient environments on plasma parameters and on surface modifications of femtosecond laser irradiated Mg and Zr have been investigated. A Ti:Sapphire laser (800 nm, 35 fs) was employed to irradiate the metallic targets under vacuum and Ar environments at various fluences. Laser induced breakdown spectroscopy analyses reveal that the optical emission spectra, excitation temperature (T_e), and electron number density (n_e) of metallic plasmas exhibit increasing trend with increasing fluence irrespective of the target under both the environments. This increasing tendency is because of the enhanced ablation rate with the increase in the fluence. However, the values of these parameters are significantly higher in the presence of Ar as compared to that of vacuum, which is attributable to confinement effects offered by the gas. The plasma parameters, T_e and n_e, have higher values in the case of Zr under both environments (vacuum and Ar) due to its higher melting point and lower thermal conductivity as compared to Mg. Field emission SEM analyses for both of the metals irradiated under vacuum exhibit a non-uniform distribution of nanoglobules, nanocones, and micrometer-sized cavities in the case of irradiated Mg, whereas for Zr, there is growth of laser induced periodic surface structures along with the formation of a deep crater. When both the metals were irradiated in Ar, a significant difference in surface morphologies of both Mg and Zr has been observed. In the case of Mg, SEM discloses the formation of micro-inhomogeneities and micrometer-sized cones covered with nanoglobules, whereas for Zr, high-spatial-frequency laser induced periodic surface structures covered with nanoroughness and micro-columns have been detected. In the present work, by exploring the optimum conditions in terms of laser fluence, environmental conditions, and material response, a correlation has been established between the calculated plasma parameters and observed micro- and nanostructures for both of the metals. This established correlation will enable us to better understand the plasma to be utilized for ion-implantation, thin film deposition, and surface structuring in a more effective manner.