Femtosecond laser-induced damage morphologies of crystalline silicon by sub-threshold pulses

The effect of sub-threshold pulses of circularly polarized Ti:sapphire femtosecond laser system on crystalline (1 0 0) silicon wafer was investigated. Surface damage morphologies were studied by irradiating the test silicon surface with pulses (peak fluence of 0.25 J/cm2) in succession. These pulses were below the single-pulse surface damage threshold. After the few initial pulses, the observed surface damage morphologies were found to be characterized by a minor phase change region and a major surface damage area at the center, corresponding to the well-known laser-induced periodic surface structure (LIPSS). Further increase in the number of pulses resulted in the formation of new surface morphologies with different features such as ablation, modification, and re-deposited materials. These features were reproducible and more distinguishable at higher number of pulses.