Temporal modulation toward femtosecond laser-induced nonlinear ionization process.

The temporal chirp of single femtosecond (fs) pulses will affect the laser-induced ionization process. By comparing the ripples induced by negatively and positively chirped pulses (NCPs and PCPs), the growth rate showed a significant difference, resulting in a depth inhomogeneity of up to 144%. A carrier density model tailored with temporal characteristics showed that NCPs could excite a higher peak carrier density, contributing to a highly efficient generation of surface plasmon polaritons (SPPs) and overall advancement of the ionization rate. Such distinction originates from their contrary incident spectrum sequences. Current work reveals that temporal chirp modulation can control the carrier density in ultrafast laser-matter interaction, which possibly brings an unusual acceleration for surface structure processing.