Recently, a new laser micromachining technique using multiple femtosecond pulses with a picosecond-to-nanosecond separation as a train group has demonstrated the ability to increase the ablation quality of dielectrics and semiconductors. However, the mechanisms involved in the technique are not fully understood. This study employs the plasma model recently developed by the authors to analyze the femtosecond pulse-train ablation of dielectrics. It is found that the transient significantly varying optical properties are the important reasons that lead to the advantages of the pulse-train technique. It has demonstrated that there exits a constant ablation-depth zone with respect to fluence, which has also been observed experimentally. By using the pulse-train technology, it is possible to obtain repeatable nanostructures, even when the laser system is subject to some fluctuations in fluences.

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