Structure-Mediated Excitation of Air Plasma and Silicon Plasma Expansion in Femtosecond Laser Pulses Ablation.

Lan Jiang,Changji Pan,Ming Li,Weina Han,Guoyan Wang,Jingya Sun,Yongfeng Lu,Feifei Wang,Kaihu Zhang,Qingsong Wang

doi:10.1155/2018/5709748

Abstract

Femtosecond laser-induced surface structures upon multiple pulses irradiation are strongly correlated with the pulse number, which in turn significantly affects successive laser-material interactions. By recording the dynamics of femtosecond laser ablation of silicon using time-resolved shadowgraphy, here we present direct visualization of the excitation of air plasma induced by the reflected laser during the second pulse irradiation. The interaction of the air plasma and silicon plasma is found to enhance the shockwave expansion induced by silicon ablation in the longitudinal direction, showing anisotropic expansion dynamics in different directions. We further demonstrate the vanishing of air plasma as the pulse number increases because of the generation of a rough surface without light focusing ability. In the scenario, the interaction of air plasma and silicon plasma disappears; the expansion of the silicon plasma and shockwave restores its original characteristic that is dominated by the laser-material coupling. The results show that the excitation of air plasma and the laser-material coupling involved in laser-induced plasma and shockwave expansion are structure mediated and dependent on the pulse number, which is of fundamental importance for deep insight into the nature of laser-material interactions during multiple pulses ablation.

Highlights

Femtosecond laser-induced plasma, as a critical process of complicated laser-material interactions, has attracted intense attention concerning the fundamental mechanisms of laser ablation and the practical applications in micro/nanofabrication [1,2,3], nanoparticle synthesis [4, 5], thin film deposition [6, 7], and laser-induced breakdown spectroscopy [8, 9]
We present a direct experimental visualization of the excitation of air plasma on femtosecond timescale induced by the reflected pulse, which is mediated by the crater structure generated by the previous pulse
We first focus on the plasma and shockwave evolution recorded during the first two pulses ablation at probe delays ranging from femtoseconds to nanoseconds with ultrafast pump-probe shadowgraphy (Figure 1)

Summary

Introduction

Femtosecond laser-induced plasma, as a critical process of complicated laser-material interactions, has attracted intense attention concerning the fundamental mechanisms of laser ablation and the practical applications in micro/nanofabrication [1,2,3], nanoparticle synthesis [4, 5], thin film deposition [6, 7], and laser-induced breakdown spectroscopy [8, 9]. Shadowgraphy images of transient plasma structure and material ejection provide insights into thermal and nonthermal laser ablation mechanisms [15,16,17,18,19]. Shadowgraphy image gives insight into the plasma dynamics generated under different ablation conditions, such as the variation of thickness of thermally grown oxide films [22, 23], air pressures [24], and in the case of excitation of air plasma at high laser intensity [24,25,26].

Methods

Results

Conclusion