Effects of Femtosecond Laser Irradiation of Metallic and Dielectric Materials in the Low-to-High Fluence Regimes

Mathew P Polek, Purdue University

Abstract

In this thesis, we studied the variation in the thermal properties of electrons and ions during femtosecond laser irradiation of copper and silicon and the resulting effects on the optical, ablative, and ion properties of each material. We established the theories needed to model the variation in heat capacity, thermal conductance, electron-ion coupling, laser absorption, and collision frequency as a function of electron and ion temperature. These theories were then implemented within our model, which we used to obtain theoretical approximations of laser absorption, melt layer formation, pressure buildup, and ion properties, which we compared to experimental results. We found that our theoretical model qualitatively matched the experimental results, and we were able to use our theoretical model to explain various features and transitions within the experimental results. At low fluences and high pulse numbers on silicon, we found that oxidation and defect formation was responsible for the formation of the ablation patterns by localization of the incident laser light around the defects, with different ablation patterns forming depending on the surface ion temperature following irradiation. For lower pulse numbers, two different types of surface structuring occurred on the surface of the silicon depending on laser fluence relative to the melting threshold. For fluences very close to the melting threshold, we saw the formation of 200 nm ripple structures and 100 nm pores caused by surface plasmon interference and deep melt layer formation respectively. As the laser fluence was raised above the ablation threshold, a different type of surface structure began to form due to a transition from thermal to non-thermal melting of the surface. For high fluences on copper, we studied the variation in ablation depth and two different ablation regimes. For fluences below 3J/cm2, ablation on copper was caused by thermalization between electrons and ions followed by phase explosion. For higher fluences, a large build-up of electron pressure caused non-thermal and shockwave ablation, resulting in a large increase in the ablation depth. It was found that the copper ion flux was independent of the ablation depth, and the ions originated from a thin surface layer comparable to the optical depth of the laser. Ion properties such as the charge state and ion velocity were related to the variation in surface electron temperature. In the case of high fluences irradiation on silicon, no transition in the ablation regimes was found due to a large electron pressure being present even at fluences below the ablation threshold. Thus, thermal, non-thermal, and shockwave ablation was found to occur even at the ablation threshold. Similar to copper, the ion flux and ion properties originated from a thin layer comparable to the optical depth, with properties dependent on electron temperature.

Degree

M.S.M.N.E.

Advisors

Hassanein, Purdue University.

Subject Area

Physics

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