Parametric Study of the Ablation Characteristics of Absorbing Dielectrics by Short Pulse Laser

Abstract

This paper investigates the effect of ablation of absorbing dielectrics by two successive ultra short pulses from an excimer laser source. The numerical model is based on two photon absorption followed by thermal degradation and diffusion. Unlike most previous studies the present formulation considers the transient nature of laser propagation within the medium. Heating of the material is dependent on light absorption by chromophores while ablation occurs through sublimation of the material from the surface. The mathematical formulation takes into consideration the saturation effects within the framework of a three level system of the electronic structure of chromophores. This involves solving a set of coupled rate equations, heat diffusion equation, and the transient radiative transport equation, using Fromm’s scheme. Results for the temperature distribution and ablation depth are obtained for different laser parameters and material properties. Parametric study of the delay time between two successive pulses, laser pulse width, laser fluence, activation energy, and the relaxation time is only performed in this paper for the purpose of brevity. The results obtained by the consideration of the transient radiative transfer equation are compared with the steady state formulation and significant differences are observed in the temperature profiles and the ablation depth.