Plasma effects in picosecond-femtosecond UV laser ablation of polymers

Abstract

Laser ablation of polyimide (PI) and polymethyl-methacrylate (PMMA) at 248 nm with pulse lengths τ ranging from 200 fs to 200 ps was investigated. The measured ablation rates show minima for pulse lengths of about 5 ps (PMMA) or 50 ps (PI).

The reflected fraction of the ablating laser pulse was measured as a function of the pulse length. In the case of PMMA maximum reflectance corresponds to a minimum ablation rate.

This behavior can be explained by a dynamic plasma reflection model: A fast build up of a dense plasma is followed by high obscuration for a brief transition time and a self-regulating opacity for the rest of the pulse. This model of plasma mediated ablation leads to a τ^1/4-dependence of the ablation rate at fixed fluence, which fits very well to the measured data, in particular if an extension to nanosecond ablation data of PI and PMMA is considered.