Tuesday, 12 October 2004 - 1:50 PM

This presentation is part of : Advanced modeling for laser-material processing

Quantum Modeling of Photon-Electron-Phonon Interaction during Ultrafast Laser Ablation of Dielectrics

Lan Jiang, Department of Mechanical and Aerospace Engineering, University of Missouri-Rolla, 121 C ME Annex, 1870 Miner Circle, Rolla, MO 65409 and Hai-Lung Tsai, Department of Mechanical and Aerospace Engineering and Engineering Mechanics, University of Missouri-Rolla, Rolla, MO 65409.

In ultrafast (<10 ps) laser ablation of dielectrics, materials are first transformed into absorbing plasma with metallic properties and, then, the subsequent laser-plasma interaction causes material removals. In this process, photon-electron-phonon interaction within the ultrafast pulse duration has decisive impacts over the ablation. This study proposes a quantum model to investigate photon-electron-phonon interaction and predict ablation threshold, depth, and crater shape. Impact ionization and photoionization including muliphoton ionization and tunnel ionization are the major competing mechanisms considered for electron generations by using the flux-doubling condition and the Keldysh's theory. The Fokker-Planck equation is used to determine electron density distribution. A modified free electron plasma model is employed to calculate the optical properties of generated plasma. Quantum statistic treatments are used to investigate electron heat capacity, relaxation time, and heating. The proposed model greatly increases accuracy of ablation depth prediction and can predict the crater shape. The predictions of threshold fluence and ablation depth for barium aluminum borosilicate and fused silica are in excellent agreement with published experimental data. The variations of optical properties in time and space domain are analyzed, which is found to be the key factor for ablation crater shape. Some interesting phenomena observed experimentally are well explained, such as, the flat bottom of the crater ablated by an ultrafast Gaussian pulse. The effects of fluence and pulse duration are also studied.

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