Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Large Scale Numerical Modeling of Laser Ablation

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Background Goal: to understand ultrafast laser (pulsewidth < s) – material interaction (application: laser micro- machining) The process of ultrafast laser-matter interaction is highly non-equilibrium. The heating rate can reach K/s, and the material can be superheated above the thermo-dynamic critical point.

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Fundamental Processes and Time Scales Involved in Ultrafast (fs) Pulsed Laser Ablation of Metal –Heating of electrons (before lattice being heated) ~ fs –Transfer of energy from electrons to the lattice and heating of the lattice to temperatures above the melting point ~ ps – the temperature of elections could be much higher than the lattice temperature –Liquid – vapor phase change, phase explosion ~ 10 – 100 ps –Melting duration ~ 100 ps – 1 ns –Cooling of the lattice ~  s

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Mechanism for material removal Phase explosion versus spinal decomposition?

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Continuum (the Two Step Energy Transfer) Model and Its Limitation For T l = 5,000 K, For T e = 10 eV, For T e = 50 eV, In laser heating, the electron temperature T e can exceed 50 eV (500,000 K!).

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University The Two Step Energy Transfer Model - cont. Kinetic relation at the solid-liquid interface Energy balance at the solid-liquid interface

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University The Two Step Energy Transfer Model - cont. Kinetic relation at the liquid-vapor interface where Energy balance at the liquid-vapor interface

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Procedure of the Finite Difference (Enthalpy) Method The electron temperature field is solved for by using the semi-implicit Crank-Nicholson scheme. The lattice temperature field and related phase changes are solved for by using an enthalpy formulation.

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Numerical Results

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Numerical Results – Cont. - Evaporation rate is very small (< 0.1 nm per pulse) - Unable to compute phase explosion

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Governing Equations: MD Simulation of Laser Melting and Ablation of an Argon Solid

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Snapshots of Atomic Positions (laser irradiates from the right hand side) (a) t=5 ps (b) t=10 ps (c) t=15 ps (d) t=20 ps (e) t=25ps (f) t=30 ps

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University MD Simulation – Phase Explosion

Center for Laser Micro-Fabrication School of Mechanical Engineering Purdue University Future Work Simulating laser ablation of ‘engineering materials’ Morse potential for fcc metals Stillinger-Weber potential for Si Large scale simulation: increase the number of molecules from the current 2,000,000 to 500,000,000. Combined MD and continuum approach.