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The physical mechanisms of short-pulse laser ablation

The physical mechanisms of short-pulse laser ablation. D. Von der Linde, K. Sokolowski-Tinten A summary report by Ryan Newson June 25, 2004. Laser Ablation. Important for materials processing Many permutations of beam parameters; many materials

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The physical mechanisms of short-pulse laser ablation

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  1. The physical mechanisms of short-pulse laser ablation D. Von der Linde, K. Sokolowski-Tinten A summary report by Ryan Newson June 25, 2004

  2. Laser Ablation • Important for materials processing • Many permutations of beam parameters; many materials • Ultrashort pulses (fs-ps) interact fundamentally different than longer pulses

  3. Importance in Our Machining Experiment • This discussion about one ultrashort pulse • Our experiment deals with a train of ultrashort pulses • Important to extend this knowledge to our regime

  4. Experiment Setup • Pump pulse angled so that “sweeping” action can be recorded • Probe pulse (weak w/ orthogonal polarization) provides time-resolved measurements

  5. Breakdown Threshold & Plasma • Previous experiments noticed there existed a threshold “breakdown” intensity of each material • Measured with similar setup, looking at reflectivity change of plasma • Ablation experiment made sure breakdown not reached (no plasma) [6] D. von der Linde, H. Schuler, J. Opt. Soc. Am. B 13 (1996) 216

  6. Physical Processes • Laser hits atoms – deposits energy to electronic states of valence & conduction bands • Energy state distribution – relaxation time tR • Energy transported macroscopically • Displacement of atoms – ablation time tA  Thermal processes dominant when tA >> tR

  7. Materials • Shown in detail is silicon, but many metals and semiconductors used • All show same results (to follow) • Hence results apply to our experiment, where aluminum is primarily used

  8. Time-Resolved Results liquid metallic Si start of ring structure surface resolidification boundary of ablated area amorphous Si

  9. Ring Pattern • Where does it occur? • Used interference microscopy (bottom) • Occurs only on ablation area

  10. Ring Pattern cont. • Physical structure or optical interference? • Varied probe pulse wavelengths • Ring spacing wavelength • Must be interference – Newton rings

  11. Hypothesis • Gas-filled bubble forming in molten material • Some problems… (not supposed to be possible)

  12. Unsteady Isentropic Expansion laser excitation & thermalization isentropic expansions hybrid gas-liquid state

  13. Ablation Layer • Speed of sound drastically lower in hybrid phase • Two steep density boundaries develop • Forms a kind of “gas bubble” hypothesized

  14. Optical Properties • Inhomogeneous phase in ablation layer difficult to model • Can approximate with Maxwell-Garnett model(right) • Calculate n=2 for Si… high enough to explain rings

  15. Summary & Application to Us • Ultrafast pulses ablate material in the fashion outlined in this paper • Ultrafast pulses eject surface material in volatile and sometimes unpredictable states • If the incident intensity is high enough, a plasma can form • If another pulse hit the material immediately after the first, it would interact with the ejected material

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