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

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

4. Experiment Setup Pump pulse angled so that “sweeping” action can be recorded Pump pulse angled so that “sweeping” action can be recorded Probe pulse (weak w/ orthogonal polarization) provides time-resolved measurements 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 Previous experiments noticed there existed a threshold “breakdown” intensity of each material Measured with similar setup, looking at reflectivity change of plasma Measured with similar setup, looking at reflectivity change of plasma Ablation experiment made sure breakdown not reached (no 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 1. Laser hits atoms – deposits energy to electronic states of valence & conduction bands 2. Energy state distribution – relaxation time t R 3. Energy transported macroscopically 4. Displacement of atoms – ablation time t A  Thermal processes dominant when t A >> t R  Thermal processes dominant when t A >> t R

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

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

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

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

11. Hypothesis Gas-filled bubble forming in molten material Gas-filled bubble forming in molten material Some problems… (not supposed to be possible) 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 Speed of sound drastically lower in hybrid phase Two steep density boundaries develop Two steep density boundaries develop Forms a kind of “gas bubble” hypothesized Forms a kind of “gas bubble” hypothesized

14. Optical Properties Inhomogeneous phase in ablation layer difficult to model Inhomogeneous phase in ablation layer difficult to model Can approximate with Maxwell-Garnett model (right) Can approximate with Maxwell-Garnett model (right) Calculate n=2 for Si… high enough to explain rings 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 ablate material in the fashion outlined in this paper Ultrafast pulses eject surface material in volatile and sometimes unpredictable states Ultrafast pulses eject surface material in volatile and sometimes unpredictable states If the incident intensity is high enough, a plasma can form 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 If another pulse hit the material immediately after the first, it would interact with the ejected material