The physical mechanisms of short-pulse laser ablation

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 gtgt 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

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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)

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Unsteady Isentropic Expansion

• laser excitation
• thermalization
• isentropic expansions
• hybrid gas-liquid state

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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 n2 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