The physical mechanisms of The physical mechanisms of short-pulse laser ablationshort-pulse laser ablation

D. Von der Linde, K. Sokolowski-D. Von der Linde, K. Sokolowski-TintenTinten

A summary report by Ryan A summary report by Ryan NewsonNewson

June 25, 2004June 25, 2004

Laser AblationLaser Ablation

Important for materials processingImportant for materials processing Many permutations of beam parameters; Many permutations of beam parameters;

many materialsmany materials Ultrashort pulses (fs-ps) interact Ultrashort pulses (fs-ps) interact

fundamentally different than longer pulsesfundamentally different than longer pulses

Importance in Our Machining Importance in Our Machining ExperimentExperiment

This discussion This discussion about one about one ultrashort pulseultrashort pulse

Our experiment Our experiment deals with a train of deals with a train of ultrashort pulsesultrashort pulses

Important to extend Important to extend this knowledge to this knowledge to our regimeour regime

Experiment SetupExperiment Setup

Pump pulse Pump pulse angled so that angled so that “sweeping” action “sweeping” action can be recordedcan be recorded

Probe pulse (weak Probe pulse (weak w/ orthogonal w/ orthogonal polarization) polarization) provides time-provides time-resolved resolved measurementsmeasurements

Breakdown Threshold & PlasmaBreakdown Threshold & Plasma

Previous experiments Previous experiments noticed there existed a noticed there existed a threshold “breakdown” threshold “breakdown” intensity of each materialintensity of each material

Measured with similar Measured with similar setup, looking at setup, looking at reflectivity change of reflectivity change of plasmaplasma

Ablation experiment Ablation experiment made sure breakdown made sure breakdown not not reached (no plasma)reached (no plasma)

[6] D. von der Linde, H. Schuler, J. Opt. Soc. Am. B 13 (1996) 216

Physical ProcessesPhysical Processes

1.1. Laser hits atoms – deposits energy to Laser hits atoms – deposits energy to electronic states of valence & conduction electronic states of valence & conduction bandsbands

2.2. Energy state distribution – relaxation time tEnergy state distribution – relaxation time tRR

3.3. Energy transported macroscopicallyEnergy transported macroscopically4.4. Displacement of atoms – ablation time tDisplacement of atoms – ablation time tAA

Thermal processes dominant when tThermal processes dominant when tAA >> t >> tRR

MaterialsMaterials

Shown in detail is silicon, but many metals Shown in detail is silicon, but many metals and semiconductors usedand semiconductors used

All show same results (to follow)All show same results (to follow) Hence results apply to our experiment, Hence results apply to our experiment,

where aluminum is primarily usedwhere aluminum is primarily used

Time-Resolved ResultsTime-Resolved Results

liquid metallic Siliquid metallic Si

start of ring start of ring structurestructure

surfacesurface

resolidificationresolidification

boundary of boundary of ablated areaablated area

amorphous Siamorphous Si

Ring PatternRing Pattern

Where does it Where does it occur?occur? Used Used

interference interference microscopy microscopy (bottom)(bottom)

Occurs only on Occurs only on ablation areaablation area

Ring Pattern cont.Ring Pattern cont.

Physical structure Physical structure or optical or optical interference?interference? Varied probe pulse Varied probe pulse

wavelengthswavelengths Ring spacing Ring spacing

wavelengthwavelength Must be Must be

interference – interference – Newton ringsNewton rings

HypothesisHypothesis

Gas-filled bubble Gas-filled bubble forming in molten forming in molten materialmaterial

Some problems… Some problems… (not supposed to (not supposed to be possible)be possible)

Unsteady Isentropic ExpansionUnsteady Isentropic Expansion

laser excitation &laser excitation &

thermalizationthermalization

isentropic isentropic expansionsexpansions

hybrid gas-liquid hybrid gas-liquid statestate

Ablation LayerAblation Layer

Speed of sound Speed of sound drastically lower in drastically lower in hybrid phasehybrid phase

Two steep density Two steep density boundaries developboundaries develop

Forms a kind of Forms a kind of “gas bubble” “gas bubble” hypothesizedhypothesized

Optical PropertiesOptical Properties

Inhomogeneous phase Inhomogeneous phase in ablation layer in ablation layer difficult to modeldifficult to model

Can approximate with Can approximate with Maxwell-Garnett Maxwell-Garnett modelmodel(right)(right)

Calculate n=2 for Si… Calculate n=2 for Si… high enough to explain high enough to explain ringsrings

Summary & Application to UsSummary & Application to Us

Ultrafast pulses ablate material in the Ultrafast pulses ablate material in the fashion outlined in this paperfashion outlined in this paper

Ultrafast pulses eject surface material in Ultrafast pulses eject surface material in volatile and sometimes unpredictable volatile and sometimes unpredictable statesstates

If the incident intensity is high enough, If the incident intensity is high enough, a plasma can forma plasma can form

If another pulse hit the material If another pulse hit the material immediately after the first, it would immediately after the first, it would interact with the ejected materialinteract with the ejected material