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7/30/2019 Slides - Laser Ablation
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
Fundamentals & ApplicationsFundamentals & Applications
Samuel S. Mao
Department of Mechanical Engineering
University of California at Berkeley
Advanced Energy Technology Department
Lawrence Berkeley National Laboratory
March 10, 2005
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
What is Laser Ablation?
Mass removal by coupling laser
energy to a target material
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Is it important?Is it important?
target
substrate
material
plume
) Film deposition
* oxide/superconductor films
* nanocrystals/nanotubes
lase
rabla
tion
target
plasma lens
optical spectrometer
mass spectrometer
)Materials characterization
* semiconductor doping profiling
* solid state chemical analysis
target transparent solid
microstructure
)Micro structuring
* direct wave guide writing
* 3D micro fabrication
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Is it important?Is it important?
) Film deposition
* oxide/superconductor films
* nanocrystals/nanotubes
lase
rabla
tion
100 m
)Materials characterization
* semiconductor doping profiling
* solid state chemical analysis
)Micro structuring
* direct wave guide writing
* 3D micro fabrication
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
lase
rabla
tion
Do we really understand?Do we really understand?
plasmaplasma
targettarget
laserlaser
beambeam Laser ablation is still
largely unexplored at the
fundamental level.
J. C. Miller & R. F. Haglund,
Laser Ablation and Desorption
(Academic, New York, 1998)
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
) What is happening?
nanosecondpicosecond microsecondfemtosecond
10-15 s 10-12 s 10-9 s 10-6 s
lase
rpulse
target
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
??
ExperimentsExperiments -- ultrafast imagingultrafast imaging
ablationlaser beam
target
CCD
probe beam
pump beam
delay time
imaginglaser beam
)Pump-probe technique
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
1 fs = 10 15 s
Femtosecond Time ScaleFemtosecond Time Scale
laserpulse
glass
air
100 fs,800 nm
E = 30 J
)Ultrafast imaging - time dependent energy transfer
self-focusing
C
Velectronic excitatione-h plasma
100m
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Femtosecond Time ScaleFemtosecond Time Scale
)Peak electron number density ne - time dependence
0 500 1000 1500 2000 25001x10
19
2x1019
3x1019
4x1019
5x1019
6x1019
Ne,max
(cm-3)
time (fs)
flattened peak electron number density
no breakdown
0
5
0 fs
z (m)el
ectronnumberd
ensity(1019 cm-3)
0
5
333 fs
0
5
667 fs
0
5
1000 fs
0
5
1333 fs
0
5
1667 fs
0 100 200 300 400 500
0
5 2000 fs
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Femtosecond Time ScaleFemtosecond Time Scale
)Fundamental processes
laserpulse
9 Nonlinear absorption
9 Nonlinear optics
Self-focusing - intensity dependence of refractive index
Electronic excitation - interband absorption
C
V
positive refractive index change
negative refractive index change
0
z
suppress self-focusing
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
) What is happening?
femtosecond picosecond nanosecond microsecond
laserpulse
target
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
50 m
(laser pulse: 35 ps, fluence: 90 J/cm
2
)
t = 50 ps
50 m
(laser pulse: 35 ps, fluence: 60 J/cm
2
)
t = 50 ps
ablationlaser
pulse
target Cu
(air)
1 ps = 10 12 s
Picosecond Time ScalePicosecond Time Scale
) Picosecond imaging
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
)Threshold behavior
(laser pulse length 35 ps; pictures taken at 20 ps)
85 J/cm2
plasma onset
110 J/cm2
regime-2
40 J/cm2
regime-1
50 m
9 Threshold for picosecond plasma formation same asthe threshold for ablation efficiency reduction:
~ 85 J/cm2 (laser fluence)
~ 1012 W/cm2 (power density)
(threshold for direct laser-induced air breakdown: ~ 1013 W/cm2)
0 100 200 300 400 5000
5
10
15
20
25
ab
lationdepth(m)
laser fluence (J/cm2)
1 2
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
)Ultrafast interferometry
r
z
t = 15 ps interference pattern
50 m
target
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
)Electron number density
0 50 100 150 200 2500.0
2.0x1019
4.0x1019
6.0x1019
8.0x10
19
1.0x1020
1.2x1020
Ne
(cm
-3)
Z (m)
z
t = 15 ps
air density
9 A large electronnumber density!(close to target surface)
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
)Longitudinal (z) expansion
z
(laser energy: 10 mJ)
50m
0 20 40 60 80 100 1200
100
200
300
400
500
z(m)
t (ps)
longitudinal plasma extent vs. time
9 longitudinal expansion is suppressed (t > 50 ps)!
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
)Lateral (r) expansion
(t > 50 ps: expansion only in lateral direction)
lateral plasma radius vs. time
(laser energy: 10 mJ)
r 0 500 1000 1500 2000 25000
10
20
30
40
50
r(m)
t (ps)
9 lateral expansion follows a power law!
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
Distribution of laser energy (100%):
~~ 50% absorbed by the picosecond micro-plasma
~~ 50% reaching target surface
2/1
4/1
tr
o
E
E: energy deposition density (laser axis)
0: ambient gas (air) density
similarity relation(2D blast wave - line energy source)
10 100 10001
10
100
t1/2
10.0 mJ
7.5 mJ
r(m)
time (ps)
t1/2 power law
)Energy deposition to picosecond plasma
plasma onset
85 J/cm2 110 J/cm2
50 m
40 J/cm2
0 100 200 300 400 5000
5
10
15
20
25
ablationdepth(m
)
laser fluence (J/cm2)
reduced efficiency
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
)Theoretical model (laser-solid-gas interaction)electron
ion (gas)
atom (gas)
gas (1atm)
before laser irradiation
Cu
laserbeam
after laser irradiation
Cuelectron
Cu atom
photon
9 laser heating of target (metal) electron heating - absorption of laser energy
lattice heating - electron-phonon collisions
9 plasma development above target surface electron emission (seed)
impact ionization of gas
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
) What is happening?
femtosecond picosecond nanosecond microsecond
laserpulse
target
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
1 ns = 10 9 s
Nanosecond Time ScaleNanosecond Time Scale
)Plasma evolution picosecond to nanosecond
z time dependence
(35 ps, 7 mJ)
z laser energy dependence
(35 ps, 2 ns)
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Nanosecond Time ScaleNanosecond Time Scale
)Plasma development
plasma advancement:~ 10
6
cm/s~ 10 m every 1 ns (1000 ps)
solid
laser
pulse
shock wave
plasma
vapor
target
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Nanosecond Time ScaleNanosecond Time Scale
)Plasma shielding nanosecond
solid
l
aser
10 100
0.1
1
10
ablationdepth(m)
laser fluence (J/cm2)
25 ns, 248 nm laser ablation of Cu
(single pulse, in air, 100 m spot diameter)
Experiment
0 1 2 3 4 5 6 7 8 9 10 11 12
without plasma
t (ns)
Theory
100 GW/cm2
30 GW/cm2
20 GW/cm2
3 ns, 1064 nm laser ablation of Si
(single pulse)
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
) What is happening?
femtosecond picosecond nanosecond microsecond
las
erpulse
target
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
1 s = 10 6 sMicrosecond Time ScaleMicrosecond Time Scale
)Plume evolution
z below threshold
10 ns 160 ns 760 ns 1.6 s 4.9 s64 ns100 m
(3 ns, 1.8x1010 W/cm2)
z above threshold
4.2 s1.3 s860 ns200 ns70 ns5 ns100 m
(3 ns, 2.1x1010 W/cm2)
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Microsecond Time ScaleMicrosecond Time Scale
18 GW/cm2
0.0
+2.0
-2.0
-4.0
-6.0
abla
tiondepth
(m)
21 GW/cm2
0.0
+2.0
-2.0
-4.0
-6.0
ablationdepth
(m)
4.2 s4.9 s
)Threshold behavior
1010
1011
0
5
10
15
20
25
ablation
depth(m)
laser intensity (W/cm2)
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Microsecond Time ScaleMicrosecond Time Scale
)Theoretical model
solid
~ 1 ns
solid
superheated
liquid layer
~ 100 ns
solid~ 1 s
109
1010
1011
5
10
15
20experiment
theory
ablation
depth(m)
laser intensity (W/cm2
)
)]11
(exp[)2( 2/1
0 TTk
mLTmk
mp
t
x
bB
evBb
x
=
=
Normal vaporization (Hertz-Knudsen equations)
ablation below threshold: normal evaporation
ablation above threshold: normal evaporation and explosive boiling
)exp()( xIx
Tk
xt
TC laser +
=
Explosive boiling (heat diffusion Tmax~ Tc)
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
) What is happening?
femtosecond picosecond nanosecond
las
erpulse
target
microsecond
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
)Fundamental processes
laser
electronic
plasma
electronicexcitation
solid
heated zone
plasma
vapor
liquid
fspsnssdroplets
absorption/excitationfs
ionization (photon)
conductionps
radiation
ionization (shock wave)
vaporizationconvection
melting
ns
boilings
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
Applications
20 40 60 80 100 120 140 160 180
0.0
0.5
1.0 ns laser, 266 nm
fs laser, 266 nm
Zn/Curatio
time (s)
micro-analysis nano-material
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Applications ofApplications ofUltrafastUltrafastLaser AblationLaser Ablation
)Ultrafast laser ablation (pulse < tthermal) FEL capability! Non-thermal ablation regime
E
+
-fs laser
ion
electron
target
9 Reduced dependence on thermal properties
9 Reduced larger cluster particles generation
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
)Micro Analysis
20 40 60 80 100 120 140 160 1800.0
0.5
1.0ns laser, 266 nmfs laser, 266 nm
M
Ssignal:Zn/
Curatio
time(s)
Mass Spectrometry - Laser ablation of brass (CuZn alloy)
ns
fs
The problem of nanosecond laser ablation
Applications ofApplications ofUltrafastUltrafastLaser AblationLaser Ablation
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MicroMicro--Analysis ApplicationAnalysis Application
New magnetic film material
(data storage applications)
0 10 20 30 40 50102
103
104
105
106
107
108
109
1010
depth profile (bulk material)
Cu
Fe
elementcounts(a.u.)
time (s)
laser
depth profiling
sputtering target
0.00
0.05
0.10
0.15
0.20
151050
Cu/Feratio
time (s)
surface profiling (film material)
laser
surface profiling
deposited film
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
Applications ofApplications ofUltrafastUltrafastLaser AblationLaser Ablation
)Nano Material
The problem of nanosecond laser ablation
50 m
50 m
Nanowires with large particles
1 m
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
NanoNano--Material ApplicationMaterial Application
) Pulsed laser deposition ZnO nanowire growth
Setup
substratetemperature control
ultrafast laser
gas
gas
target
fabrication chamber
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
NanoNano--Material ApplicationMaterial Application
excitation
source266 nm (Nd:YAG)
ZnO nanowiresapphire
substrate
~ 100 nm
ZnO nanowire:
natural laser cavity
370 375 380 385 390 395 4000
500
1000
1500
2000
2500
3000
above threshold
below threshold
intensity(a.u
.)
wavelength (nm)
[Science 292 (2001) 1897]
10-3
10-2
10-1
100
101
105
106
lasing
spontaneous
emissionintensity(a.u.)
excitation energy (mJ)
)Nanolaser spectra
UV lasing
(room temperature)
)Nanowire nanolaser
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University of California at Berkeley Q Lawrence Berkeley National Laboratory
AcknowledgementsAcknowledgements
U. S. Department of Energy
Yanfeng Zhang
Quanming LuPeidong Yang
Richard Russo
Xianglei Mao