University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
Fundamentals & ApplicationsFundamentals & Applications
Samuel S. MaoDepartment of Mechanical Engineering
University of California at BerkeleyAdvanced Energy Technology DepartmentLawrence Berkeley National Laboratory
March 10, 2005
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
What is “Laser Ablation”?
Mass removal by coupling laser energy to a target material
University of California at Berkeley Lawrence Berkeley National Laboratory
Is it important?Is it important?
target
substrate
materialplume
Film deposition* oxide/superconductor films* nanocrystals/nanotubes
laser ablation
target
plasma lens
optical spectrometer
mass spectrometerMaterials characterization
* semiconductor doping profiling* solid state chemical analysis
target transparent solid
microstructure
Micro structuring* direct wave guide writing* 3D micro fabrication
University of California at Berkeley Lawrence Berkeley National Laboratory
Is it important?Is it important?
Film deposition* oxide/superconductor films* nanocrystals/nanotubes
laser ablation
100 µm
Materials characterization* semiconductor doping profiling* solid state chemical analysis
Micro structuring* direct wave guide writing* 3D micro fabrication
University of California at Berkeley Lawrence Berkeley National Laboratory
laser ablation
Do we really understand?Do we really understand?
plasmaplasma
targettarget
laser laser 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)
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
What is happening?
nanosecondpicosecond microsecondfemtosecond10-15 s 10-12 s 10-9 s 10-6 s
laser
pul
se
target
University of California at Berkeley Lawrence Berkeley National Laboratory
??
ExperimentsExperiments -- ultrafast imagingultrafast imaging
ablation laser beam
target
CCD
probe beam
pump beam
delay time
imaging laser beam
Pump-probe technique
University of California at Berkeley Lawrence Berkeley National Laboratory
1 fs = 10 –15 sFemtosecond Time ScaleFemtosecond Time Scale
laser
pul
se
glassair
100 fs,800 nmE = 30 µJ
Ultrafast imaging - time dependent energy transfer
self-focusing
C
Velectronic excitation
e-h plasma
100 µm
University of California at Berkeley Lawrence Berkeley National Laboratory
Femtosecond Time ScaleFemtosecond Time Scale
0
50 fs
z (µm)
elec
tron
num
ber
dens
ity (1
019 c
m-3)
0
5333 fs
0
5667 fs
0
51000 fs
0
51333 fs
0
51667 fs
0 100 200 300 400 5000
5
2000 fs
0
z
Electron number density ne – time/space dependencelas
er p
ulse
22
2
1 τωωτα+
= p
nc
0
2
εω
e
ep m
en=
• Transmittance (probe beam)
• Absorption coefficient
• Plasma frequency
dII ed α−=
0
ne ~ 1019 cm-3
University of California at Berkeley Lawrence Berkeley National Laboratory
Femtosecond Time ScaleFemtosecond Time Scale
Peak electron number density ne - time dependence
0 500 1000 1500 2000 25001x1019
2x1019
3x1019
4x1019
5x1019
6x1019
Ne,
max
(cm
-3)
time (fs)
flattened peak electron number density
no breakdown
0
50 fs
z (µm)
elec
tron
num
ber
dens
ity (1
019 c
m-3)
0
5333 fs
0
5667 fs
0
51000 fs
0
51333 fs
0
51667 fs
0 100 200 300 400 5000
5
2000 fs
University of California at Berkeley Lawrence Berkeley National Laboratory
Femtosecond Time ScaleFemtosecond Time Scale
Fundamental processeslas
er p
ulse
Nonlinear absorption
Nonlinear opticsSelf-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
University of California at Berkeley Lawrence Berkeley National Laboratory
Femtosecond Time ScaleFemtosecond Time Scale
Propagation of electronic excitation in glass
-500 0 500 1000 1500 2000 2500
-500
50100150200250300350400450500
v = 1.8x108 m/sn = 1.65
z (µ
m)
t (fs)
250 µJ 120 µJ 60 µJ 30 µJ 12 µJ
100 fs,800 nmE = 30 µJ
[Appl. Phys. A 79, 1695–1709 (2004)]
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
What is happening?
femtosecond picosecond nanosecond microsecond
laser
pul
se
target
University of California at Berkeley Lawrence Berkeley National Laboratory
50 µm
(laser pulse: 35 ps, fluence: 90 J/cm2)
t = 50 ps
50 µm
(laser pulse: 35 ps, fluence: 60 J/cm2)
t = 50 ps
ablation laser pulse
target Cu
(air)
1 ps = 10 –12 sPicosecond Time ScalePicosecond Time Scale
Picosecond imaging
University of California at Berkeley 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 µm
Threshold for picosecond plasma formation – same as the 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
abla
tion
dept
h (µ
m)
laser fluence (J/cm2)
1 2
University of California at Berkeley Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
Ultrafast interferometry
r
z
t = 15 ps interference pattern
50 µm
target
University of California at Berkeley 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
8.0x1019
1.0x1020
1.2x1020
Ne (
cm-3)
Z (µm)
z
t = 15 ps
air density
A large electron number density!(close to target surface)
University of California at Berkeley Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
Longitudinal (z) expansion
z
(laser energy: 10 mJ)
50 µ
m
0 20 40 60 80 100 1200
100
200
300
400
500
z (µ
m)
t (ps)
longitudinal plasma extent vs. time
longitudinal expansion is suppressed (t > 50 ps)!
University of California at Berkeley 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)
r0 500 1000 1500 2000 2500
0
10
20
30
40
50
r (µ
m)
t (ps)
lateral expansion follows a power law!
University of California at Berkeley 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
∝
ρE: energy deposition density (laser axis)ρ0: 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 (µm
)
time (ps)
t1/2 power law
Energy deposition to picosecond plasma
plasma onset
85 J/cm2 110 J/cm2
50 µm
40 J/cm2
0 100 200 300 400 5000
5
10
15
20
25
abla
tion
dept
h (µ
m)
laser fluence (J/cm2)
reduced efficiency
University of California at Berkeley Lawrence Berkeley National Laboratory
Picosecond Time ScalePicosecond Time Scale
Theoretical model (laser-solid-gas interaction) electronion (gas)
atom (gas)
gas (1atm)
before laser irradiationCu
laser beam
after laser irradiationCu
electron
Cu atom
photon
laser heating of target (metal) • electron heating - absorption of laser energy• lattice heating - electron-phonon collisions
plasma development above target • surface electron emission (seed)• impact ionization of gas
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
What is happening?
femtosecond picosecond nanosecond microsecond
laser
pul
se
target
University of California at Berkeley Lawrence Berkeley National Laboratory
1 ns = 10 –9 sNanosecond Time ScaleNanosecond Time Scale
Plasma evolution – picosecond to nanosecond
time dependence
(35 ps, 7 mJ)
laser energy dependence
(35 ps, 2 ns)
University of California at Berkeley Lawrence Berkeley National Laboratory
Nanosecond Time ScaleNanosecond Time Scale
Plasma development
plasma advancement:
~ 106 cm/s~ 10 µm every 1 ns (1000 ps)
solid
laser pulse
shock wave
plasma
vapor
target
University of California at Berkeley Lawrence Berkeley National Laboratory
Nanosecond Time ScaleNanosecond Time Scale
Plasma shielding – nanosecond
solid
laser
10 100
0.1
1
10
abla
tion
dept
h (µ
m)
laser fluence (J/cm2)
25 ns, 248 nm laser ablation of Cu(single pulse, in air, 100 µm 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)
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
What is happening?
femtosecond picosecond nanosecond microsecond
laser
pul
se
target
University of California at Berkeley Lawrence Berkeley National Laboratory
1 µs = 10 –6 sMicrosecond Time ScaleMicrosecond Time Scale
Plume evolution
below threshold
10 ns 160 ns 760 ns 1.6 µs 4.9 µs64 ns
100 µm
(3 ns, 1.8x1010 W/cm2)
above threshold4.2 µs1.3 µs860 ns200 ns70 ns5 ns
100 µm
(3 ns, 2.1x1010 W/cm2)
University of California at Berkeley Lawrence Berkeley National Laboratory
Microsecond Time ScaleMicrosecond Time Scale
18 GW/cm2
0.0
+2.0
-2.0
-4.0
-6.0
ablat
ion
dept
h(µ
m)
21 GW/cm2
0.0
+2.0
-2.0
-4.0
-6.0
ablat
ion
dept
h(µ
m)
4.2 µs4.9 µs
Threshold behavior
1010 10110
5
10
15
20
25
abla
tion
dept
h (µ
m)
laser intensity (W/cm2)
University of California at Berkeley Lawrence Berkeley National Laboratory
Microsecond Time ScaleMicrosecond Time Scale
Theoretical model
solid~ 1 ns
solid
superheated liquid layer
~ 100 nssolid
~ 1 µs
109 1010 1011
5
10
15
20 experiment theory
abla
tion
dept
h (µ
m)
laser intensity (W/cm2)
)]11(exp[)2( 2/1
0 TTkmL
Tmkmptx
bB
evBb
x
−=∂∂ −
=
πρ
β
• Normal vaporization (Hertz-Knudsen equations)
ablation below threshold: normal evaporation
ablation above threshold: normal evaporation and explosive boiling
)exp()( xIxTk
xtTC laser ααρ −+
∂∂
∂∂
=∂∂
• Explosive boiling (heat diffusion – Tmax~ Tc)
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
What is happening?
femtosecond picosecond nanosecond
laser
pul
se
target
microsecond
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
Fundamental processes
lase
relectronic
plasma
electronicexcitation
solid
heated zone
plasma
vapor
liquid
fspsnsµsdroplets
absorption/excitationfs
ionization (photon)conductionps
radiationionization (shock wave)vaporizationconvectionmelting
ns
boilingµs
University of California at Berkeley Lawrence Berkeley National Laboratory
Laser AblationLaser Ablation
Applications
20 40 60 80 100 120 140 160 1800.0
0.5
1.0 ns laser, 266 nm fs laser, 266 nm
Zn/C
u ra
tio
time (s)
micro-analysis nano-material
University of California at Berkeley Lawrence Berkeley National Laboratory
Applications of Applications of UltrafastUltrafast Laser AblationLaser Ablation
Ultrafast laser ablation (τpulse < tthermal) – FEL capability!• Non-thermal ablation regime
E
+
-fs laser
ion
electron
target
Reduced dependence on thermal propertiesReduced larger cluster particles generation
University of California at Berkeley Lawrence Berkeley National Laboratory
Micro Analysis
20 40 60 80 100 120 140 160 1800.0
0.5
1.0 ns laser, 266 nm fs laser, 266 nm
MS
signa
l: Zn
/Cu
ratio
time(s)Mass Spectrometry - Laser ablation of brass (CuZn alloy)
ns
fs
The problem of nanosecond laser ablation
Applications of Applications of UltrafastUltrafast Laser AblationLaser Ablation
University of California at Berkeley Lawrence Berkeley National Laboratory
MicroMicro--Analysis ApplicationAnalysis ApplicationNew 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
elem
ent c
ount
s (a.
u.)
time (s)
laser
depth profiling
sputtering target
0.00
0.05
0.10
0.15
0.20
151050
Cu/
Fe r
atio
time (s)
surface profiling (film material)laser
surface profiling
deposited film
University of California at Berkeley Lawrence Berkeley National Laboratory
Applications of Applications of UltrafastUltrafast Laser AblationLaser Ablation
Nano Material
The problem of nanosecond laser ablation
50 µm
50 µm
Nanowires with large particles
1 µm
University of California at Berkeley Lawrence Berkeley National Laboratory
NanoNano--Material ApplicationMaterial Application
Pulsed laser deposition – ZnO nanowire growth
Setup
substratetemperature control
ultrafast lasergas
gastarget
fabrication chamber
University of California at Berkeley Lawrence Berkeley National Laboratory
NanoNano--Material ApplicationMaterial Application
excitation source
266 nm (Nd:YAG)
ZnO nanowiresapphiresubstrate
~ 100 nm
ZnO nanowire: natural laser cavity
370 375 380 385 390 395 4000
500
1000
1500
2000
2500
3000
above threshold below threshold
inte
nsity
(a.u
.)
wavelength (nm)
[Science 292 (2001) 1897]
10-3 10-2 10-1 100 101
105
106
lasing
spontaneousem
issi
on in
tens
ity (a
.u.)
excitation energy (mJ)
Nanolaser spectra
UV lasing
(room temperature)
Nanowire nanolaser
University of California at Berkeley Lawrence Berkeley National Laboratory
AcknowledgementsAcknowledgements
U. S. Department of Energy
Yanfeng ZhangQuanming LuPeidong YangRichard RussoXianglei Mao
