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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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Laser AblationLaser Ablation

What is Laser Ablation?

Mass removal by coupling laser

energy to a target material

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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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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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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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??

ExperimentsExperiments -- ultrafast imagingultrafast imaging

ablationlaser beam

target

CCD

probe beam

pump beam

delay time

imaginglaser beam

)Pump-probe technique

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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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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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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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Laser AblationLaser Ablation

) What is happening?

femtosecond picosecond nanosecond microsecond

laserpulse

target

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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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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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Picosecond Time ScalePicosecond Time Scale

)Ultrafast interferometry

r

z

t = 15 ps interference pattern

50 m

target

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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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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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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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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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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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Laser AblationLaser Ablation

) What is happening?

femtosecond picosecond nanosecond microsecond

laserpulse

target

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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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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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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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Laser AblationLaser Ablation

) What is happening?

femtosecond picosecond nanosecond microsecond

las

erpulse

target

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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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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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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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Laser AblationLaser Ablation

) What is happening?

femtosecond picosecond nanosecond

las

erpulse

target

microsecond

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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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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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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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)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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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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NanoNano--Material ApplicationMaterial Application

) Pulsed laser deposition ZnO nanowire growth

Setup

substratetemperature control

ultrafast laser

gas

gas

target

fabrication chamber

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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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AcknowledgementsAcknowledgements

U. S. Department of Energy

Yanfeng Zhang

Quanming LuPeidong Yang

Richard Russo

Xianglei Mao