Ultrafast Laser Ablation and Plasma Diagnostics

1
 Ultrafast Laser Ablation and Plasma Diagnostics

• Zhiyu Zhang
• Center for Ultrafast Optical Science
• Department of Electrical Engineering and Computer
  Science
• University of Michigan, Ann Arbor, MI

2
Outline

• Introduction and Motivation
• Multi-diagnostic comparison of plasmas by
  ultrafast laser and nanosecond laser ablation
• Multi-pulse ablation
• Application of double and multi-pulse ablation
  isotope enrichment and clusters formation
• Summary

3
Ultrafast Laser Applications Materials
Processing (I)
Above Ablation Threshold

• Micromachining metal, semiconductor, dielectric,
  dental tissue, 100nm lateral, 10nm vertical
  Appl. Phys. A 68 369, Appl. Phys. A 68 403
• DNA transfection Nature 418 290
• Patterning biomaterials Appl. Surf. Sci. 208-209
  245
• Fresnel zone plate fabrication Opt. Exp. 10 978
• Surface microstructureing APL 81 1999
• Photomask repair J. Vac. Sci. Technol. B 21 204

4
Ultrafast Laser Applications Materials
Processing (II)
Below Ablation Threshold

• Waveguide direct writing Opt. Lett. 21 1729
• Phase transition JETP Lett. 76 461
• 3D micro-photopolymerization
• Nature 412 697
• Nonlinear optical coefficient enhancement APL
  81 1585
• Precipitating nanoparticles inside glasses APL
  81 3040

5
Ultrafast Laser in Materials Processing
6
Ultrafast Laser System
10 Hz X 10,000 200 ps
Beam Splitter
Stretcher
7
Ultrafast Laser System
100 fs 10 Hz 100 mJ
8
High-Repetition Rate Commercial Ultrafast Laser
Laser 1 kHz, 1 mJ/pulse, 150 fs
9
Experimental Setup
10
Electrostatic Energy Analyzer
Select ions with E/q2.254 ?Vs
11
Time-of-Flight Spectrum BN

• Laser 100 fs, 780 nm, 50 J/cm2

12
Time-Resolved OESAl Ablated with 100-fs Pulses

• 100 ns Delay Increment
• 50 ns Acquisition Window
• 100, 780nm, 1J/cm2 Laser Pulses
• Continuum Emission at Early Time

13
Langmuir Probe I-V Curve

• Plasma parameters related to the different
  regions of the I-V curve
• Probe theory developed to extract the parameters
  from the I-V curve
• Time-resolved analysis (1ms resolution)

14
HeNe Probe Laser Deflection
Plume
Quad Detector
Vout
HeNe
j Ion or Neutral component
Self-similar expansion model for both ion and
neutral species
C.L. Enloe et al, Rev. Sci. Instrum. 58, 1597
(1987)
15
HeNe Deflection Signal
BN Ablation 6ns, 18 J/cm2
Parameters Vion 4.76x106cm/s Vneutral 1.04x106c
m/s Nion 5x1014 Nneutral 1x1017
16
In-situ Analysis Plasma Diagnostic Technique

• Electrostatic Energy Analyzer
• Charge state, Ion energy distribution
• Langmuir Probe
• Electron temperature, Plasma and floating
  potential, Density of electrons and ions
• Spatial and temporal resolution (1ms)
• Optical Emission Spectroscopy
• Plasma temperature, Excited states
• ns time resolution
• Laser Deflection Probe
• Ion to neutral ratio
• Hydrodynamic component

Target Metal (Al), Nitrides (BN), Oxide (SnO2)
17
Energy Spectrum of AlEffect of Laser Pulse Width

• 6 ns,1.06 mm
• 1.3J/cm2

80 fs, 780 nm 0.4J/cm2
Al Target Ablated in Vacuum
18
Time-Resolved Electron Temperature Optical
Emission Spectroscopy

• Laser Pulses
• 6ns
• 1.06mm
• 1.4J/cm2
• 100fs
• 780nm
• 0.4J/cm2

19
Time-Resolved Electron Temperature Langmuir
Probe
6 ns
80 fs
Laser pulses 80fs 780nm 0.4J/cm2 6ns
1.06mm 1.3J/cm2 Langmuir Probe 10cm away from
Al target
20
HeNe Deflection Ion to Neutral Ratio
100 fs Laser Neutral Energy4 eV Ion Energy 85eV
6 ns Laser Neutral Energy4 eV Ion Energy 100eV
21
Plume Properties Modified by Laser Pulse

• Pulse width plasma shielding and heating effects
  
• Pulse contrast enhance the high energy component
  in the plume (Appl. Surf. Sci 2000)
• Pulse intensity optimum ablation efficiency (MRS
  2002)
• Multi-Pulse enhanced ion properties (APL 2003)

22
Ion Yield and Energy in Ablation Plumes
3.3kJ/cm2

• 10Hz
• TiSapphire
• Same focus spot
• Varying pulse
• energy
• Ion Energy is converted from TOF signal

0.8kJ/cm2
23
Laser Wave in Plasma
ncr
Light Wave in Inhomogeneous Plasma s-pol
Collisional absorption p-pol Collisional
Resonance absorption Collisional
absorption Resonance absorption
w laser frequency L plasma scale
length vie electron-ion collision
frequency c speed of light
24
Modify the Ablation Dynamics by Double Pulse
Ablation

• Saturation of absorption occurs as single- pulse
  energy increases due to plasma reflection
• Use an appropriately time-delayed secondary laser
  pulse to add energy into the plasma during the
  early stages of its expansion.
• Examine the effect on the plasma and the clusters

25
Double pulse experiment

• Initial gradient scale length of a plasma
• Two identical 300fs pulses tens of ps delay
• Ref C. Y. Cote et. al Phy. Rev. E 56, 992 (1997
  )
• Thin Film Deposition reducing particulate
• CO2 and YAG lasers 4ns delay
• Ref S. Witanachchi, et. al Appl. Phys. Lett. 66
  1469 (1995)
• X-ray emission
• Weak pre-pulse, 2ns delay
• Ref H. Nakano et. al Appl. Phys. Lett. 79 24
  (2001)

26
Time-of-Flight Single vs. Double-Pulse
Laser 110fs, 780nm, p-polarized
1x1016 W/cm2
050ps
2x1016 W/cm2
27
Ion Yield and Energy vs. Delay
28
Optical Emission Spectroscopy Si Target
29
Double-Pulse Enhanced Optical Emission
Laser 130fs, 780nm, p-pol
1x1016 W/cm2
050ps
30
Langmuir Probe Double-Pulse Enhanced Ion Density
Laser 130fs, 780nm, p-pol
1x1016 W/cm2
050ps
31
Charge State Distribution Single vs.
Double-Pulse
Average Charge Single 2.5 Double
5-ps 3.1 Double 10-ps 3
32
Resonance Absorption of Time-Delayed Pulse
33
Plasma Scale-Length Results
34
Ion Yield vs. Laser Intensity (Double-Pulse)
Maximum Intensity 1x1016 W/cm2
35
Setup for Generation of 4-pulse Sequence
36
Silicon Ablation by Multi-pulse
Laser Intensity 5x1015 W/cm2 Four-Pulse t15ps,
t3 is a variable
37
Isotopic Enrichment Double-Pulse Enhancement
BN Ablation 120 fs, 2 x 1016 W/cm2, Dt 10 ps
Isotope Ratio
Ion Yield
38
Time-resolved Magnetic Field
Magnetic field by RA Laser Intensity 1x1016 W/cm2
Toroidal magnetic field Laser Intensity 5x1018
W/cm2
Borghesi et.al. PRL v81,p112
Sandhu et.al. PRL v89,p225002
39
Questions

• Cluster component in the plume?
• Surface roughness is caused by growth effect or
  clusters formed in plume?
• High intensity femtosecond plasma bursts result
  in dense cluster formation?

40
Cluster Formation for Cu/NiLaser intensity 2 x
1016 W/cm2 Tm 1300 C
41
Cluster Formation for SmCo
42
Cluster Formation for NdFeB
43
Cluster Size Distribution Ni
Ni RT, 10Hz Laser, Single Pulse, 2x1016W/cm2
Average Size 242 nm 155nm
44
Cluster Size Distribution Ni
Ni RT, 10Hz Laser, Double Pulse, 1x1016W/cm2,
5ps Delay
Average Size 99nm41nm
45
Cluster Size Distribution Ge
Single full-energy pulse
46
Cluster Size Distribution Ge
Double pulse
47
Clusters Formation Mechanism (I)

• Condensation of ns laser ablation plume
• High background pressure, 1 10 Torr
• Hydrodynamic expansion and thermodynamic
  condensation
• Plume nonuniformity gives cluster size
  distribution
• Average size less than 10 nm

48
Clusters Formation Mechanism (II)

• Photomechenical Effects - fragmentation
• Clusters are formed by fragmentation due to the
  strain associated with gradient of expansion
  velocity
• Cluster size determined by fragmentation time
• Average size less than 10 nm for 1ps
  fragmentation time

49
Clusters Formation Mechanism (III)

• Photothermal Effects phase explosion
• Superheating leads to homogeneous nucleation
• Mixture of vapor and liquid forms clusters
• Average size possible 100 nm

50
Femto-Jet nozzle
Hagena Equation
Femtosecond laser ablation P0 103 104 atm a
small, supersonic expansion Density very
large k large for heavy atoms
d jet throat diameter a jet expansion half
angle p0 backing pressure T0 initial gas
temperature k material dependent constant
number of atoms per clusters Nc ? (G)2.02.5
51
Ablation PlumeFemto-Jet Nozzle
Cluster condensation

52
Femto-Jet Application

• Cluster-Laser Interaction
• X-ray emission
• Higher energy ions (MeV) and high charge
• Femto-Jet
• High backing pressure 103 104 atm (normal nozzle
  10 atm)
• Supersonic expansion
• Not limited to gas species
• Jitter-free interaction

53
Summary

• Comparative study of ultrafast laser and
  nanosecond laser ablation
• Plume in far field ions, neutrals and clusters
• Plasma can be optically pumped with resonance
  absorption of a time-delayed laser pulse
• Application of multi-pulse ablation isotope
  separation enhancement and cluster formation

54
Acknowledgements
Peter Pronko Paul VanRompay John Nees Gerard
Mourou Roy Clarke Steve Yalisove Xiaoqing Pan
Paul Torek Chris Stewart Kyoungsik
Kim Seung-Whan Bahk Wei Tian Juan Dominguez