20121219 01 osu chowdhury public
DESCRIPTION
understanding femtosecond laser -matter interactionTRANSCRIPT
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Understanding the femtosecond laser-solid
interaction
near and beyond the material damage threshold.
Enam Chowdhury
Department of Physics
The Ohio State University
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Outline
• Goals
• Background
• Proposed Experimental Thrusts
• Modeling efforts to date
• Summary
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Ultimate Goal
NeedNeedNeedNeed material material material material XXXX, , , ,
with property with property with property with property YYYY and and and and
structure structure structure structure ZZZZ
Raw material
Femtosecond Laser
with optimized
Parameters
Complete
Understanding of Laser-
Matter Interaction
XYZXYZXYZXYZ
BRI Topic: BRI Topic: BRI Topic: BRI Topic: techniques for ultrafast- laser
processing (e.g. machining, patterning)
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Ultimate Goal II
Ever Increasing Ever Increasing Ever Increasing Ever Increasing
demand for optics demand for optics demand for optics demand for optics
for high power for high power for high power for high power
femtosecond femtosecond femtosecond femtosecond
applicationsapplicationsapplicationsapplications
Femtosecond Laser
Damage Studies of
systems
Optics with Optics with Optics with Optics with
High LDT in High LDT in High LDT in High LDT in
femtosecond femtosecond femtosecond femtosecond
regimeregimeregimeregime
Complete
Understanding of Laser-
Matter Interaction
Thin films,
materials and
surface
structure
technologies
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Project Goals
• Understand dynamics of ionization: First step in interaction
• Understand creation and effect of defect states in multi-pulse interaction
• Use micro and nano scale structured surfaces to study effect of structures on laser damage
• Use innovative experiments and realistic computer models to capture dynamics of laser ablation
To To To To develop a new, fundamental understanding of intense develop a new, fundamental understanding of intense develop a new, fundamental understanding of intense develop a new, fundamental understanding of intense field laser field laser field laser field laser
ablation/damageablation/damageablation/damageablation/damage in the femtosecond regime.in the femtosecond regime.in the femtosecond regime.in the femtosecond regime.
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Team of Experts
• Chowdhury (OSU Physics): femtosecond laser matter interaction experiment (PI)
• Schumacher (OSU Physics): Simulation of femtosecond dynamics
• Shvets (UTA Physics): Simulation of femtosecond dynamics of Structured Surfaces
• Akbar (OSU MSE): Fabrication and characterization of nano-structured surfaces
• Yi (OSU IE): Fabrication and characterization of micro-machined surfaces
• Smith (PGL): Fabrication and characterization of ultra-precise periodic surface structures
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Background
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Physics of femtosecond Damage
Wellershoff et. al. App. Phys. A 69 S99 (1999)
B. C. Stuart et. al.PHY REV B 53, 1996
Fs: multi-photon ionization/non-
equilibrium electron transport and
lattice damage via electron
phonon coupling, and or non-
thermal melting
Ns: material heating via collision,
diffusion, melting and removal
femtosecondfemtosecondfemtosecondfemtosecond
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Experimental Consideration: Need for Vacuum
• Laser matter interaction with short pulse lasers, and
femtosecond laser LDT test often require focusing
the beam.
• Although most realistic machining would happen in
air, it can also act as an extra variable, where broad broad broad broad
bandwidthbandwidthbandwidthbandwidth of the pulse allows a focusing beam to
react to the non-linear index of air, causing non-linear
phase accumulation of wave-front and ultimately the
breakup of the beambreakup of the beambreakup of the beambreakup of the beam into multiple self-focusing
filaments.
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Damage Threshold of Optics
Air Fluence
(J/cm2)
Vacuum Fluence
(J/cm2)
Air Intensity
(W/cm2)
Vacuum Intensity
(W/cm2)
Protected
Au Mirror
Single shot 1.5 1.7 6.0E+13 6.7E+13
Multi shot 1.0 1.5 3.9E+13 5.8E+13
Au GratingSingle shot 0.30 1.2E+13
Multi shot 0.27 0.3 1.1E+13 1.2E+13
Dielectric
Mirror
Single shot 0.62 1.2 2.5E+13 4.7E+13
Multi shot 0.55 0.7 2.2E+13 2.6E+13
Damage Damage Damage Damage FluenceFluenceFluenceFluence on surface, corrected for on surface, corrected for on surface, corrected for on surface, corrected for coscoscoscos 450 angle projectionangle projectionangle projectionangle projection
(30-30k)
(30-30k)
(30-30k)
Chowdhury, Proc.
SPIE 2010
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Conformal Gold coating over
an Etched Silica Grating
•A A A A conformalconformalconformalconformal gold coating is gold coating is gold coating is gold coating is essential to both Diffraction essential to both Diffraction essential to both Diffraction essential to both Diffraction Efficiency and shortEfficiency and shortEfficiency and shortEfficiency and short----pulse laser pulse laser pulse laser pulse laser damage performancedamage performancedamage performancedamage performance
SUBSTRATE
PHOTORESIST
METAL
1) Gold Over Sinusoidal Resist 2) Lamellar Gold Over Binary Resist
3) Lamellar Gold over Etched Silica 4) Lamellar Etched Into Gold
• Diffraction Gratings are the weakest link in Ultra-intense lasers
• Also the most expensive
• Develop new type of Diffraction gratings with high LDT
Widely used
Used at SCARLETUsed at SCARLETUsed at SCARLETUsed at SCARLET
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SEM Examination Of Damaged Sites
• The lighter material The lighter material The lighter material The lighter material is gold. The underlying is gold. The underlying is gold. The underlying is gold. The underlying Silica grating remainsSilica grating remainsSilica grating remainsSilica grating remainsintact.intact.intact.intact.
• Full size gratings Full size gratings Full size gratings Full size gratings with damaged gold with damaged gold with damaged gold with damaged gold have been have been have been have been successfullysuccessfullysuccessfullysuccessfully stripped stripped stripped stripped and recoatedand recoatedand recoatedand recoated
The Underlying Silica Grating structure Is IntactThe Underlying Silica Grating structure Is IntactThe Underlying Silica Grating structure Is IntactThe Underlying Silica Grating structure Is Intact
ICUIL 2012, RomaniaICUIL 2012, RomaniaICUIL 2012, RomaniaICUIL 2012, Romania
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Experimental Thrusts
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Experimental System Schematics
• Extend experiments from 400 nm – 4000 nm
• Study pulse width dependence effects from 5 – 1000 fs
• Housed in a dedicated 700 sq. ft. modern Lab space
within SCARLET facilities
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Modeling femtosecond laser
interaction with solids
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Efforts in Modeling
ParametersParametersParametersParameters Molecular Dynamics (MD)Molecular Dynamics (MD)Molecular Dynamics (MD)Molecular Dynamics (MD) Hybrid ParticleHybrid ParticleHybrid ParticleHybrid Particle----inininin---- Cell (PIC) Cell (PIC) Cell (PIC) Cell (PIC)
LSPLSPLSPLSP
Laser Coupling Some prescribed energy
coupling
Self consistent field interacting
with moving charges
Ionization Can be handled Incorporated (OSU)
Inter-particle
interaction
Realistic Potentials different scattering models
including binary collisions or
Monte Carlo sampling
Spatial Dimension ~10 nm Realistic sizes
Intensity (Wcm-2) Typ. ~1014 or below arbitrary
16 ps 49 ps 184 ps 130 fs 275 fs 340 fs
Molecular DynamicsMolecular DynamicsMolecular DynamicsMolecular Dynamics LSP hybrid PIC: LSP hybrid PIC: LSP hybrid PIC: LSP hybrid PIC: This ProjectThis ProjectThis ProjectThis Project
M. D. Shirk, et al, ICALEO (2003)
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Particle-In-Cell (PIC) Simulations
� We are developing PIC simulations of planned experiments using LSP.
• Good parallelization
• Inter-particle effects can be incorporated empirically
� LSP is a commercial code developed by Voss Scientific.1 Purchase
includes the source code so it can readily be modified by the user.
• Supports 1D, 2D, 3D in various geometries.
• Hybrid operation with two fluid models: hydrodynamic treatment.
• Advance algorithms and particle pushers, including explicit and
implicit algorithms.
• Multiple collision models and rates, including binary and LMD.
• Can treat different media including conductors and dielectrics.
1 D. R. Welch, et al, Nuclear Instruments and Methods in Physics Research A 464464464464, 134 (2001).
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PIC Simulation Challenges
� Cold systems are very hard to treat due to runaway heating
instability. Initial simulation temperatures of 12 million Kelvin
and transient electron temperatures of 1 MeV are commonplace.
� Simulations involving a laser beyond a few ps are rare.
� Ablation/damage studies require a highly unusual regime for PIC:
solid density materials initially at room temperature that must be
modeled for times as long as, sometimes well exceeding, 100 ps.
� PIC can handle entire laser ablation morphology
� Entire region of interaction
� Multiple pulse modifications
� Long time evolution using fluidics package
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PIC Simulation Progress So Far
� Our initial program called for:
� Finding a stable mode of operation in 1D
� Extending to 2D
• Extending to 3D
� Results
1D - Can now run out to hundreds of ps with good energy conservation.
Observe threshold damage behavior and evaporation.
2D - Careful treatment of boundary conditions is required, including
the use of PML (perfectly matched layers).
We currently have demonstrated stable simulation of a 10
µm wide x 5 µm thick gold target with a 1013 W/cm2 laser, 2
µm waist, injected onto the grid that runs stably out to 200
ps. (Run was halted arbitrarily at 200 ps.)
~1 day using 48 cores. LSP scales well up to thousands of cores.
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Modeling femtosecond laser
interaction with nano-structure
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Modeling Parameters
• Laser Parameters: – AAA
• 1-D periodic metallic (Ag) structured surface
• VLPL (Very Large Plasma Laboratory)– 2-D Particle-In-Cell (PIC) simulation self-consistent E&M fields
• COMSOL – Multiphysics FEA solver to determine local E&M fields of complex structures with high resolution
• PIC code to benchmark COMSOL solutions
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Summary: Progress
• Dedicated lab is being setup
• 3 Students joined recently
• Laser system being designed, major components ordered
• Preliminary ionization experiments being setup with existing laser system
• Characterizing surface nano-structures
• VLPL and COMSOL benchmarked in modeling fs light interacting with periodic structures
• Hybrid PIC code LSP running stably in laser ablation regime
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Acknowledgements
• Grad Students:
– R. Mitchell (OSU Physics),
– Austin Yi (UTA Physics)
• OSU Supercomputing Center
• CVI Melles Griot for Optics
• Riq Parra, AFOSR Program Officer. This project is supported by an AFOSR grant # AFPSR-FA9550-12-1-0454