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Femtosecond laser ablation dynamics in wide band gap
crystals.
N.FedorovCEA/DSM/IRAMIS
École Polytechnique
Summary
• Introduction.– Problems of micro-machining– Proposed experiments.
• Femtosecond ablation– Single shot surface modification.– Multi shot surface modification.
• Ablation under picosecond pulse.
• Conclusion and perspectives.
Material Ejection
Stages of ablation for dielectric crystal• Excitation of electrons
• Heating of electrons by laser.
• Heating of surface.
• Vaporization.
• Cooling and condensation of material.
Femtosecond laser’s applications for micromachining.
Problem: Micro channels high profundityCondensation of vaporized material on channel border.
Detection in non-transparent material (metal):
•Crater profile
•Plasma light emission
•Electron / Ion emission.
•Light reflection modulation
craterLaser
Metal
plasma
Femtosecond laser’s applications for micromachining.
Why scintillation crystals?• Plasma emission• Induced absorption• Reflection modulation.• Self emission.• Refraction index modulation.
Possible to study density of electronic excitation inside the sample.
Plasma
Electronic excitations in dielectric
Laser
Dielectric Crystalplasma
Plasma emission
Luminescence emission
Scintillation crystals:
SiO2:H, CdWO4,ets.
Single pulse surface modification
Surfase modifications in
crater:• Periodic structure• “Mouldy” surface:
nanofibers.
Quartz monocrystal,
Irradiation by SLIC Ti:Saphire laser at CEA/Saclay 50fs 800nm 20Hz repetition rate or second harmonic (400nm)
Nano-particles and nano-fibers
• Fast cooling of plasma.• Collapsing to drops.• Drop of glass stretch a fiber.
400nm 5J/cm2 (1014W/cm2) Single shot
400nm 5J/cm2 (1014W/cm2) Single shot
Periodic structure in the crater
• Evolution of structure with number of shots
• Direction of the structure and polarization.– Polarization– Exposition.
400nm 5J/cm2 (1014W/cm2) 1 shot
400nm 5J/cm2 (1014W/cm2) 5 shots
400nm 5J/cm2 (1014W/cm2) 10 shots
Period and amplitude of structure.
• L=l/1+Sin(F)=l normal incidence• Amplitude proportional to Sinn where n is multi
photonic order n=Eg/Eph. For SiO2 Eg=9eV, Ti:Saphire 800nm: Eph=1.55eV
• n(800nm)=6, n(400nm)=3.
200 400 600 800 1000 1200
0 .5
1 .0
1 .5
SEM image brightness amplitude
Period 800nm
Fitting by Sin6
AFM measurement is required.
Polarization.
• Literature: Structure is parallel to polarization• 400nm: Structure is parallel to polarization• 800nm: Structure is perpendicular to polarization
400nm
800nm
Polarization.
Verification of polarization.• Vertical – horizontal• Horizontal – vertical• Circular-circular.
800nm
800nm circular polarization
800nm
Polarization.
800nm Long exposition (50J/cm2 x 20Hz : 1015W/cm2) :
Appearance of parallel structure.
800nm
Polarization, picosecond pulse duration.
800nm Long exposition (40J/cm2 : 2*1013W/cm2) pulse duration 2ps:
Appearance of parallel structure.800nm
Different pulse durations.
• Femtoseconds (50fs)– Excitation of electrons.– Absorption of laser pulse by electrons– VaporizationAll processes on the surface
• Picoseconds (2ps)– Amorphization– Darkening– Absorption by amorphous dark volumeHeating of big volume.
800nm 40J/cm2 (1013W/cm2) 1 shot
Very weak modification
800nm 40J/cm2 (1013W/cm2) 5 shots
Parallel and perpendicular structures.
800nm 40J/cm2 (1013W/cm2) 10 shots
Dark spot in the center
800nm 40J/cm2 (1013W/cm2) 12 shots
Beginning of boiling in the center
800nm 40J/cm2 (1013W/cm2) 15 shots
Boiling in the center
800nm 40J/cm2 (1013W/cm2) 20 shots
Boiling all the crater.
800nm 40J/cm2 (1013W/cm2) multi shots
Cracks around craterStrong heating in the volume under surfase
Conclusions.
• Collapsing of plasma to nano-particles.• Stretching of fibers of glass.• In the case of multi photonic absorption creation
of structure perpendicular to light polarization.• Creation of parallel structure after long
exposition or single photon absorption.• Amplitude of structure is proportional to Sin
power coefficient of nonlinearity.• Long pulse duration gives amorphization,
darkening and heating of volume under surface.
Perspectives
Electron density distribution study
• AFM study to amplitude of structure in crater.
• Installation of Intensified CCD Camera for luminescence and plasma emission studies.
• Time resolved imaging of plasma reflection
Merci de votre attention
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