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

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