Transcript Femtosecond laser ablation dynamics in wide band
Femtosecond laser ablation dynamics in wide band gap crystals.
N.Fedorov
CEA/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 profundity Condensation of vaporized material on channel border.
Detection in non-transparent material (metal): •Crater profile •Plasma light emission •Electron / Ion emission.
•Light reflection modulation Metal Laser plasma crater
Femtosecond laser’s applications for micromachining.
Why scintillation crystals?
• Plasma emission • Induced absorption • Reflection modulation.
• Self emission.
• Refraction index modulation.
Plasma Electronic excitations in dielectric Possible to study density of electronic excitation inside the sample.
Scintillation crystals: SiO 2 :H, CdWO 4 ,ets.
plasma Laser Dielectric Crystal Plasma emission Luminescence emission
Single pulse surface modification
Quartz monocrystal, Irradiation by SLIC Ti:Saphire laser at CEA/Saclay 50fs 800nm 20Hz repetition rate or second harmonic (400nm) Surfase modifications in crater: • Periodic structure • “Mouldy” surface: nanofibers.
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/cm 2 (10 14 W/cm 2 ) Single shot
Periodic structure in the crater
• Evolution of structure with number of shots • Direction of the structure and polarization.
– Polarization – Exposition.
400nm 5J/cm
2
(10
14
W/cm
2
) 1
shot
400nm 5J/cm
2
(10
14
W/cm
2
) 5
shots
400nm 5J/cm
2
(10
14
W/cm
2
) 10
shots
Period and amplitude of structure.
• L=l /1+Sin( F )= l normal incidence 1.5
• Amplitude proportional to Sin n where n is multi photonic order n=E g /E ph . For SiO 2 800nm: Eph=1.55eV
Eg=9eV, Ti:Saphire • n(800nm)=6, n(400nm)=3.
1.0
0.5
SEM image brightness amplitude Period 800nm Fitting by Sin 6 AFM measurement is required. 200 400 600 800 1000 1200
Polarization.
• Literature: Structure is parallel to polarization • 400nm: Structure is parallel to polarization • 800nm: Structure is perpendicular to polarization 400nm
Polarization.
Verification of polarization.
• Vertical – horizontal • Horizontal – vertical • Circular-circular.
800nm circular polarization 800nm
Polarization.
800nm Long exposition (50J/cm 2 10 15 W/cm 2 ) : x 20Hz : Appearance of parallel structure.
Polarization, picosecond pulse duration.
800nm Long exposition (40J/cm 2 : 2*10 13 W/cm 2 ) pulse duration 2ps: Appearance of parallel structure.
Different pulse durations.
• Femtoseconds (50fs) – Excitation of electrons.
– Absorption of laser pulse by electrons – Vaporization All processes on the surface • Picoseconds (2ps) – Amorphization – Darkening – Absorption by amorphous dark volume Heating of big volume.
800nm 40J/cm
2
(10
13
W/cm
2
) 1
shot Very weak modification
800nm 40J/cm
2
(10
13
W/cm
2
) 5
shots Parallel and perpendicular structures.
800nm 40J/cm 2 (10 13 W/cm 2 ) 10 shots Dark spot in the center
800nm 40J/cm 2 (10 13 W/cm 2 ) 12 shots Beginning of boiling in the center
800nm 40J/cm 2 (10 13 W/cm 2 ) 15 shots Boiling in the center
800nm 40J/cm 2 (10 13 W/cm 2 ) 20 shots Boiling all the crater.
800nm 40J/cm 2 (10 13 W/cm 2 ) multi shots Cracks around crater Strong 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