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Femtosecond laser ablation dynamics in wide band gap crystals.

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.

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Femtosecond laser ablation dynamics in wide band gap crystals.

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  1. Femtosecond laser ablation dynamics in wide band gap crystals. N.Fedorov CEA/DSM/IRAMIS École Polytechnique

  2. 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.

  3. 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.

  4. Metal Laser crater plasma 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

  5. Dielectric Crystal plasma Laser Luminescence emission Plasma emission 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: SiO2:H, CdWO4,ets.

  6. Single pulse surface modification Quartz monocrystal, Irradiation by SLIC Ti:Saphire laser at CEA/Saclay 50fs 800nm 20Hz repetition rateor second harmonic (400nm) Surfase modifications in crater: • Periodic structure • “Mouldy” surface: nanofibers.

  7. 400nm 5J/cm2 (1014W/cm2) Single shot Nano-particles and nano-fibers • Fast cooling of plasma. • Collapsing to drops. • Drop of glass stretch a fiber.

  8. 400nm 5J/cm2 (1014W/cm2) Single shot

  9. Periodic structure in the crater • Evolution of structure with number of shots • Direction of the structure and polarization. • Polarization • Exposition.

  10. 400nm 5J/cm2 (1014W/cm2) 1 shot

  11. 400nm 5J/cm2 (1014W/cm2) 5 shots

  12. 400nm 5J/cm2 (1014W/cm2) 10 shots

  13. 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. SEM image brightness amplitude Period 800nm Fitting by Sin6 AFM measurement is required.

  14. Polarization. • Literature: Structure is parallel to polarization • 400nm: Structure is parallel to polarization • 800nm: Structure is perpendicular to polarization 400nm 800nm

  15. Polarization. Verification of polarization. • Vertical – horizontal • Horizontal – vertical • Circular-circular. 800nm circular polarization 800nm 800nm

  16. Polarization. 800nm Long exposition (50J/cm2 x 20Hz : 1015W/cm2) : Appearance of parallel structure. 800nm

  17. Polarization, picosecond pulse duration. 800nm Long exposition (40J/cm2 : 2*1013W/cm2) pulse duration 2ps: Appearance of parallel structure. 800nm

  18. 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.

  19. 800nm 40J/cm2 (1013W/cm2) 1 shot Very weak modification

  20. 800nm 40J/cm2 (1013W/cm2) 5 shots Parallel and perpendicular structures.

  21. 800nm 40J/cm2 (1013W/cm2) 10 shots Dark spot in the center

  22. 800nm 40J/cm2 (1013W/cm2) 12 shots Beginning of boiling in the center

  23. 800nm 40J/cm2 (1013W/cm2) 15 shots Boiling in the center

  24. 800nm 40J/cm2 (1013W/cm2) 20 shots Boiling all the crater.

  25. 800nm 40J/cm2 (1013W/cm2) multi shots Cracks around crater Strong heating in the volume under surfase

  26. 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.

  27. 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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