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. 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 crater Laser Metal plasma

5. 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 Crystal plasma Plasma emission Luminescence emission Scintillation crystals: SiO 2 :H, CdWO 4,ets.

6. 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)

7. 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

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

10. 400nm 5J/cm 2 (10 14 W/cm 2 ) 1 shot

11. 400nm 5J/cm 2 (10 14 W/cm 2 ) 5 shots

12. 400nm 5J/cm 2 (10 14 W/cm 2 ) 10 shots

13. Period and amplitude of structure. L=l/1+Sin(F)=l normal incidence Amplitude proportional to Sin n where n is multi photonic order n=E g /E ph. For SiO 2 Eg=9eV, Ti:Saphire 800nm: Eph=1.55eV n(800nm)=6, n(400nm)=3. SEM image brightness amplitude Period 800nm Fitting by Sin 6 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 800nm circular polarization 800nm

16. Polarization. 800nm Long exposition (50J/cm 2 x 20Hz : 10 15 W/cm 2 ) : Appearance of parallel structure. 800nm

17. Polarization, picosecond pulse duration. 800nm Long exposition (40J/cm 2 : 2*10 13 W/cm 2 ) 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/cm 2 (10 13 W/cm 2 ) 1 shot Very weak modification

20. 800nm 40J/cm 2 (10 13 W/cm 2 ) 5 shots Parallel and perpendicular structures.

21. 800nm 40J/cm 2 (10 13 W/cm 2 ) 10 shots Dark spot in the center

22. 800nm 40J/cm 2 (10 13 W/cm 2 ) 12 shots Beginning of boiling in the center

23. 800nm 40J/cm 2 (10 13 W/cm 2 ) 15 shots Boiling in the center

24. 800nm 40J/cm 2 (10 13 W/cm 2 ) 20 shots Boiling all the crater.

25. 800nm 40J/cm 2 (10 13 W/cm 2 ) 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