1. Coherent Phase Control of Electronic Transitions in Gallium Arsenide Robert J. Gordon, Sima Singha, and Zhan Hu Department of Chemistry University of Illinois at Chicago FRISNO 11 Aussois, France March 31, 2011

2. F. Crim Passive Control

3. Active Control

4. JPC

5. Outline Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions

6. Cut in Decemet’s Membrane 6 ns, 1064 nm 30 ps, 1064 nm Vogel, et al., Invest. Ophthalmol. Vis. Sci. 35, 3033 (1997)

7. Surface Modification with Ultrafast Pulses Stoian, et al., Appl.Phys. Lett. 80, 353 (2002)

8. SEM images of the ablation craters on GaAs 1, 5 and 5+1 pulse trains

11. Outline Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions

12. Phys. Rev. B 82, 115205 (2010) LIBS/Photoluminescence Spectrum

13. Effect of Laser Polarization

14. PL Signal at 450.8 nm

15. Control Landscape

16. Effects of Polarization and Incidence Angle

17. Effect of Laser Fluence

18. Effect of Laser Phase

19. Outline Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions

20. Closed Loop Control Sine phase optimized for 390-450 nm sine phase optimized for 420-440 nm random phase optimized for 390-450 nm J. Phy. Chem. A (in press)

21. 20100528-115537PRB paper graph Optimum Pulse Shapes for Open and Closed Loops

22. Effect of Laser Fluence

23. Effect of Laser Polarization on Optimized PL Spectrum

24. Effect of Laser Phase on Open-Loop Spectrum

25. Effect of Laser Phase on Closed-Loop Spectrum

26. Outline Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions

27. Mechanistic Questions Where does the new band come from? How is it possible to excite optical phonons at fluences above the threshold for melting? How does light couple to the plasma? How does energy couple to the phonons? Where does the coherence come from?

29. Ratio of double pulse to single pulse fluorescence as a function of delay time and total energy Si App. Phys. Lett. 90, 131910 (2007), J. Appl. Phys. 104, 113520 (2008)

30. Dispersion relation for a light wave in a plasma: Critical density: Index of refraction: Total reflection: Light Propagation in a Plasma

31. Brunel or vacuum heating

32. Comparison of Closed and Open-Loop Pulses

33. Conclusions Coherent control of carrier recombination was achieved at fluences well above the damage threshold. The primary mechanism for open loop control appears to be phonon-hole scattering, with trapping of carriers in the L-valley. Brunel (ponderomotive) heating launches ballistic electrons that excite the phonons. Effect of laser phase suggests a competition between photoemission and phonon excitation. Random phase optimization appears to converge to a different control pathway.

34. Yaoming Lu, Youbo Zhao, Slobodan Milasinovic John Penczak, Sima Singha, Zhan Hu Supported by NSF, USAF Surgeon General, UIC

36. Time Delay Scans

37. Properties of the Optimum Pulse vs. Fluence