Transcript PPT
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 Passive Control F. Crim Active Control Outline • • • • • Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions Cut in Decemet’s Membrane 6 ns, 1064 nm 30 ps, 1064 nm Vogel, et al., Invest. Ophthalmol. Vis. Sci. 35, 3033 (1997) Surface Modification with Ultrafast Pulses Stoian, et al., Appl.Phys. Lett. 80, 353 (2002) SEM images of the ablation craters on GaAs 1, 5 and 5+1 pulse trains Outline • • • • • Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions LIBS/Photoluminescence Spectrum Phys. Rev. B 82, 115205 (2010) Effect of Laser Polarization PL Signal at 450.8 nm Control Landscape Effects of Polarization and Incidence Angle Effect of Laser Fluence Effect of Laser Phase Outline • • • • • Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions 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) Optimum Pulse Shapes for Open and Closed Loops PRB paper graph 20100528-115537 Effect of Laser Fluence Effect of Laser Polarization on Optimized PL Spectrum Effect of Laser Phase on Open-Loop Spectrum Effect of Laser Phase on Closed-Loop Spectrum Outline • • • • • Motivation and methods Results from open loop experiments Results from closed loop experiments Proposed mechanism Conclusions 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? Ratio of double pulse to single pulse fluorescence as a function of delay time and total energy Si<111> App. Phys. Lett. 90, 131910 (2007), J. Appl. Phys. 104, 113520 (2008) Light Propagation in a Plasma • Dispersion relation for a light wave in a plasma: 2 pe 2 2 L L pe me ncr 4e 2 L 2 • Critical density: • Index of refraction: • Total reflection: k c 2 L ne n 1 1 ncr 2 2 pe 2 L ( z ) sin ; ne ncr cos 2 2 Brunel or vacuum heating Comparison of Closed and Open-Loop Pulses 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. Yaoming Lu, Youbo Zhao, Slobodan Milasinovic John Penczak, Sima Singha, Zhan Hu Supported by NSF, USAF Surgeon General, UIC A sin 2 m m0 / T Time Delay Scans Properties of the Optimum Pulse vs. Fluence