Coherent Phase Control of Electronic Transitions in Gallium - PowerPoint PPT Presentation

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 ω = ω + 2 2 2 2 k c • Dispersion relation for L pe L a light wave in a ⇒ ω ≥ ω plasma: L pe ω 2 m = e L n • Critical density: π cr 2 4 e ω 2 n ε = = − = − pe 2 • Index of refraction: e n 1 1 ω 2 n cr L ε = θ = θ • Total reflection: 2 2 ( z ) sin ; n n cos e cr

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.

YaomingLu, Youbo Zhao, Slobodan Milasinovic John Penczak, SimaSingha, Zhan Hu Supported by NSF, USAF Surgeon General, UIC

[ ] ( ) ( ) ψ ω = π − + ϕ A sin 2 m m / T 0

Time Delay Scans

Properties of the Optimum Pulse vs. Fluence