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Coherent Phase Control of Electronic Transitions in
Gallium Arsenide
Robert J. Gordon, Sima Singha, and Zhan HuDepartment of Chemistry
University of Illinois at Chicago
FRISNO 11 Aussois, FranceMarch 31, 2011
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F. Crim
Passive Control
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Active Control
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JPC
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Outline
• Motivation and methods
• Results from open loop experiments
• Results from closed loop experiments
• Proposed mechanism
• Conclusions
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Cut in Decemet’s Membrane
6 ns, 1064 nm
30 ps, 1064 nm
Vogel, et al., Invest. Ophthalmol. Vis. Sci. 35, 3033 (1997)
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Surface Modification with Ultrafast Pulses
Stoian, et al., Appl.Phys.Lett. 80, 353 (2002)
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SEM images of the ablation craters on GaAs 1, 5 and 5+1 pulse trains
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Outline
• Motivation and methods
• Results from open loop experiments
• Results from closed loop experiments
• Proposed mechanism
• Conclusions
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Phys. Rev. B 82, 115205 (2010)
LIBS/Photoluminescence Spectrum
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Effect of Laser Polarization
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PL Signal at 450.8 nm
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Control Landscape
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Effects of Polarization and Incidence Angle
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Effect of Laser Fluence
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Effect of Laser Phase
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Outline
• Motivation and methods
• Results from open loop experiments
• Results from closed loop experiments
• Proposed mechanism
• Conclusions
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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)
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20100528-115537PRB paper graph
Optimum Pulse Shapes for Open and Closed Loops
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Effect of Laser Fluence
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Effect of Laser Polarization on Optimized PL Spectrum
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Effect of Laser Phase on Open-Loop Spectrum
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Effect of Laser Phase on Closed-Loop Spectrum
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Outline
• Motivation and methods
• Results from open loop experiments
• Results from closed loop experiments
• Proposed mechanism
• Conclusions
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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?
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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)
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• Dispersion relation for a light wave in a plasma:
• Critical density:
• Index of refraction:
• Total reflection:
peL
LpeL ck
2222
2
2
4 e
mn Lecr
2
22 11
L
pe
cr
e
n
nn
22 cos;sin)( cre nnz
Light Propagation in a Plasma
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Brunel or vacuum heating
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Comparison of Closed and Open-Loop Pulses
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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.
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Yaoming Lu, Youbo Zhao, Slobodan MilasinovicJohn Penczak, Sima Singha, Zhan Hu
Supported by NSF, USAF Surgeon General, UIC
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TmmA /2sin 0
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Time Delay Scans
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Properties of the Optimum Pulse vs. Fluence