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

F. Crim

Passive Control

Active Control

JPC

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

Phys. Rev. B 82, 115205 (2010)

LIBS/Photoluminescence Spectrum

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)

20100528-115537PRB paper graph

Optimum Pulse Shapes for Open and Closed Loops

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)

• 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

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 MilasinovicJohn Penczak, Sima Singha, Zhan Hu

Supported by NSF, USAF Surgeon General, UIC

TmmA /2sin 0

Time Delay Scans

Properties of the Optimum Pulse vs. Fluence