[Phys-seminars] 2011-04-12 Lasers Seminar
ibar at bgu.ac.il
Tue Apr 5 17:27:57 IDT 2011
TIME: 3:30pm (Tue)
PLACE: Physics building (#54) room 207
Coherent Phase Control of Electronic Transitions in Gallium Arsenide
Prof. Robert Gordon, Department of Chemistry, University of Illinois at
Much of the history of coherent control has dealt with manipulating
energy transfer and product branching ratios of isolated molecules.
In an attempt to extend the techniques of coherent control to
condensed matter, we have used trains of ultrashort laser pulses to
excite the optical phonons of a GaAs(100) crystal at fluences well
above the damage threshold. In an open loop experiment,1 trains of
three equally spaced pulses were focused on the crystal. For
arbitrary pulse spacing, a conventional laser-induced breakdown
spectrum is observed. When the spacing is set equal to an integer
multiple of the phonon period, however, a photoluminescence (PL)
band is generated by recombination of electrons and holes in the
L-valley of the Brillouin zone. An analysis of the effects of laser
polarization and angle of incidence shows that the plasma created by
the laser pulse plays a key role in scattering the carriers. The
principle mechanism appears to be impulsive excitation of the lattice
beneath the molten surface by ballistic electrons driven into the
plasma by the ponderomotive force. Decay of the longitudinal optical
(LO) phonons provides the requisite momentum to scatter the carriers
into the L-valley. In a closed loop experiment, selected regions of
the PL spectrum were enhanced by using an evolutionary algorithm and
a spatial light modulator (SLM) to optimize the pulse shape.2 A sine
phase function produced an order of magnitude increase in signal
after only one generation and converged after 15 generations to a
train of three equally spaced pulses approx. 0.8 ps apart. In
contrast, random programming of individual pixels of the SLM resulted
in a gradual evolution to a more complex pulse approx. 15 ps long
after 80 generations.
1 Z. Hu, S. Singha, and R. J. Gordon, Phys. Rev. B 82, 115205 (2010).
2 S. Singha, Z. Hu, and R. J. Gordon, J. Phys. Chem. A (Victoria Buch
memorial issue, in press).
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