[Phys-seminars] 2011-04-12 Lasers Seminar

ilana ibar at bgu.ac.il
Tue Apr 5 17:27:57 IDT 2011


Lasers Seminar 

  DATE: 12-04-2011 

  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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