Femtosecond Coherent Control of Spin with Light

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Date/Time:Friday, 20 Aug 2010 from 4:10 pm to 5:00 pm
Location:Room 18 Physics Hall
Channel:Condensed Matter Physics
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Prof. Ilias Perakis, Department of Physics University of Crete, Heraklion, Crete

Femtosecond nonlinear optical spectroscopy provides the means to investigate physical phenomena that occur within a time window comparable to the fundamental timescales of electronic and atomic motion. This opportunity offers a unique view of non-equilibrium properties of cooperative condensed matter systems that are not accessible with conventional time-integrated experimental techniques such as linear optical spectroscopy or inelastic neutron scattering.

In the femtosecond temporal regime, the system is far away from thermodynamic equilibrium. Issues such as quantum coherence, non-thermal populations, and the validity of the free energy concept then come into play. An important goal is control of magnetic systems at the quantum level using femtosecond optical pulses. Such an approach goes beyond traditional investigations of optical properties and can lead to the design of fast photonic devices with a functional role in information storage and processing.
Using density matrix equations of motion, we predict a femtosecond collective spin tilt triggered by nonlinear, near-ultraviolet, coherent photoexcitation of (Ga,Mn)As ferromagnetic semiconductors with linearly polarized light. This dynamics results from carrier coherences and nonthermal populations excited in the {111} equivalent directions of the Brillouin zone and triggers a subsequent uniform precession. Our mechanism explains recent ultrafast pump-probe experiments.