FEMTOSECOND PROBES OF MAGNETIC PHENOMENA - SPIN-POLARIZATION SPECTROSCOPY IN DILUTE-MAGNETIC-SEMICONDUCTOR QUANTUM-WELLS

The spin-scattering times of charge carriers in dilute-magnetic- semiconductor multiple quantum-well structures are observed directly in a time-resolved experiment. Luminescence measurements with a temporal resolution of 150 fs, obtained via the sum-frequency-generation optical gating technique, monitor the evolution of carrier populations optically prepared to have nonequilibrium spin polarizations. This method enables measurements of spin scattering on shorter time scales than have hitherto been accessible. Spin-relaxation times of a few picoseconds are measured for quantum-confined electrons, and these times are found to be surprisingly insensitive to the presence of magnetic ions in the wells. The strong carrier-ion spin-spin-exchange mechanism is therefore not the dominant spin-scattering channel in these structures, contrary to theoretical expectations.