FEMTOSECOND FARADAY-ROTATION IN SPIN-ENGINEERED HETEROSTRUCTURES

We have developed a new femtosecond-resolved optical technique with which one may examine magnetic spin dynamics in near-atomic scale structures by employing a highly sensitive, ultrafast measurement of the Faraday rotation. We apply this spectroscopy to novel band-gap-engineered II-VI diluted magnetic semiconductor heterostructures of two types to demonstrate the ability to simultaneously monitor electronic and magnetic interactions in a quantum geometry. The experiments show that these dynamics evolve on widely different time scales, and reveal the onset and decay of magnetization due to carrier spin scattering with femtosecond temporal resolution (300 fs). Although photoexcited carriers recombine within several hundred picoseconds, they leave behind a magnetic ''footprint'' that persists considerably longer and relaxes through an entirely different spin-lattice mechanism. Time-resolved magnetic measurements reveal the unusual dynamical properties of low-dimensional systems incorporating magnetic spins as compared to traditional semiconductor heterostructures.