Scott A. Crooker

Time-resolved Faraday rotation spectroscopy of spin dynamics in digital magnetic heterostructures

A time-resolved resonant Faraday rotation spectroscopy is employed to study the dynamical interplay between local magnetic moments and photoexcited carrier spins in quantum-confined semiconductor geometries. This highly sensitive technique functions as an energy selective, noninvasive, all-optical probe of spin dynamics ranging from femtosecond to microsecond timescales and is particularly suited to low-dimensional systems having small numbers of magnetic spins. Carrier spin-scattering rates, lifetimes, and the orientation and relaxation of perturbed magnetic ions are directly observed in the time domain. The utility of this technique is demonstrated through the study of a newly developed class of magnetic heterostructure, in which fractional monolayer planes of magnetic Mn/sup 2+/ ions are incorporated digitally into nonmagnetic II-VI ZnSe-ZnCdSe quantum wells. These digital magnetic heterostructures (DMH) possess large g-factors and exhibit enormous low-field resonant Faraday rotations in excess of 1.7/spl times/10/sup 7/ deg/T/spl middot/cm at low temperatures. Time-resolved Faraday rotation measurements identify a wealth of unexpected electronic and magnetic spin dynamics that are different from those generated in traditional semiconductors or alloyed diluted magnetic semiconductor structures.

Femtosecond synchronization of two passively mode‐locked Ti:sapphire lasers

We describe a method of reducing the timing jitter between two passively mode‐locked femtosecond titanium:sapphire laser systems, enabling femtosecond‐resolved measurements with independently tunable pump and probe wavelengths. The scheme supplements a commercially available system (Coherent Synchrolock) which locks two Ti:sapphire lasers to a master clock. By selecting only those pulses which are temporally coincident within a user‐specified range, a timing jitter reduction between the two lasers from ∼3 ps to <200 fs FWHM can be achieved, as measured by optical cross correlation. The timing jitter between the lasers is easily varied, allowing optimization of the tradeoff between temporal resolution and throughput depending on experimental needs.

Spin transport and optically-probed coherence in magnetic semiconductor heterostructures

Molecular beam epitaxy is used to “spin engineer” an environment wherein quantum-confined electronic states in a wide band gap II–VI semiconductor quantum well (Zn1−xCdx Se) are strongly exchange-coupled to systematic 2D distributions of localized spins (Mn2+ ions). Magneto-optical spectroscopy of undoped structures demonstrates that such a scheme successfully produces well-confined excitonic states whose Zeeman splitting in modest magnetic fields greatly exceeds the inhomogeneous line widths. In modulation-doped structures, a combination of magneto-transport and magneto-optical measurements shows the formation of a “magnetic” two-dimensional electron gas characterized by spin gaps which are much larger than Landau level gaps. This results in a novel quantum Hall system which can be highly spin polarized even at large filling factors. Time-resolved Faraday/Kerr effect measurements in the Voigt geometry probe the electronic spin dynamics of the exciton/electron gas, revealing terahertz and gigahertz oscillations that originate from the coherent spin precession of electrons and local moments, respectively.