Coherence and manipulation of spin states in semiconductor nanostructures

Ultrafast optical techniques are developed to study and control carrier spin dynamics in semiconductor quantum dots and wells. Systematic studies of spin relaxation were performed in CdSe quantum dots (QDs) ranging from 22–80Å in diameter. A fundamental motivation for the experiments was to identify operative spin scattering pathways for carriers in semiconductors with the discrete bandstructure resulting from quantum confinement. Because carriers are physically isolated from one another by imbedding the QDs in an insulating matrix, these studies explore a novel system where homogeneous spin-spin interactions may be negligible.; Disparate transverse and longitudinal spin relaxation was characterized by applying a magnetic field either perpendicular or parallel to the observation direction. Nanosecond-scale transverse spin lifetimes were observed that exhibit relatively little dependence on temperature up to 300K. Spin precession occurs at distinct frequencies that are attributed to electron and exciton spins. Transverse spin relaxation is shown to be limited by inhomogeneous dephasing through a proportionate reduction in spin lifetime with field. In contrast, longitudinal spin relaxation times increase with field, with a component that persists for >20μs at low temperatures <50K. Similar measurements in a related system of CdS_1−xSe_x QDs suggest a possible connection between the μs-scale dynamics and interactions of carriers with nuclear spins.; In an effort to identify and eliminate the limitations on transverse spin lifetimes imposed by sample inhomogeneity, an all-optical method was developed with the potential for producing pulse sequences familiar from conventional magnetic resonance that can reverse certain types of dephasing. An effective magnetic field was generated in a semiconductor quantum well by a ‘tipping’ pulse whose energy was tuned below the bandgap. Through the resultant optical Stark effect, magnetic field strengths of order 20T are achievable within the ∼200fs pulse width. Coherent rotations of electron spins by these laser pulses up to 90° have been demonstrated by monitoring a change in the amplitude of spin precession. Current limitations on the technique due to the excitation of carriers by the sub-resonant pulse have been fully explored as functions of the laser intensity, energy, and polarization.