Optical injection and coherent control of ballistic spin currents, carrier density, and carrier spin in semiconductors.

摘要

In this dissertation, extensive two-color coherent optical control of the density and motion of charge and spin is demonstrated in semiconductors. The two colors are ultrashort optical pulses of ∼100 femtosecond duration: a fundamental pulse at frequency o with wavelength ∼1.5 mum and its second harmonic at 2o with wavelength ∼0.75 mum. The o (2o) photon energy is below (above) the semiconductor band gap energy, and injects carriers through two- (one-) photon absorption. Two-color control arises through interference processes requiring both pulses. The phases and polarizations of these pulses---and the crystal symmetry and orientation---are used to independently control the carrier- and spin-density and the motion of charge and spin.; Three currents are demonstrated. The first is a spin-polarized charge current, in which spin and charge move in the same direction. The second is a pure spin current, in which spin-up and spin-down carriers travel in opposite directions, leading to a net motion of spin without a net motion of charge. The third is an unpolarized charge current that was previously observed. Charge currents are investigated using surface electrodes. Both charge and spin currents are studied with a spatially resolved optical probing technique that tracks carrier density, spin, and position. Adjusting the relative phases of o and 2o controls the direction or magnitude of each current.; In addition, the phases and polarizations of o and 2o, as well as the crystal symmetry and orientation, are used to independently control the overall carrier density and spin in off-axis-oriented semiconductors. This control is also monitored with an optical probing technique.; Two processes contribute to the observed two-color coherent control. Quantum interference between transition amplitudes associated with one- and two-photon absorption produces charge and spin populations that have asymmetric momentum distributions, thus generating charge and spin currents. Both quantum interference and a cascaded process---in which frequency conversion between o and 2o leads to a modulation of the one-photon carrier- and spin-injection---lead to control of the overall densities of charge and spin.