Using two-color time-resolved Faraday rotation and ellipticity, we demonstrate ultrafast optical control of electron spins
in GaAs quantum wells and InAs quantum dots. In quantum wells, a magnetic-field induced electron spin polarization is
manipulated by off-resonant pulses. By measuring the amplitude and phase of the spin polarization as a function of pulse
detuning, we observe the two competing optical processes: real excitation, which generates a spin polarization through
excitation of electron-hole pairs; and virtual excitation, which can manipulate a spin polarization through a stimulated
Raman process without exciting electron-hole pairs. In InAs quantum dots, the spin coherence time is much longer, so that
the effect of many repetitions of the pump pulses is important. Through real excitation, the pulse train efficiently polarizes
electron spins that precess at multiples of the laser repetition frequency, leading to a "mode-locking" phenomenon. Through
virtual excitation, the spins can be partially rotated toward the magnetic field direction, leading to a sensitive dependence of
the spin orientation on the precession frequency and detuning. The electron spin dynamics strongly influence the nuclear
spin dynamics as well, leading to directional control of the nuclear polarization distribution.
Spin g-factors and lifetimes were studied with picosecond pump-probe techniques for a set of samples of InAs quantum
dots of uniform height. The samples were grown by MBE with a cap and flush sequence to produce a height of 2.5 nm.
Remote doping provided electrons in the dots. Electron coherence was excited by a fast pump pulse and detected
through the Faraday rotation of a probe pulse. The results show an in plane g-factor of 0.427 and lifetimes around 1 ns
that shorten for increasing magnetic fields. For an undoped sample, signals from singly charged and neutral dots are
observed and simulated to provide the hole g-factor and parameters for the neutral exciton. The undoped sample also
exhibits signals for negative delays attributed to mode-locking of the spin coherence to the optical pulse train. This
observation indicates that the true spin coherence lasts at least 12 ns.
Recent theoretical analysis and experimental investigations indicate that the physics of clusters deposited on semiconductor surfaces such as Silicon may be a promising future avenue for nanostructure science. Clusters of small number (5 - 10) of atoms in free space have also been shown to have interesting energy structures as well as magnetic and electrical moments. We report on the formation of Mn islands on Si(111) surfaces and their optical scattering response. We show that Mn islands of diameter 15 to 30nm exhibit paramagnetism at low temperatures, while thick films of Mn do not. In addition, our experiments verify previous theoretical suggestions that polarized optical scattering can be used to detect magnetism in small clusters. We will discuss some of these along with possible future applications of cluster physics.
Neutron irradiation of sapphire with 1 x 1022 neutrons(<EQ MeV)/m2 increases the c-axis compressive strength by a factor of 3 at 600 degree(s)C. The mechanism of strength enhancement is the retardation of r-plane twin propagation by radiation-induced defects. 1-B and Cd shielding was employed during irradiation to filter our thermal neutrons (<EQ1 eV), thereby reducing residual radioactivity in the sapphire to background levels in a month. Yellow-brown irradiated sapphire is nearly decolorized to pale yellow by annealing at 600 degree(s)C with no loss of mechanical strength. Annealing at sufficiently high temperature (such as 1200 degree(s)C for 24 h) reduces the compressive strength back to its baseline value. Neutron irradiation decreases the flexure strength of sapphire at 600 degree(s)C by 0-20% in some experiments. However, the c- plane ring-on-ring flexure strength at 600 degree(s)C is doubled by irradiation. Elastic constants of irradiated sapphire are only slightly changed by irradiation. Infrared absorption and emission and thermal conductivity of sapphire are not affected by irradiation at the neutron fluence used in this study. Defects that might be correlated with strengthening were characterized by electron paramagnetic resonance spectroscopy. Color centers observed in the ultraviolet absorption spectrum were not clearly correlated with mechanical response. No radiation-induced changes could be detected by x-ray topography or x-ray diffraction.