The atomistic pseudopotential method is used to accurately predict the electron-hole exchange-induced fine structure (FS) and polarization anisotropy in million-atom ${\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ quantum dots of various shapes and compositions. The origin of the FS splittings is clarified using a simple model where the effects of atomistic symmetry and spin-orbit interaction are separately evident. Remarkably, polarization anisotropy and FS splittings are shown to occur, even in a cylindrically symmetric dot. Furthermore, “dark excitons” are predicted to be partially allowed. Trends in splittings among different shapes and compositions are revealed.

Near-field photoluminescence imaging spectroscopy of naturally occurring GaAs quantum dots (QDs) is presented. We successfully mapped out center-of -mass wave functions of an exciton confined in a GaAs QD in real space due to the enhancement of spatial resolution up to 30 nm. As a consequence, we discovered that the spatial profile of the exciton emission, which reflects the shape of a monolayer-high island, differs from that of biexciton emission, due to different distributions of the polarization field for the exciton and biexciton recombinations. This novel technique can be extensively applied to wave function engineering in the design and the fabrication of quantum devices.

The size dependence of the lowest electron-hole state in semiconductor microcrystals is calculated using the variational principle with a three-parameter Hylleraas-type wave function. For very small particles the Coulomb interaction may be treated as a perturbation. For larger particles the size dependence of the energy is much sharper than that expected in previous work.

We present a tight-binding calculation of the interband optical transitions in semiconductor quantum dots (QD’s). The calculated optical-absorption spectra are in good agreement with the existing experimental spectra of CdS and CdSe QD’s. We establish a correspondence between the tight-binding (TB) energy levels and those calculated using the spherical multiband effective-mass approximation (EMA). Consequently, the comparatively stringent selection rules of the latter are applicable to a large extent. Thus we formulate a convenient and quantitatively accurate description of the optical absorption in QD’s in terms of the TB energy levels and multiband EMA quantum numbers.

We have observed ferromagnetism in dilute $(\sim 1–4\mathrm{at.}\%)$ Mn-doped crystalline ZnO nanowires at temperatures up to $400\mathrm{K}$ . Arrays of freestanding single crystal ZnO:Mn nanowires were fabricated by Au-catalyzed vapor-liquid-solid growth. Structure and compositional analyses revealed that Mn was incorporated into the ZnO lattice. From the observed saturation magnetization, the magnetic moment per Mn atom is estimated to be between $0.3{\mu }_B$ and $1.2{\mu }_B$ . Photoluminescence measurements show a strong suppression of defect related midgap emission, indicative of an interplay between Mn doping and native point defects.

The exciton and two-exciton states in semiconductor quantum dots much larger in size than the exciton Bohr radius are investigated, and the energies and oscillator strengths of several exciton and biexciton states are calculated. The presence of weakly correlated exciton-pair states are identified and these have a large oscillator strength increasing proportional to the volume of the quantum dot. These states are shown to play a crucial role in determining the nonlinear optical response of large quantum dots. The weakly correlated exciton-pair states are found to cause a cancellation effect in the third-order nonlinear optical susceptibility at the exciton resonance, providing a consistent understanding of the experimentally observed saturation of the mesoscopic enhancement of the excitonic optical nonlinearity. The excited-state absorption in quantum dots is also studied and the excitation of the weakly correlated exciton-pair states is found to dominate the spectrum. The spectral features in the pump-probe spectroscopy are predicted in detail. The biexciton binding energy and oscillator strength are obtained in good agreement with experimental results on CuCl quantum dots. Also, the good correspondence of the excited-state absorption spectra between the theory and experiments provides convincing evidence for the presence of the weakly correlated exciton-pair states.