A recent experiment on Stark effect spectroscopy in self-assembled quantum dots (SADs) has demonstrated the existence of an inverted electron-hole alignment due to the presence of gallium diffusion in InAs SADs and has established a relation between the Stark shift and the vertical electron-hole separation. The theoretical interpretation of these experimental results is based on the assumption that the applied electric field can be treated by the second-order perturbation theory, which results in a quadratical dependence of the transition energy on the applied electric field. While this relation is well satisfied in many quantum systems including single SADs and quantum well structures. But this relation is not valid for vertically coupled SAD structures, the asymmetric Stark shift of experimental measurement has shown existence of built-in dipole moment in InAs/GaAs QDs. Here we present a theoretical investigation of the factors influencing the sign and magnitude of the built-in dipole moment in realistic QD structures, including cubical, pyramidal and truncated pyramidal shape. The comparable results gave a reasonable interpretation for the experimental results.
Our calculations consist of two basic steps. First, the strain is calculated for particular dot geometry with Green function methods. Second, the single-electron states are calculated using an electronic Hamiltonian, which depends on the strain. The computed states can then be used to determine various quantities, such as optical transition strengths, or exciton binding energies. In the article, we use Green function for the strain calculation, and plane-waves expansion in the envelope function approximation for the electronic structure calculations. The theoretical results agree well with the available experimental data. Our calculated results are useful for the application of QDs to photoelectric devices.
In this paper, performance of holographic storage based on doubly doped Fe:Mn:LiNbO<sub>3</sub> crystal with 0.03 wt% Fe<sub>2</sub>O<sub>3</sub> and 0.1 wt% MnO dopants which was great contrastive against previous investigations was studied theoretically and experimentally. We established the coupling differential equations using the band-transport model. Using practical experiment parameters, we numerically calculated and explained the time-dynamic developing process of the holographic storage, and analyzed how the oxidation-reduction degree of the crystal affects the space charge field within the crystal. Only the oxidative crystals can accomplish nonvolatile holographic storage but not the reductive crystal. For the oxidative crystal, its remaining magnitude of the space charge field increases with the increase of oxidative degree. Further more we measured the diffraction efficiencies of four specimens with different degree of oxidation-reduction and realized holographic multiplying and holographic fixing in the oxidation crystal successfully. Based on the experiments, we also calculated both the dynamic range parameter M/# of the oxidized doubly doped Fe:Mn:LiNbO<sub>3</sub> crystal and the recording sensitivity of various crystals. The experimental results coincided with the theoretical analysis very well. Diffraction efficiency of fixing grating increased with oxidation, photorefractive sensitivity decreases with oxidation, higher the concentration of doped manganese and oxidation, the larger the effective dynamic range of holographic storage system is, where holograms can be stored permanently.