Band structure and optical properties of lead tungstate, and multiphoton microscopy of lead tungstate and gallium nitride: A combined theoretical and experimental dissertation

This dissertation covers two major parts: (1) band structure calculation on four scheelite crystals and optical property calculation on PbWO₄ ; (2) multiphoton excitation microscopy study on PbWO₄ and GaN.; Density functional calculations using the linearized augmented plane wave (LAPW) method were carried out for PbWO₄ as well as for CaMoO ₄, CaWO₄, and PbMoO₄ in order to determine their ground state electronic properties. The results indicate that CaMoO₄ and CaWO₄ have direct band gaps at the center of the Brillouin zone, while PbMoO₄ and PbWO₄ have band extrema at wave vectors away from the zone center with possibly indirect band gaps. The valence and conduction bands near the band gap are dominated by molecular orbitals associated with the $MoO-a4$ and $WO-a4$ ions, where α ≈ 2. In the Pb materials, the Pb 6s states form narrow bands 1eV below the bottom of the valence bands and also hybridize with states throughout the valence bands, while the Pb 6p states hybridize with states throughout the conduction bands. In the Ca materials, the Ca 3d states contribute to a high density of states 3–4eV above the bottom of the conduction bands.; Dielectric constants and reflectivity of PbWO₄ are also studied. The reflectivity calculated from the single-particle band structure compares with the experimental reflectivity unexpectedly well, including the sharp peak at the absorption threshold. And the exciton binding energy of PbWO ₄ is discussed and estimated as well as its band gap.; By using of a home-made multiphoton excitation scanning microscope, the luminescence spectra in both PbWO₄ crystals and epitaxial films of GaN have been resolved in three spatial dimensions. Inhomogeneities have been observed in both MOVPE- and MBE-grown GaN samples with occasional intense spots of 200 nm to 800 nm size (FWHM). Axial scans suggest non-radiative recombination (dead-layer) zones at the interfaces, the band-to-band photoluminescence image can be generally described as dim over most of the surface. In PbWO₄ samples, no distinct spectroscopic features are resolved in the surface and inside the bulk under the limit of the current system resolution. The explanation of the rapid falloff of luminescence intensity in z-scan is suggested.