Tsuyoshi Okuno

Photoluminescence Properties of Eu3+-Doped ZnO Nanoneedles

Eu3+-doped ZnO nanoneedles were fabricated by thermal evaporation. In order to include Eu in ZnO nanostructures, europium nitrate was used to produce starting droplets from which ZnO nanoneedles were grown. Photoluminescence spectra included a sharp blue peak corresponding to the ZnO band gap and a broad red band due to defect states. In addition, sharp intra-4f transitions of Eu3+ ions were observed. Energy transfer from the ZnO matrix to Eu3+ was also observed. Study of the photoluminescence excitation revealed absorption tail states below the ZnO band gap induced by the inclusion of Eu ions in the ZnO nanoneedles. The location of Eu ions was assumed to be near the surfaces of the ZnO nanoneedles.

Homogeneous line width of Praseodymium ions in various inorganic materials

Persistent spectral hole burning and fluorescence line narrowing spectroscopy was applied to Pr 3+ doped silicate glass, yttria stabilized zirconia, and yttria, to investigate homogeneous line width (Γ). In the temperature (T) dependence of Γ, T-linear components are commonly found below 10 K in these three materials, and their magnitudes are found to become larger as the degree of disorder increases

Persistent spectral hole-burning of Pr3+ ions in yttria stabilized zirconia: a new hole-burning material

Abstract Persistent spectral hole-burning has been observed in Pr 3+ doped yttria stabilized zirconia (YSZ) and used to measure the homogeneous linewidth (Г h ). Holes can be burned only in the lowest crystal field component of the 3 H 4 → 1 D 2 transition of Pr 3+ ions (6115 A, inhomogeneous linewidth: ∼100 cm -1 ) and observed below 25 K. Homogeneous linewidth of the transition is found to be 155 MHz at 4.2 K and to have a T 1.20±0.1 -dependence. Such a temperature dependence is quite similar to that usually observed in organic-molecules-doped polymers and coupling of electrons to the two level system (TLS) is suggested for the phase relaxation process.

Intense Green Luminescence from Eu2+-doped ZnO Microstructures

We fabricated Eu2+-doped ZnO microstructures by thermal evaporation. Their structures were micrometer-size grains with smooth surfaces or nanometer-size needles. Morphology depended on the temperature region where structures were grown. A strong green luminescence at room temperature is demonstrated. This is attributed to the efficient energy transfer from the ZnO matrix to Eu2+ ions and their 5d–4f transitions.

Yellow Photoluminescence of Europium Thiosilicate on Silicon Substrate

We report the fabrication of europium thiosilicate (Eu2SiS4) on a silicon substrate. Europium sulfide (EuS) is thermally evaporated on a silicon substrate. It is sealed in a vacuum with sulfur and then heated at 650 °C. Eu2SiS4 shows intense yellow photoluminescence, and its wavelength (peak 570 nm) and width (60 nm) are reproducible. This corresponds to the 4f65d–4f7 transition of Eu2+. The photoluminescence efficiency of Eu2SiS4 is measured and estimated to be 0.5%.

Optical Properties of Silicon Nanowires Fabricated by Electroless Silver Deposition

Vertically aligned silicon nanowires are fabricated on silicon substrates by electroless silver deposition, and their optical properties are investigated. The diameter of the nanowires ranges from 100 to 500 nm and their length ranges from 30 to 100 ?m. Red photoluminescence appears at approximately 700 nm at room temperature. Its peak and intensity vary along the wire positions in Raman microscopy measurements. The photoluminescence intensity is high at the top of the wire. In this region, oxidation is enhanced and the Raman scattering spectra of the optical phonon mode tend to be broadened. The origin of the red photoluminescence is assumed to be defect states located at the interface between the core of the silicon nanowires and the surrounding silicon oxide.

Luminescence Properties of Rare-Earth-Doped Thiosilicate Phosphors on Silicon Substrate

The luminescence properties of rare-earth-doped thiosilicate phosphors are reported. These thiosilicate materials are fabricated in phosphor layers on silicon substrates. For Eu2+-doped calcium thiosilicate, yellow (560 nm) and red (650 nm) bands are obtained in the photoluminescence spectrum, which is almost the same as that for the corresponding powder sample. The energy transfer efficiency from Eu2+ to Er3+ in Eu2SiS4:Er3+ on Si substrates is improved by optimization of the annealing conditions. In addition, the insulation of electroluminescence devices using BaSi2S5:Eu2+ on silicon-on-insulator substrates is improved using a high-dielectric-constant polymer as a transparent insulating layer.

1.5-µm Intraband Transitions in PbSe Quantum Dots

Intraband transitions around 1.5 mum are studied in PbSe quantum dots at room temperature. Femtosecond pump-probe measurements reveal induced absorption by photoexcited carriers up to higher energy levels and its relaxation. A dominant decay component has a decay time of 10-40 ps, which is determined by the relaxation of carriers at the lowest absorption peak. The induced absorption is observed in a broad spectral range between 0.77 and 2.5 eV.

Defect-induced persistent hole burning in MgO-doped Pr3+:YAG systems

Abstract Persistent spectral holes have been found in Pr3+:YAG, when a small amount of MgO was co-doped. The holes are stable against temperature cycling up to 100 K. The initial hole burning rate shows a roughly linear dependence on the concentration of Mg2+ ions in the matrix. This suggests that the persistent hole is induced by the defects associated with the co-doping of the Mg2+ ions.

Observation of two types of spectral holes in MgO-doped Y2O3:Pr3+ crystals

Abstract In addition to a narrow spectral hole due to optical pumping to hyperfine sublevels in the ground state, an anomalously broad persistent spectral hole was burned in Y 2 O 3 :Pr 3+ crystals which contain a small amount of MgO. This broad hole was stable against 300 K-annealing. Burning of this broad hole is ascribed to the rearrangement of local structure around optical centers.