Mitsuo Yamaga

Long-lasting phosphorescence in Ce3+-doped Ca2Al2SiO7 and CaYAl3O7 crystals

Long-lasting phosphorescence is observed in Ce3+-doped Ca2Al2SiO7 (CASM) and CaYAl3O7 (CYAM) crystals at room temperature. UV irradiation at 365 nm produces broadband emissions with peaks at 417 nm for CASM and 425 nm for CYAM, which persist for not less than 1 h after the removal of the irradiating light. Based on the time dependence of the intensity of the phosphorescence, the long-lasting phosphorescence in these crystals is considered to be due to the electron–hole recombination at shallow traps, which occurs by a thermally activated process.

Optical properties of long-lasting phosphorescent crystals Ce3+-doped Ca2Al2SiO7 and CaYAl3O7

Abstract UV excitation of Ce3+-doped melilite crystals Ca2Al2SiO7 and CaYAl3O7 produces long-lasting phosphorescence besides intrinsic Ce3+ luminescence with lifetime of 40 ns. The intensity of the phosphorescence was measured as functions of temperature and time. The temperature dependence of the intensity integrated in time domain supports the upward-hopping mechanism of the trapped electron and the recombination of electron and hole at localized hole sites Ce4+.

Energy levels and symmetry of Ce3+ in fluoride and oxide crystals

Single crystal samples of Ce3+-doped fluorides [LiCaAlF6 (LiCAF), LiYF4 (LYF), NaCaYF6 (NCYF)], and oxides [Ca2Al2SiO7 (CASM), CaYAlO4 (CYA)] have been grown in inert and reducing atmospheres, with the intent of studying the effects of symmetry and ligand coordination on the dopant energy levels. The optical absorption spectra of Ce3+ in these crystals consist, at most, of five overlapping bands. The luminescence is also a broadband due to strong electron–phonon interaction in the 5d excited state. The energies of the lowest absorption band and the luminescence band decrease along the sequence LiCAF, LYF, NCYF, CASM, and CYA. This trend may be explained by the nephelauxetic effect and lowering symmetry of the anion-coordinate polyhedra of Ce3+ in these crystals. The lowest energy peak in the absorption spectrum of Ce3+ observed in CASM is coincident with the third harmonic (355 nm) of a Q-switched pulsed YAG laser.

Optical Spectroscopy Study on β-Ga2O3

Undoped β-Ga2O3 single crystals were grown by the floating zone technique under different conditions, and subsequently annealed in O2. Excitation and luminescence measurements were correlated with optical absorption, Hall- and electron-paramagnetic resonance measurements. It is shown, that the UV luminescence does not have an intrinsic nature, as stated by other authors. UV and blue luminescence are related to the concentration of oxygen vacancies, and thus to the electrical conductivity. They are assigned to donor-acceptor transitions. On the contrary, a green luminescence was observed after band-to-band excitation, and it is attributed to self-trapped or bound excitons.

Optical and electron spin resonance spectroscopy of Ti3+ and Ti4+ in Al2O3

The absorption spectrum of Ti3+:Al2O3 crystals grown in a reducing atmosphere consists of the main blue‐green absorption band and a weak infrared band. This infrared absorption decreases in intensity on annealing the as‐grown samples in a reducing atmosphere. The analysis of electron spin resonance spectra of the as‐grown samples, in which the infrared absorption band is observed, indicates that the center associated with the band is a cluster involving Ti3+ and Ti4+ ions with a neighboring, charge compensating Al3+ vacancy. The coexistence of a Ti4+ ion and an Al3+ vacancy in the neighborhood of the Ti3+ ion weakens the crystal field at this ion more than a single Ti4+ ion, giving rise to a red shift of the Ti3+ absorption.

Temperature dependence of the lifetime of Cr3+ luminescence in Garnet crystals I

The lifetimes of the R-lines from the 2E level and the broad bands from the 4T2 level of Cr3+ ions in various garnet crystals vary with the energy separation between the 2E and 4T2 levels, which change systematically with the composition of host crystal. The trend of the Cr3+ lifetimes, as well as their temperature dependences in garnet crystals, is explained by zero-point vibration and phonon-assisted tunnelling between the 2E and 4T2 states of Cr3+ ion.

The magnetic and optical properties of in

Electron spin-resonance (ESR) spectra of -doped (LiCAF) crystals measured at low temperature reveal three distinct centres in the crystal. The most intense spectrum is fitted to a spin Hamiltonian with trigonal symmetry and is associated with ions that substitute for ions in the LiCAF structure. Two other spectra have orthorhombic symmetry: they are assigned to ions with charge-compensating -ion vacancies near to the impurity ions. The and components of the polarized optical absorption and luminescence spectra of in LiCAF show there to be at least two centres. The intensities of optical transitions of are calculated using the eigenfunctions of the ground and excited states of in trigonal and orthorhombic symmetry and are compared with the observed polarizations of the absorption and luminescence.

Optical and structural studies on BaMgF4 : Ce3+ crystals

Single crystals BaMgF 4 (BMF:Ce 3+ ) doped with Ce 3+ and codoped with Ce 3+ and Na + were grown by the Czochralski technique. The optical and electron spin-resonance (ESR) spectra of Ce 3+ in the BMF:Ce 3+ crystals have been measured at room and low temperatures, respectively. The experimental results show that there are, at least, two distinct Ce 3+ -substitutional sites in the BMF:Ce 3+ crystals. In order to identify the Ce 3+ -substitutional sites, we have taken X-ray oscillation photographs. The additional satellite reflections observed in the photographs suggest that the BMF:Ce 3+ crystals have a commensurate superstructure with twofold periodicity along the a-, b- and c-axes of the basic structure, respectively. The superstructure can explain the presence of two distinct Ce 3+ -substitutional sites in the BMF:Ce 3+ crystals.

Electro paramagnetic resonance and optical spectra of Ti3+-doped YAlO3

The electron paramagnetic resonance (EPR), optical absorption and luminescence spectra of Ti3+ ions in YAlO3 have been measured at low temperatures. The optical absorption spectrum is composed of two broad bands with peak wavelengths of 434 and 492 nm, the energy separation of which is due to splitting of the excited 2E state. The broad-band luminescence with a peak at a wavelength of 605 nm at low temperatures is accompanied by a single sharp zero-phonon line at 540 nm. The absorption and luminescence spectra are strongly polarized. The spin-Hamiltonian parameters of Ti3+ ions substituting for Al3+ ions in sites with orthorhombic symmetry were determined from the orientation dependence of the EPR spectra to be gx=1.795(5), gy=1.850(5) and gz=1.950(5). The experimental g-tensor is compared with that calculated in terms of the mixing of the higher components of the 2T2 ground states and the 2E excited states into the lowest 2T2 ground state by spin-orbit interaction, the energies of these levels having been estimated from the broad-band absorption spectrum and the zero-phonon lines of the emission spectrum.

Electron paramagnetic resonance and optical spectra of Cr3+-doped YAlO3

This paper reports electron paramagnetic resonance (EPR) measurements of Cr3+ ions in YAlO3 (YAP) at 1.6 K and room temperature along with their optical absorption and luminescence spectra The polarization of the excitation spectrum of the Cr3+ luminescence has also been measured. The EPR spectrum is ascribed to substitutional Cr3+ ions at Al3+ sites. The spin-Hamiltonian parameters determined from the EPR spectrum of Cr3+ ions in nearly orthorhombic sites in YAP are gx = 1.981, gy = 1.981, gz = 1.981, b20 = +445 * 10-4 cm-1 and b22 = -278 * 10-4 cm-1. The anisotropy of the Cl3+ EPR spectrum is discussed in terms of the electrostatic potential of the surrounding O2- and Y3+ ions in YAP crystals and is compared with that of Ti3+ ions in the same host crystal.