Yong Gyu Choi

1.3 μm emission and multiphonon relaxation phenomena in PbOBi2O3Ga2O3 glasses doped with rare-earths

Abstract The 1.3 μm fluorescent emissions from PbOBi 2 O 3 Ga 2 O 3 glasses doped with Pr 3+ or Dy 3+ were measured. Modified Judd—Ofelt analysis was applied to the Pr 3+ -doped glasses to obtain intensity parameters and radiative properties. Emission from the Pr 3+ : 1 G 4 → 3 H 5 transition in heavy metal oxide (HMO) glass was centered at the wavelength of 1320 nm with a spectral bandwidth of 100 nm. The lifetime of the 1 G 4 level was 53 μs with a quantum efficiency of 9%. Excited-state absorption near the 1.3 μm emission were comparable to those in fluoride glasses. Multiphonon relaxation rates in PbOBi 2 O 3 Ga 2 O 3 glasses calculated from the lifetimes were similar to those of fluoride glasses, but were the smallest among oxide glasses.

Comparative study of energy transfers from Er3+ to Ce3+ in tellurite and sulfide glasses under 980 nm excitation

We have demonstrated that the population feeding from the 4I11/2 level to the 1.5 μm fluorescence emitting 4I13/2 level of Er3+ ions in low phonon energy glass hosts can be enhanced by codoping with Ce3+ under optical pumping at 980 nm. The nonradiative energy transfer Er3+: 4I11/2; Ce3+: 2F5/2→Er3+: 4I13/2; Ce3+: 2F7/2, occurs in the form of phonon-assisted energy transfer, and therefore the feeding rates are faster in the tellurite glasses, which have a comparatively higher phonon energy than in the sulfide glasses. The cross-relaxation process for 4I13/2: 4I13/2→4I15/2: 4I9/2, which lowers the population density of the 4I13/2 manifold and causes a deleterious effect in the 1.5 μm fluorescence intensity, is more severe in the sulfide glasses. Population feeding rate from the 4I11/2 to the 4I13/2 level is significantly enhanced by way of cerium codoping into tellurite glasses, which promises an efficient 980 nm pumped broadband Er3+-doped fiber amplifier.

Emission properties of the Er3+:4I11/2→4I13/2 transition in Er3+- and Er3+/Tm3+-doped Ge–Ga–As–S glasses

Abstract Ge–Ga–As–S glasses were investigated to develop efficient host materials for the Er 3+ :2.7 μm fiber lasers. Ge30Ga1As9S60 and Ge30Ga2As6S62 (all in at.%) glasses show good thermal stability and high rare-earth solubility up to 0.3 mol%. Mid-infrared emission with a peak wavelength of 2.76 μm and bandwidth of ∼120 nm was observed in Er3+- and Er3+/Tm3+-doped Ge30Ga2As6S62 glasses. Codoping of Tm3+ significantly reduced the lifetime of the Er 3+ : 4 I 13/2 level due to the energy transfer of Er 3+ : 4 I 13/2 → Tm 3+ : 3 F 4 . Thus, the population inversion between the 4 I 11/2 and 4 I 13/2 levels in Er3+ became possible. In Er3+-doped glasses, the rate of cross-relaxation for the 4 I 11/2 level was approximately 11 times faster than that for the 4 I 13/2 level.

1.6 μm emission from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses

1.6 μm emission originated from Pr3+: (3F3,3F4)→3H4 transition in Pr3+- and Pr3+/Er3+-doped selenide glasses were investigated under an optical pump of a conventional 1480 nm laser diode. The measured peak wavelength and full width at half maximum of the fluorescent emission were ∼1650 and >100 nm, respectively. A moderate lifetime of the thermally coupled upper manifolds (∼212±5 μs) together with a high stimulated emission cross section of ∼(3±1)×10−20 cm2 promises to be useful for 1.6 μm band fiber-optic amplifier that can be pumped with an existing high-power laser diode. Codoping of Er3+ significantly enhanced the emission intensity by way of a nonradiative Er3+: 4I13/2→Pr3+: (3F3,3F4) energy transfer.

Oxidation state and local coordination of chromium dopant in soda-lime-silicate and calcium-aluminate glasses

Abstract We have analyzed Cr K-edge X-ray absorption spectra of Cr-doped soda-lime-silicate and calcium-aluminate glasses of which the compositions were 15Na2O–15CaO–70SiO2 and 10MgO–55CaO–35Al2O3 in mole%, respectively. It was estimated that the amount of Cr4+ formed in the aluminate glass was ∼85±10% out of the total nominal chromium content, i.e., 0.06 mole%, and the remaining chromium ions were in 6+ oxidation state. In the silicate glass, Cr3+ was accompanied by Cr6+ the portion of which was relatively small compared to that in aluminate glass. Short-range structural units of chromium in the aluminate glass were [CrO4] and severely distorted [CrO6] with a Cr–O bond distance of ∼1.72 A , while [CrO6] polyhedra prevailed in the silicate glass with an average Cr–O length of ∼1.96 A .

Effect of heat treatment on photoluminescence behavior of Zn2SiO4:Mn phosphors

Abstract The thermal treatment in reducing atmospheres gives rise to the increase both in emission intensity and 10% decay time in Zn 2 SiO 4 :Mn phosphors. The present investigation aims to take account of such changes in association with the structural change. For this sake, X-ray absorption spectroscopy techniques such as XANES and EXAFS were conducted to the Zn 2 SiO 4 :Mn phosphors. The Zn 2 SiO 4 :Mn phosphors were fired in the air and then thermally treated in two different reducing atmospheres (hydrogen or carbon). The photoluminescent (PL) behavior was closely related to the X-ray absorption data. The XANES and EXAFS prove that the oxidation state (+2) remains identical regardless of whether or not the samples are treated, but that the Mn–O distance was reduced by the heat treatment. In order to give a plausible interpretation to the change in PL results, two possible suggestions are presented. Firstly, it is conceivable that the thermally activated diffusion process of manganese ions splits Mn–Mn pair during the heat treatment. Another possibility is that the thermal treatment annealed out some quenching site, which is related with defects and impurities. Such hypotheses can be rationalized systematically by considering the results from lifetime measurement, Debye–Waller factor calculation, and XANES pre-edge peaks.

Ga K-edge EXAFS analysis on the coordination of gallium in PbO–Ga2O3 glasses

Abstract Ga K-edge extended X-ray absorption fine structure spectra of ( X )PbO–(1− X )Ga 2 O 3 glasses, where X is 0.7, 0.75 and 0.80 in mole fraction were recorded at room temperature to understand the coordination scheme of gallium in glasses. Analyses of the spectra indicate that most of the gallium forms GaO 4 tetrahedra at the Ga–O bond distance between 0.1856 and 0.1861 nm with a negligible amount (less than 5% of total Ga-polyhedra) of GaO 6 octahedra. The longer Ga–O bond distance in glasses compared to that in β-Ga 2 O 3 crystal composed of similar GaO 4 tetrahedra indicated the presence of oxygens bonded to three independent cations.

Ho3+: (5S2,5F4)→5I5 transition in fluoride glasses

Abstract We propose that Ho 3+ and Ho 3+ /Yb 3+ -doped fluoride glasses are a good candidate material for fiber amplifiers operating at 1340–1400 nm band. Upconversion excitation in the wavelengths of 880–920 and 970–990 nm is achieved for Ho 3+ -single-doped glasses, while wavelength region of 910–1000 nm for Ho 3+ /Yb 3+ -codoped glasses. Ho 3+ concentration is preferably less than 0.5 mol% to avoid lifetime decrease of ( 5 S 2 , 5 F 4 ) manifold. Yb 3+ codoping enhances the upconversion excitation efficiency, however as Yb 3+ concentration increases, back energy transfer from holmium to ytterbium slightly diminishes the fluorescing level lifetime. Some transition-metal ions and rare-earths ions doped in the clad part of a fluoride fiber waveguide, for example, may quench unwanted amplified spontaneous emissions at ∼540 nm and ∼750 nm.

Controlling fluorescence lifetime of rare-earth element in amorphous inorganic solids via very small compositional adjustments

Fluorescence lifetime of hypersensitive 4f-4f transitions of rare-earth elements embedded in amorphous inorganic solids can be dramatically modified by compositional adjustment of the hosts tantamount to not more than 1 mol % without any elaborated thermal treatments. It is possible to modify a spontaneous emission rate of Dy3+:(F11∕26,H9∕26)→H15∕26 transition in chalcogenide Ge–As–S glasses through selective addition of low levels of Ga and CsBr. Along with the change of the spontaneous emission rate, multiphonon relaxation rate involved in the (F11∕26,H9∕26) state also significantly varies upon the minute compositional adjustment. The combination of these effects results in the measured lifetime of the fluorescing (F11∕26,H9∕26) level being greatly enhanced. Such behaviors are attributed to the hypersensitive nature associated with the H15∕26↔F11∕26 transition and preferential coordination of bromine in the nearest-neighboring shell of the Dy3+ ions, which is formed spontaneously during the vitrificatio...

Emission properties of Ho3+∕Tb3+ Co-doped in Ge30Ga2As8S60 glass

Emission properties of Ho3+ and Ho3+∕Tb3+ doped Ge30Ga2As8S60 glasses were investigated. Population densities of the I55 and I75 levels and the gain coefficients for the 1.6μm emission were calculated. Upon the addition of more than 0.05mol% Tb3+, population inversion between the I55 and I75 levels was achieved while it was not possible in the glass doped with Ho3+ only. The positive optical gain in the 1620–1750nm wavelength region was obtained with the addition of 0.3mol% Tb3+ ions.