Woon Jin Chung

Phosphor in glasses with Pb-free silicate glass powders as robust color-converting materials for white LED applications

Phosphor-in-glass (PiG) typed robust color converters were fabricated using Pb-free silicate glasses for high-power white LED applications. SiO2-B2O3-RO(R=Ba,Zn) glass powder showed good sintering behavior and high visible transparency under the sintering condition of 750 °C for 30 min without noticeable interaction with phosphors. By simply changing the thickness of the PiG plate or mixing ratio of glass to Y3Al5O12:Ce3+ phosphor, CIE chromaticity coordinates of the LED can be easily controlled. Enhanced thermal quenching property of PiG compared to phosphor with conventional silicone resin suggests its prominent feasibility for high-power/high-brightness white LEDs.

Control of chromaticity by phosphor in glasses with low temperature sintered silicate glasses for LED applications

Phosphor-in-glass (PiG) color converters for LED applications were fabricated with a mixture of phosphors, Y₃Al₅O₁₂:Ce³⁺ (yellow) and CaAlSiN₃:Eu²⁺ (red). The low sintering temperature (550°C) of SiO₂-Na₂O-RO (R=Ba, Zn) glass powder enabled the inclusion of CaAlSiN₃:Eu²⁺ (red) phosphor which cannot be embedded with conventional glass powders for PiGs. By simply varying the mixing ratio of glass to phosphors as well as the ratio of yellow to red phosphors, the facile control of the CIE chromaticity coordinates and correlated color temperature of the LED following the Planckian locus has been achieved. Phosphors were well distributed within the glass matrix without noticeable reactions, preserving the enhanced thermal quenching property of the PiG compared to those with silicone resins, for LEDs.

Enhanced room-temperature emission in Cr4+ ions containing alumino-silicate glasses

The Bi2O3 modified alumino-silicate glasses, when excited at 800 nm exhibited a broadband emission of Cr4+ in the range of 1.2 to ∼1.6 μm at room temperature. The mean lifetime of (1E−3A2)→3T1 transition in Cr4+-doped glass at RT was as long as ∼426 μs, enabling the realization of both tunable lasers and broadband fiber amplifiers. The Cr4+ emission at RT also showed the dependency on the excitation wavelength and site distribution within the chosen glass host. A photoinduced charge transfer process from the Cr3+ ion into Cr4+ ion appears to be responsible for the excitation of Cr4+.

Sensitizing effect of Tm3+ on 2.9 μm emission from Dy3+-doped Ge25Ga5S70 glass

Abstract Energy transfer mechanisms and the effect of Tm3+ addition on the intensity of the Dy3+:2.9 μm emission from Ge25Ga5S70 glass were investigated. Intensity of the 2.9 μm fluorescence from Dy3+ increased with increasing Tm3+ content when the Dy3+ concentration remained constant, which indicates the presence of an efficient energy transfer between Tm3+ and Dy3+. A decrease in the measured lifetimes of the Tm3+:3F4 level and a wide spectral overlap between the Tm3+:1.8 μm emission and the absorption of Dy 3+ : 6 H 15 2 → 6 H 11 2 also supported the proposed energy transfer scheme. Analyses of the emission intensities and decay times suggested that a direct energy transfer from the Tm3+:3F4 level to Dy 3+ : 6 H 11 2 level occurred after a fast diffusion-limited migration of the excitation energy among Tm3+.

Structural and optical characterizations of multi-layered and multi-stacked PbSe quantum dots

A multi-layered nanocomposite thin film was fabricated to increase the density of PbSe quantum dots. It consisted of low-loss polymer layers and multi-stacked PbSe quantum dot layers in series. The solutions of a UV-curable low-loss polymer and PbSe quantum dots were spin-coated layer-by-layer. An arrayed and multi-stacked PbSe quantum dot layer was self-assembled by spin-coating and solvent evaporation. The multi-stacked layers of PbSe quantum dots have a face-centred cubic structure with an average {111} plane distance of ~6.1?nm, determined by small-angle x-ray scattering. The average diameter of PbSe quantum dots is ~5?nm and the average interparticle spacing distance is ~2.5?nm, characterized by transmission electron microscopy. The density of PbSe quantum dots in the single-arrayed layer is ~1.8 ? 1012?cm?2 and the density in the three-layered thin film of multi-stacked PbSe QDs is >1.6 ? 1013?cm?2. The peak intensities of absorption and photoluminescence were increased with increasing number of PbSe quantum dot layers. The time of luminescence decay to e?1 is 138?ns at room temperature. We investigated the structures of multi-layered and multi-stacked PbSe quantum dot thin films as well as their optical properties, and then suggested their photonic applications.

Room temperature persistent spectral hole burning in x-ray irradiated Eu3+-doped borate glasses

Irradiation of x-rays has induced room-temperature persistent spectral hole burning (PSHB) in Eu3+-doped borate glasses melted under an inert atmosphere. Defects were formed by x-ray irradiation and these defects, especially electron trapping centers near rare-earth ions in glasses, were responsible for the PSHB. Electrons were released from these defects upon irradiation of a burning light. Photoreduction of Eu3+ to Eu2+ by trapping these electrons resulted in the formation of persistent spectral holes.

A novel hybrid silica wide-band amplifier covering S+C+L bands with 105-nm bandwidth

We report a novel hybrid optical amplifier covering S+C+L bands with 105-nm total bandwidth using a silica fiber. The principle of amplification is based on the stimulated radiative transition of Er-ions for C-band and on the stimulated Raman scattering for S- and L-band, respectively. In this letter, we analyze the amplification characteristics for two types of active fiber mediums through numerical simulation. One is a silica fiber configured with Er-doped cladding and Ge-doped core and the other is a medium consisting of Er-doped fiber and dispersion-compensating fiber. By optimizing parameters such as fiber length and pump power, we newly achieve wide-band amplification with 105-nm bandwidth showing a flat gain characteristic over the entire S+C+L bands.

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.

Facile one-step fabrication of 2-layered and 4-quadrant type phosphor-in-glass plates for white LEDs: an insight into angle dependent luminescence

Phosphor-in-glass (PiG) when used in combination with a blue light emitting diode (LED) chip in a remote phosphor configuration offers precise tuning and yields higher luminous efficiencies at elevated temperatures, compared to the conventional conformal LED packaging. However, drawbacks such as spectral overlapping of the constituent phosphors and resultant reabsorption remain unresolved in multi phosphor color conversion plates. These issues were solved up to a desired extent by arranging different color-emitting PiGs, via cutting and reassembly. However, the interface of the multicolored plates acted as a dissipative layer. In this work, a novel fabrication technique was proposed to overcome this drawback by eliminating the interfacial layer through a one-step process. PiGs were fabricated using glass frits at a low softening temperature of 600 °C. As a result, a higher efficacy of the studied prototypes, i.e., a horizontal 2-layered PiG and a 4-quadrant PiG, was obtained as compared with their counterparts. The angular dependency of the luminescence of the segmented 4-quadrant type PiG was studied, and the results were discussed.

The effect of Er3+-ion concentration on the Er3+ : 4I13/2 → 4I15/2 transition in tellurite glasses

The effect of Er3+ concentration on the Er3+ : 4I13/2 → 4I15/2 emission in tellurite glasses has been investigated. The full width at half-maximum increased with the increasing concentration of Er2O3 in tellurite glasses. The effect of local structure of Er3+ ions and related spectroscopic changes are taken into account to explain the line broadening. Inhomogeneous broadening due to the distribution of crystal field around the Er3+  ion has little effect on the absorption spectra. Highly efficient energy trapping between the ions was identified from the time-resolved analysis of the fluorescence decay and is found to be responsible for the extended lifetime at intermediate concentrations of Er3+ ions in tellurite glasses. The effect of temperature on spectral line shape has been determined for analysing the contribution of Boltzmann population on line broadening. The increased population of the overlying Stark sublevels at the 4I13/2 energy level via direct pumping and/or interaction between Er3+ ions were also found to be significant for enhancement in spectral line shape at higher concentrations of Er3+ ions in tellurite glasses. It was observed that at higher concentrations of Er3+ ions in glass also enhance the local symmetry of ions, which is apparent from the absorption band of the Er3+ : 4I15/2 → 2H11/2 hypersensitive transition.