Gandham Lakshminarayana

Enhanced cooperative quantum cutting in Tm 3+ -Yb 3+ codoped glass ceramics containing LaF 3 nanocrystals

Tm(3+)-Yb(3+) codoped transparent oxyfluoride glass ceramics containing LaF(3) nanocrystals were obtained by thermal treatment on the as-made glasses. The formation of LaF(3) nanocrystals and the incorporation of Tm(3+) and Yb(3+) into LaF(3) nanocrystal lattice were confirmed by X-ray diffraction and high resolution transmission electron microscopy. Infrared quantum cutting involving Yb(3+) 950-1100 nm ((2)F(5/2)--> (2)F(7/2)) emission was achieved upon the excitation of the (1)G(4) energy level of Tm(3+) at 468 nm. We measured the photoluminescence properties of these glass ceramics. We also investigated the thermal treatment duration dependent quantum efficiency, and found that the quantum efficiency is 13% increased for the 0.5Tm(3+)-4Yb(3+) doped glass ceramic with a maximum value of 144%, and 16% increased for the 0.5Tm3+-8Yb3+ doped glass ceramic with a maximum value of 162%, respectively.

White light emission from Sm3+/Tb3+ codoped oxyfluoride aluminosilicate glasses under UV light excitation

In this paper, we report on the absorption and photoluminescence properties of oxyfluoride aluminosilicate and boro-aluminosilicate glasses codoped with Sm3+ and Tb3+ ions. The differential thermal analysis profiles of these glasses have been obtained to confirm their thermal stability. From the measured absorption spectrum, Judd?Ofelt (J?O) intensity parameters (?2, ?4 and ?6) have been evaluated for the Sm3+ ion. When excited by ultraviolet light these glasses emit a combination of blue, green and orange?red wavelengths forming white light. The ratio of the intensities of orange?red to green emissions can be tuned by varying both the concentration of the Sm3+ ion and the composition of the glass matrix. The excitation and emission spectra have shown a self-quenching effect for the Sm3+ ions and an efficient energy transfer from Tb3+?:?5D4 ? Sm3+?:?4G5/2 was observed which was also confirmed by the decay lifetime measurements.

Cooperative downconversion in Yb 3+ –RE 3+ (RE=Tm or Pr) codoped lanthanum borogermanate glasses

We report on cooperative downconversion in Yb(3+)-RE(3+) (RE=Tm or Pr) codoped lanthanum borogermanate glasses (LBG), which are capable of splitting a visible photon absorbed by Tm(3+) or Pr(3+) ions into two near-infrared photons. The results indicate that Pr(3+)-Yb(3+) is a more efficient ion couple than Tm(3+)-Yb(3+) in terms of cooperative downconversion. We have obtained a highest quantum yield of 165% and 138% for Pr(3+)-Yb(3+) and Tm(3+)-Yb(3+) codoped LBG glasses under 468 nm excitation, respectively. However, ultraviolet light excitation to the charge transfer band of Yb(3+) does not result in quantum splitting as rapid relaxation from the charge transfer band to 4f(13) levels of Yb(3+) dominates.

Co-operative downconversion luminescence in Tm3+/Yb3+?:?SiO2?Al2O3?LiF?GdF3 glasses

Oxyfluoride aluminosilicate glasses in the composition of 50SiO2?20Al2O3?20LiF?10GdF3?0.5TmF3?xYbF3 (x = 0, 1.0, 2.5, 5, 7.5, 10, 15, 20, 25 and 30?mol%) have been prepared to study their thermal and optical properties. From the differential thermal analysis measurements, glass transition temperatures and onset crystallization temperatures have been evaluated and from them glass stability factors were calculated. Glass stabilities decreased gradually with fluoride content increment in all the studied glasses. The photoluminescence and decay measurements have also been carried out for all these glasses. In these glasses, an efficient near infrared quantum cutting with optimal quantum efficiency approaching 187% has been demonstrated, by exploring the co-operative downconversion mechanism from Tm3+ to Yb3+, with 467?nm (Tm3+?:?3H6 ? 1G4) excitation wavelength. These glasses are promising materials to achieve high efficiency silicon based solar cells by means of downconversion in the visible part of the solar spectrum.

Enhanced broadband near-infrared luminescence and optical amplification in Yb-Bi codoped phosphate glasses

Yb–Bi codoped phosphate glass was prepared and its properties were compared with Bi-doped phosphate glass. The broadband infrared luminescence intensity from Yb–Bi codoped glass was ∼32 times stronger than that of Bi-doped glass. The single-pass optical amplification was measured on a traditional two-wave mixing configuration. No optical amplification was observed in Bi-doped glass, while apparent broadband optical amplification between 1272 and 1336nm was observed from Yb–Bi codoped glass with 980nm laser diode excitation. The highest gain coefficient at 1272nm of Yb–Bi codoped glass reached to 2.62cm−1. Yb–Bi codoped phosphate glass is a promising material for broadband optical amplification.

Cyan-white-red luminescence from europium doped Al 2 O 3 -La 2 O 3 -SiO 2 glasses

Aluminum-lanthanum-silicate glasses with different Eu doping concentration have been synthesized by conventional melt-quenching method at 1680°C in reductive atmosphere. Under 395nm excitation, samples with low Eu doping concentration show mainly the cyan broad emission at 460nm due to 4f65d1-4f7 transition of Eu2+; and the samples with higher Eu doping concentration show mainly some narrow emissions with maximum at 616nm due to 5D0-7FJ (J=0, 1, 2, 3, 4) transitions of Eu3+. Cyan-white-red tunable luminescence under 395nm excitation has been obtained by changing the Eu doping concentration.

Photoluminescence of Pr3+-, Dy3+- and Tm3+-doped transparent nanocrystallized KNbGeO5 glasses

In this paper, we present the photoluminescence properties of Pr3+-, Dy3+- and Tm3+-doped potassium?niobium?germanate glasses and glass ceramics. From the x-ray diffraction measurement, the glass structure was established. These glasses have shown strong absorption bands in the near-infrared region. Compared with Pr3+-, Dy3+- and Tm3+-doped glasses, their respective glass ceramics have shown stronger emissions due to the presence of the K3.8Nb5Ge3O20.4 crystalline phase. For Pr3+ -doped glass, two weak emission bands centred at 616?nm(3P0 ? 3H6) and 648?nm(3P0 ? 3F2) and for glass ceramic strong emission bands centred at 533?nm(3P0 ? 3H5), 616?nm(3P0 ? 3H6) and 648?nm(3P0 ? 3F2) along with weak emissions at 688?nm(3P1 ? 3F3), 709?nm(3P0 ? 3F3) and 732?nm(3P0 ? 3F4) have been observed with 450?nm(3H4 ? 3P2) excitation wavelength. Among them, 648?nm(3P0 ? 3F2) has shown bright red emission. For the Pr3+ -doped glass and glass ceramic, upconversion emission spectra have also been measured. Bright blue upconversion luminescence was observed under 1D2 level excitation. With regard to Dy3+?:?glass, a weak emission band centred at 576?nm(4F9/2 ? 6H13/2) and for glass ceramic a blue emission band centred at 485?nm(4F9/2 ? 6H15/2) and a bright fluorescent yellow emission at 576?nm(4F9/2 ? 6H13/2) have been observed, apart from 4F9/2 ? 6H11/2(664?nm) emission transition with an excitation at 389?nm(6H15/2 ? 4I13/2, 4F7/2) wavelength. Emission bands of 1G4 ? 3F4(650?nm), 3F3 ? 3H6(707?nm) and 1G4 ? 3H5(774?nm) transitions have been observed for the Tm3+?:?glass and glass ceramic, with an excitation at 3H6 ? 1G4(473?nm). Among them, the 1G4 ? 3F4 transition (650?nm) shows bright red emission. The stimulated emission cross-sections of all the emission bands of Pr3+, Dy3+ and Tm3+?:?glasses and glass ceramics have been computed based on their measured full width at half maximum (??) and lifetimes (?m).

Energy transfer between silicon-oxygen-related defects and Yb3+ in transparent glass ceramics containing Ba2TiSi2O8 nanocrystals

We report on the conversion of near-ultraviolet radiation of 250-350 nm into near-infrared emission of 970-1100 nm in Yb3+-doped transparent glass ceramics containing Ba2TiSi2O8 nanocrystals due to the energy transfer from the silicon-oxygen-related defects to Yb3+ ions. Efficient Yb3+ emission (F-2(5/2)-> F-2(7/2)) was detected under the excitation of defects absorption at 314 nm. The occurrence of energy transfer is proven by both steady state and time-resolved emission spectra, respectively, at 15 K. The Yb2O3 concentration dependent energy transfer efficiency has also been evaluated, and the maximum value is 65% for 8 mol % Yb2O3 doped glass ceramic. These materials are promising for the enhancement of photovoltaic conversion efficiency of silicon solar cells via spectra modification.

Cooperative downconversion luminescence in Pr 3+ /Yb 3+ :SiO 2 –Al 2 O 3 –BaF 2 –GdF 3 glasses

Oxyfluoride aluminosilicate glasses with compositions of 50SiO 2 –20Al 2 O 3 –20BaF 2 –10GdF 3 –0.5PrF 3 – x YbF 3 ( x = 0, 1.0, 2.5, 5, 7.5, 10, 15, 20, 25, and 30 mol%) have been prepared to study their thermal and optical properties. From the differential thermal analysis (DTA) measurement, glass-transition temperatures and onset crystallization temperatures have been evaluated and from them, glass-stability factors against crystallization were calculated. Glass stabilities were decreased gradually with fluoride content increment in all the studied glasses. The photoluminescence and decay measurements have also been carried out for these glasses. In these glasses, an efficient near-infrared (NIR) quantum cutting with optimal quantum efficiency approaching 160% have been demonstrated, by exploring the cooperative downconversion mechanism from Pr 3+ to Yb 3+ with 481 nm ( 3 P 0 → 3 H 4 ) excitation wave length. These glasses are promising materials to achieve high-efficiency silicon-base solar cells by means of downconversion in the visible part of the solar spectrum.

Conversion of near-ultraviolet radiation into visible and infrared emissions through energy transfer in Yb2O3 doped SrO–TiO2–SiO2 glasses

We demonstrated the conversion of near-ultraviolet radiation of 250–350 nm into visible emission of 450–600 nm and near-infrared emission of 970–1100 nm in the Yb2O3 doped transparent 40SrO-20TiO2-40SiO2 glasses. The observed broad visible emission band centered at 510 nm is associated with silicon-oxygen-related defects in the glassy matrix, and the near-infrared emission originated from the Yb3+ F25/2→F27/2 transition is due to the energy transfer from silicon-oxygen-related defects to Yb3+. The energy transfer process was studied by both the steady state spectra and the time-resolved spectra of Yb3+ at 15 K. The temperature dependent energy transfer rate was calculated. The Yb2O3 concentration dependent energy transfer efficiency has also been evaluated, and the maximum value is 56% for 12 mol % Yb2O3 doped glass.