Optical properties and upconversion in Yb3+—Tm3+ co-doped oxyfluoride glasses and glass ceramics

A theoretical study has explored the changes brought upon the electron density of the open (C_2v) and cyclic (D_3h) forms of ozone by protonation. Although protonation results in a strong deformation of the electron density of ozone with notably different charge distributions for the stable forms of O₃H⁺, the electric dipole polarizability is almost stable for the open protonated species. The polarizability of the cyclic form is less affected by protonation than that of the open one. The anisotropy of the dipole polarizability discriminates very clearly between open and closed protonated ozone. We expect the present findings to advance our understanding of the chemical reactivity of O₃H⁺.     

Spectroscopic investigation and Judd–Ofelt analysis of silver nanoparticles embedded Er3+-doped tellurite glass

A series of silver nanoparticles (NPs) embedded zinc-tellurite glass is prepared by melt-quenching technique. The transmission electron microscopic images reveal spherical as well as anisotropic silver NPs having average diameter in the range of 14–48 nm. The Er³⁺-free glass sample containing AgCl exhibits surface plasmon resonance (SPR) band of Ag NPs centered at ∼ 501 nm. From Judd–Ofelt analysis, it is found that by increasing the concentration of NPs, the value of Ω₂ is enhanced suggesting increased covalency and decreased symmetry around the Er³⁺ ions. Integrated emission cross-section (IEC) is enhanced as the concentration of silver NPs is increased up to 0.5 mol% AgCl. Fourier infrared spectra show that the intensity of the vibrational band of the water molecule and fundamental stretching band of OH group are suppressed. Furthermore, under an excitation wavelength of 786 nm, three prominent upconversion emissions are observed at 520 nm, 550 nm and 650 nm which are attributed to ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. The upconversion emissions are enhanced significantly by introduction of silver NPs. The enhancement is mainly attributed to the local field effect of silver NPs. Studied nanocomposites are potential candidates for the development of solid state lasers.     

Luminescence properties of Pr3+-doped transparent oxyfluoride glass–ceramics containing BaYF5 nanocrystals

Transparent oxyfluoride glass–ceramics containing BaYF₅ nanocrystals were successfully synthesized by appropriate heat-treatment on the SiO₂–Al₂O₃–Na₂O–BaF₂–Y₂O₃–Pr₆O₁₁ precursor glass. The structure and luminescence properties of the precursor glass and glass–ceramics were investigated by DSC, XRD, TEM, optical transmission, photoluminescence, decay time and radioluminescence spectra. The XRD results indicate that the BaYF₅ nanocrystals can percitated in the precursor glass and the sharper emission peaks of Pr³⁺ in glass ceramic suggests that Pr³⁺ ions are incorporated into the BaYF₅ nanocrystals. The higher the heat-treatment temperature is, the more the Pr³⁺ ions are centered into BaYF₅ nanocrystals, which results in the optimal concentration of Pr³⁺ in glass ceramic changes on heat-treatment temperature. It is notable that the emission intensity of both photoluminescence and radioluminescence for 0.1 mol% Pr³⁺ in the glass ceramic (GC665) are stronger than those in the precursor glass. The mechanism of enhanced luminescence is also discussed.     

Energy transfer and frequency upconversion in Yb3+–Er3+-doped PbO-GeO2 glass containing silver nanoparticles

We report the infrared-to-visible frequency upconversion in Er³⁺–Yb³⁺-codoped PbO-GeO₂ glass containing silver nanoparticles (NPs). The optical excitation is made with a laser at 980 nm in resonance with the ²F_5/2→²F_7/2 transition of Yb³⁺ ions. Intense emission bands centered at 525, 550, and 662 nm were observed corresponding to Er³⁺ transitions. The simultaneous influence of the Yb³⁺→Er³⁺ energy transfer and the contribution of the intensified local field effect due to the silver NPs give origin to the enhancement of the whole frequency upconversion spectra.     

Optical gain by upconversion in Tm–Yb oxyfluoride glass ceramic

Evidence of positive optical gain is observed in Tm³⁺–Yb³⁺-codoped oxyfluoride glass ceramic in an upconversion pump and probe experiment. The ¹G₄ level of the Tm³⁺ ions is populated by an upconversion mechanism under excitation of the Yb³⁺ ions at 975 nm with a high-power pulsed laser and give rise to an intense emission from the ¹G₄ to the ³F₄ levels. The ¹G₄→³F₄ electronic transition is stimulated with a low signal at 650 nm as a probe.     

Broadband 2.84 μm luminescence properties and Judd–Ofelt analysis in Dy3+ doped ZrF4–BaF2–LaF3–AlF3–YF3 glass

2.84 μm luminescence with a bandwidth of 213 nm is obtained in Dy³⁺ doped (ZrF₄–BaF₂–LaF₃–AlF₃–YF₃) ZBLAY glass. Three intensity parameters and radiative properties have been determined from the absorption spectrum based on the Judd–Ofelt theory. The 2.84 μm emission characteristics and energy transfer mechanism upon excitation of a conventional 808 nm laser diode are investigated. The prepared Dy³⁺ doped ZBLAY glass possessing high predicted spontaneous transition probability (45.92 s^?1) along with large calculated emission cross section (1.17×10^?20 cm²) has potential applications in 2.8 μm laser.     

Er^3＋/Tm^3＋/Yb^3＋ tridoped oxyfluoride glass ceramics with efficient upconversion white-light emission

A novel Tm^3＋/Yb^3＋ triply-doped glass ceramics containing BaF2 nano-crystals are successfully prepared. Fluoride nanocrystals BaF2 are successfully precipitated in glass matrix, which is affirmed by the X-ray diffraction results. The intense blue （476 nm）, green （543 nm）, and red （656 nm） emissions of the glass ceramics are simultaneously observed at room temperature under 980-am excitation, and the emission luminescence intensity increases significantly compared with the precursor glass, which is attributed to the low phonon energy of fluoride nanocrystals when rare-earth ions are incorporated into the precipitated BaF2 nanocrystals. Under 980-nm excitation at 400 mW, the international commission on illumination （CIE） chromaticity coordinate （X = 0.278, Y = 0.358） of the tridoped oxyfluoride glass ceramics＇ upconversion emissions is close to the standard white-light illumination （X = 0.333, Y= 0.333）. The results indicate that Tm^3＋/Yb^3＋ triply doped glass ceramics can act as suitable materials for potential three-dimensional displays applications.     

Spectroscopic properties and mechanism of Tm^3＋/Er^3＋/Yb^3＋ co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals

Transparent Tm^3＋/Er^3＋/yb^3＋ co-doped oxyfluorogermanate glass ceramics containing BaF2 nanocrystals are prepared. Under excitation of a 980-nm laser diode （LD）, compared with the glass before heat treatment, the Tm^3＋/Er^3＋/yb^3＋ co-doped oxyfluorogermanate glass ceramics can emit intense blue, green and red up-conversion luminescence and Stark- split peaks; X-ray diffraction （XRD） and transmission electron microscope （TEM） results show that BaF2 nanocrystals with an average diameter of 20 nm are precipitated from the glass matrix. Stark splitting of the up-conversion luminescence peaks in the glass ceramics indicates that Tm^3＋, Er^3＋ and （or） Yb^3＋ ions are incorporated into the BaF2 nanocrystals. The up-conversion luminescence intensities of Tm^3＋, Er^3＋ and the splitting degree of luminescence peaks in the glass ceramics increase significantly with the increase of heat treat temperature and heat treat time extension. In addition, the possible energy transfer process between rare earth ions and the up-conversion luminescence mechanism are also proposed.     

Determination of energy constants for Yb3＋/Er3＋ transfer and upconversion codoped phosphate glass

The energy transfer and cooperation upconversion processes are investigated in Yb3＋/Er3＋ codoped phos- phate glass. Based on the measured curves of output power versus incident power, the laser and spectroscopic parameters of the glass are fitted and analyzed. We focus on the resonant energy transfer constant k from Yb3＋ to Er3＋ as well as the cooperation upconversion coefficient Cup from 4113/2 of Era＋. The fitted k and Cup can give almost the same results for different thicknesses of glass disk with the same dop- ing concentrations. The determination of these parameters is helpful for the development of Yb3＋/Er3＋ codoped laser glass.     

Do Eu dopants prefer the precipitated LaF3 nanocrystals in glass ceramics?

Previous experimental results, using techniques associated with transmission electron microscopy (TEM), suggested that rare‐earth dopants prefer the precipitated fluoride nanocrystals to the glass matrix. By contrast, this work shows that contradicting results can be obtained under different experimental conditions. In the silicate glass containing LaF₃ nanocrystals, the Eu dopants distribute indistinguishably in the precipitated LaF₃ and the silicate glass matrix. However, electron beam exposure during the measurement can relocate Eu, leading to the false conclusion that apparently Eu prefers the precipitated LaF₃ nanoparticles to the glass matrix. Fortunately, the artefact can be avoided by shortening the exposure time. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature measurements inside an Er3+–Yb3+ co-doped fluoride crystal heated by a NIR laser diode and probed by red-to-green upconversion

The temperature of a transparent Cd_0.7Sr_0.3F₂: Er³⁺(4%)–Yb³⁺(6%) crystalline plate 0.3 mm thick heated by a near-infrared (974 nm) laser diode and probed by a red (652 nm) laser was accurately evaluated as a function of the infrared power absorbed by the Yb³⁺ ions.The green emission generated by the Er³⁺ ions directly excited by the red laser consists of three major lines (coming from three individual Stark levels in thermal equilibrium) whose intensities were measured according to the absorbed infrared power and the distance between the heated and probed volumes, to evaluate the heating induced by the excitation of Yb³⁺ and Er³⁺ ions at 974 nm by applying the Boltzmann's equation linking the populations of emitting levels to the temperature. In the case where the Yb³⁺ ions excited by the laser diode are situated at a distance of about 0.5 mm from the edge of the crystal and for an absorbed infrared power of 100 mw, the crystal's edge temperature is reaching 80 °C after 20 s of continuous excitation at 974 nm.     

Efficient UV–visible up-conversion emission in Er3+/Yb3+ co-doped La2O3 nano-crystalline phosphor

A nano-crystalline La₂O₃: Er³⁺/Yb³⁺ phosphor sample has been synthesized through the solution combustion route using urea as a reducing agent. Thermal, structural and optical characterizations have been carried out to explore several of its properties. By thermal analysis one concludes to the presence of moisture and hydroxide phases [La(OH)₃ and LaOOH] of lanthanum in the as-synthesized sample, which further changes to La₂O₃ phase above 600°C temperature. Up-conversion (UC) study shows the intense emission bands in the UV, blue, green and red regions. This paper also reports the first observation of UC emission bands extending up to the UV (240 nm) region on excitation with 976 nm wavelength. Heat treatment of the samples shows a change in the crystallite phase along with crystallite growth and relative UC luminescence intensities. The input pump power dependence shows the involvement of up to four photons.     

Blue upconversion luminescence in Tm~(3+)/Yb~(3+) co-doped new oxyfluoride tellurite glass

Solid-state blue and green light sources are desirablefor high-density optical storage, color displays, optoelec-tronics, and medical diagnostics[1-4]. Tm3+ is the moststudied in rare-earth ions for blue laser operation basedupon upconversion[5,6]. One approach to improving theluminescence efficiency of Tm3+ is to co-dope it withother rare-earth ions[7]. The choice of host materials is avery important factor to obtain high efficient upconver-sion emission. In this case, the main goal of the c…     

Up conversion luminescence of Yb3+–Er3+ codoped CeO2 nanocrystals with imaging applications

The effects of Yb³⁺ doping on up conversion in Yb³⁺–Er³⁺ co-doped cerium oxide nanocrystals are reported. Green emission around 545 and 560 nm attributed to the ²H_11/2, ⁴S_3/2→⁴I_15/2 transitions and red emission around 660 and 680 nm due to ⁴F_9/2→ ⁴I_15/2 transitions under 975 nm excitation were studied at room temperature. Both green and red emission intensities increase as the Yb³⁺ concentration increases from 0%. Emission strength starts to decrease after the Yb³⁺ concentration exceeds a critical amount. The green emission strength peaks around 1% Yb³⁺ concentration while the red emission strength peaks around 4%. An explanation of competition between different decay mechanisms is presented to account for the luminescence dependence on Yb³⁺ concentration. Also, the application of up converting nanoparticles in biomedical imaging is demonstrated.     

Enhancement of up-conversion Yb3＋/Tm3＋/Er3＋ tri-doped tellurite glass luminescence properties in transparent oxyfluoride ceramics

Optically transparent Er3＋/Tm3＋/Yb3＋ tri-doped oxyfluoride tellurite based nano-crystallized glass ceramics with the batching composition of 73TeO2-15ZnO-7ZnF2-3YF3-1.5YbF3-0.3ErF3-0.2TmF3 （mol%） is prepared by a conventional melting quenching and the subsequent heat treatment processes. The sizes of grown nano-crystals in glass matrix appear to be smaller than 100 nm from the scanning electron mi- croscope measurement. Visible up-conversion luminescence of the as melted glass and glass ceramics is investigated. The three-color up-conversion luminescence intensities by 980-nm pumping are increased significantly due to the heat treatment, and the blue intensity increases with a higher magnitude than other wavelengths after heat treatment.     

Investigation of 2.0 μm emission in Tm and Ho co-doped oxyfluoride tellurite glass

This paper reports 2.0 μm emission properties of Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass exited by 808 nm laser diode (LD). Mid-infrared transmittance property of glass was investigated by Fourier transform infrared (FTIR) spectrometer. The real chemical composition of investigated glass was identified by X-ray photoelectric spectroscopy (XPS). Thermal stability of the glass was determined by differential thermal analysis (DTA) measurement. The Judd-Ofelt parameters, spontaneous radiative transition probabilities, branching ratios and radiative lifetime of Ho³⁺ were calculated based on the absorption spectra by using Judd-Ofelt theory. Results indicate that the maximum 2.0 μm emission intensity attributed to the ⁵I₇→⁵I₈ transition of Ho³⁺ was achieved at 1.5 mol% Tm₂O₃ and 1 mol% Ho₂O₃ concentrations in oxyfluoride tellurite glass. OH⁻ absorption at 3000 cm⁻¹ was greatly depressed by introduction of 10 mol% F⁻. The maximum absorption and stimulated emission cross-section of Ho³⁺ near 2.0 μm are 7.0×10⁻²¹ cm² at 1950 nm and 8.8×10⁻²¹ cm² at 2048 nm, respectively. The calculated radiative lifetime of 4.4 ms for ⁵I₇→⁵I₈ transition and large stimulated emission cross-section of the Tm³⁺/Ho³⁺ co-doped oxyfluoride tellurite glass indicate that the glass has a potential application in efficient 2.0 μm laser.