Energy transfer and upconversion luminescence in Tm/Yb co-doped lanthanum-zinc-lead-tellurite glasses

A series of Tm³⁺/Yb³⁺ co-doped lanthanum-zinc-lead-tellurite (TPZL) glasses pumped by a 980 nm laser diode (LD) were demonstrated to obtain a high efficiency of infrared-to-visible upconversion. Effects of PbO content on the thermal stability, structure and upconversion properties of Tm³⁺/Yb³⁺ co-doped TPZL glasses had been investigated. The efficient visible upconversion fluorescences corresponding to the ¹G₄→³H₆, ¹G₄→³F₄ and ³H₄→³H₆ transitions of Tm³⁺ were observed under 980 nm excitation. The upconversion intensities of blue, red and near infrared emissions in Tm³⁺/Yb³⁺ co-doped TPZL glasses were obviously enhanced with increasing PbO content. The dependence of upconversion intensities on excitation power and the possible upconversion mechanisms had been evaluated by a proper rate equation model. Population density in different levels and coefficients of the energy transfer rate C_Di (i=2, 4, 6) between Tm³⁺ and Yb³⁺ were estimated by fitting the simulated curves to the measured ones. The obtained three energy transfer coefficients C_D2, C_D4, and C_D6 were determined to be 5.7×10⁻¹⁷, 1.3×10⁻¹⁶ and 8.6×10⁻¹⁷ cm³/s, respectively.     

Composition dependent frequency upconversion luminescence in Er-doped oxychloride germanate glasses

Er³⁺-doped oxychloride germanate glasses have been synthesized by conventional melting and quenching method. Structural and thermal stability properties were obtained based on the Raman spectra and differential thermal analysis, indicating that PbCl₂ plays an important role in the formation of glass network and has an important influence on the maximum phonon energy and thermal stability of host glasses. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were observed at room temperature. With increasing PbCl₂ content, the intensity of green (525 and 546 nm) emissions increases significantly, while the red (657 nm) emission increases slowly. The results indicate that PbCl₂ has more influence on the green emissions than the red emission in oxychloride germanate glasses. The possible upconversion luminescence mechanisms has also been estimated and discussed.     

The microscopic interaction parameters for Er/Ho energy transfer in tellurite glasses

We investigate the energy transfer between Er³⁺/Ho³⁺ in tellurite glasses. The main channels of energy transfer between Er³⁺/Ho³⁺ are analyzed in detail. The microscopic interaction parameters of resonant and non-resonant (phonon-assisted) energy transfer parameters via Er³⁺→Ho³⁺ are calculated. The result shows that the resonant energy transfers Er³⁺(²H_11/2(⁴S_3/2))→Ho³⁺(⁵F₄(⁵S₂)) and Er³⁺(⁴F_9/2)→Ho³⁺(⁵F₅) are very efficient and non-resonant energy transfers Er³⁺(⁴I_13/2)→Ho³⁺(⁵I₇) and Er³⁺(⁴I_11/2)→Ho³⁺(⁵I₆), which are a phonon-assisted energy transfer process because of energy mismatch are also existed and cannot be neglected.     

Intense upconversion luminescence in ytterbium-sensitized thulium-doped novel oxychloride bismuth-germanium glass

Structural and up-conversion fluorescence properties in ytterbium-sensitized thulium-doped novel oxychloride bismuth-germanium glass have been studied. The structure of novel bismuth-germanium glass was investigated by peak-deconvolution of Raman spectrum, and the structural information was obtained from the peak wave numbers. The Raman spectrum investigation indicates that PbCl₂ plays an important role in the formation of glass network, and has an important influence on the up-conversion luminescence. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions ¹G₄→³H₆ and ¹G₄→³H₄, respectively, were observed at room temperature. The possible up-conversion mechanisms are discussed and estimated. This novel oxychloride bismuth-germanium glass with low maximum phonon energy (∼730 cm⁻¹) can be used as potential host material for up-conversion lasers.     

Broadband amplification and upconversion luminescence properties of Er/Yb co-doped fluorophosphate glass

Broadband and upconversion properties were studied in Er³⁺/Yb³⁺ co-doped fluorophosphate glasses. Large Ω₆ and S_ed/(S_ed+S_md) values and the flat gain profile over 1530-1585 nm indicate the good broadband properties of the glass system. And a premise of using Ω₆ as a parameter to estimate the broadband properties of the glasses is proposed for the first time to our knowledge. Results showed that fluorescence intensity, upconversion luminescence intensity, the intensity ratio of red/green light (656 nm/545 nm) are closely related to the Yb³⁺:Er³⁺ ratio and Er³⁺ concentration, and the corresponding calculated lifetime of ⁴F_9/2 and ⁴S_3/2 states for red and green upconversion samples proves this conclusion. The upconversion mechanism is also discussed.     

Comparative investigation of up-conversion luminescence in Tm-doped oxide-chloride germanate and tellurite glasses

Tm³⁺-doped oxide-chloride germanate and tellurite glasses have been synthesized by conventional melting method. Intense up-conversion luminescence emissions were simultaneously observed at room temperature in these glasses. The possible up-conversion mechanisms are discussed and estimated. However, in these Tm³⁺-doped glasses, tellurite glass showed weaker up-conversion emissions than germanate glass, which is inconsistent with the prediction from the difference of maximum phonon energy between tellurite and germanate glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Our results confirm that, besides the maximum phonon energy, the phonon density of host glasses is also an important factor in determining the up-conversion efficiency.     

Spectral performance and intensive green upconversion luminescence in Er/Yb-codoped NaY(WO4)2 crystal

Using Czochralski (CZ) pulling method, an Er³⁺/Yb³⁺-codoped NaY(WO₄)₂ crystal was prepared. Absorption spectra, emission spectra and excitation spectra of this crystal were measured at room temperature. Some optical parameters, such as intensity parameters, spontaneous emission probabilities and lifetimes, were calculated from absorption spectra with Judd-Ofelt (J-O) theory. Upconversion luminescence excited by a 970 nm diode laser was studied. In this crystal, green upconversion luminescence is particularly intensive. Energy transfer mechanisms that play an important role in upconversion processes were analyzed. Two cross-relaxation processes: ⁴G_11/2 + ⁴I_9/2 → ²H_11/2 (or ⁴S_3/2) + ²H_11/2 (or ⁴S_3/2), and ⁴G_11/2 + ⁴I_15/2 → ²H_11/2 (or ⁴S_3/2) + ²I_13/2, which contribute to the intensive green luminescence under 378 nm excitation, were put forward. Background energy transfer ⁴G_11/2(Er³⁺) + ²F_7/2(Yb³⁺) → ⁴F_9/2(Er³⁺) + ²F_5/2(Yb³⁺) was also demonstrated.     

Comparative investigation of up-conversion luminescence in Tm/Yb-codoped germanate-niobic and germanium-bismuth glasses: effect of phonon density on up-conversion emission

Tm³⁺/Yb³⁺-codoped germanate-niobic (GN) and germanium-bismuth (GB) glasses have been synthesized by conventional melting and quenching method. Intense blue and weak red emissions centered at 477 and 650 nm, corresponding to the transitions ¹G₄→³H₆ and ¹G₄→³H₄, respectively, were observed at room temperature. The possible up-conversion mechanisms are discussed and estimated. GN glass showed a weaker up-conversion emission than GB glass, which is inconsistent with the prediction from the difference of maximum phonon energy between GN and GB glasses. In this paper, Raman spectroscopy was employed to investigate the origin of the difference in up-conversion luminescence in the two glasses. Compared with phonon side-band spectroscopy, Raman spectroscopy extracts more information including both phonon energy and phonon density. For the first time, our results reveal that, besides the maximum phonon energy, the phonon density of host glasses is also an important factor in determining the up-conversion efficiency.     

NIR luminescence from Er/Yb, Tm/Yb, Er/Tm and Nd ions-doped zincborotellurite glasses for optical amplification

In this paper, we report the near-infrared luminescence from the Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, Er³⁺/Tm³⁺ and Nd³⁺ ions-doped TeO₂-ZnO-B₂O₃-Li₂O-Na₂O glasses for optical amplification. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) profiles of the host glass matrix have been carried out. From the DSC thermogram, glass transition (T_g), crystallization (T_c) and melting (T_m) temperatures have been evaluated. The near-infrared spectra of Er³⁺/Yb³⁺, Tm³⁺/Yb³⁺, Er³⁺/Tm³⁺ and Nd³⁺ ions-doped glasses have shown full-width at half-maxima (FWHM) around 58, 127, 87 and 35 nm, respectively. These glasses with better thermal stability and broad near-infrared emissions should have potential applications in broadly tunable laser sources and broadband optical amplification at low-loss telecommunication windows.     

Spectroscopy of infrared transitions of Pr ions in Ga-Ge-Sb-Se glasses

Electronic transitions of Pr³⁺ ions in Ga-Ge-Sb-Se glasses corresponding to emissions in the infrared region were studied by means of absorption and emission spectroscopies and fluorescence lifetimes measurements. Transition probabilities, radiative lifetimes, branching ratios, and quantum efficiencies of most of the emission transitions including the infrared ones occurring around 1.3, 1.7, and 2.4 μm were estimated based on a standard Judd-Ofelt analysis.     

Upconversion luminescence of Tm/Yb-codoped oxyhalide tellurite glasses

Thermal stability, Raman spectra and blue upconversion luminescence properties of Tm³⁺/Yb³⁺-codoped halide modified tellurite glasses have been studied. The results showed that the mixed halide modified tellurite glass (TFCB) has the best thermal stability, the lowest phonon energies and the strongest upconversion emissions. The effect of halide on upconversion intensity is observed and discussed and possible upconversion mechanisms are evaluated. The intense blue upconversion luminescence of Tm³⁺ in TFCB glass may be a potentially useful material for developing upconversion optical devices.     

Strong cooperative upconversion luminescence of ytterbium doped oxyfluoride nanophase vitroceramics

The strong 479.1 nm blue cooperative upconversion luminescence of ytterbium Yb³⁺ ion doped oxyfluoride nanophase vitroceramics (Yb:FOV) is studied in this article. It is found that the 479.1 nm blue cooperative upconversion luminescence strength of Yb(5):FOV is 230 times greater than that of fluoride glass Yb(3):ZBLAN. The large enhancement on cooperative upconversion blue luminescence of this article results from the comprehensive improvement on the aspects of better coupled chance of the Yb³⁺-Yb³⁺ cluster, less cross-relaxation, better concentration contribution of Yb³⁺ activator, non-saturation, and better upconversion luminescence efficiency.     

Structural and upconversion fluorescence properties of Er/Yb-codoped oxychloride lead-germanium-bismuth glass

Structural and infrared-to-visible upconversion fluorescence properties of Er³⁺/Yb³⁺-codoped oxychloride lead-germanium-bismuth glass have been studied. The Raman spectrum investigation indicates that PbCl₂ plays an important role in the formation of glass network, and has an important influence on the upconversion luminescence owing to lower phonon energy. Intense green and red emissions centered at 525, 546, and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546, and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs under 975 nm excitation.     

Discrimination between thermal quenching of the fluorescence and Auger upconversion processes using thermal lens technique

In this work we used the Thermal Lens (TL) technique to discriminate two important processes responsible to reduce the upper-state population and fluorescence quantum efficiency (η) in Cr³⁺ doped colquiriite crystals: the thermal quenching of the fluorescence (TQF) and the Auger upconversion (ETU). We observed a nonlinear increase of the TL signal with laser power due to the decrease of η by ETU and/or TQF. The analysis of these curves allowed the determination of the thermal load, the increase of the crystal temperature as a function of the pump excitation and the discrimination between ETU and TQF processes.     

Eu luminescence in tellurite glasses with gold nanostructures

Luminescence properties of Eu³⁺ doped TeO₂-PbO-GeO₂ glasses containing gold nanoparticles (NPs) were investigated. The emission spectra of the samples exhibited enhancement of Eu³⁺ luminescence due to the presence of gold NPs. The emission at 614 nm, due to the Eu³⁺ hypersensitive transition ⁵D₀-⁷F₂, is much influenced by the gold NPs and increases by ≈100% for samples heat-treated at 350 °C during 41 h.     

Upconversion luminescence and mechanisms in Yb-sensitized Tm-doped oxyhalide tellurite glasses

Effect of Yb₂O₃ content on upconversion luminescence and mechanisms in Yb³⁺-sensitized Tm³⁺-doped oxyhalide tellurite glasses were investigated under 980 nm excitation. Intense blue and relatively weak red upconversion emission centered at 476 and 649 nm corresponding to the transitions ¹G₄→³H₆ and ¹G₄→³H₄ of Tm³⁺, respectively, are simultaneously observed at room temperature. The results show that upconversion blue and red emission intensities of Tm³⁺ first increase, reach its maximum at Yb₂O₃%=3 mol%, and then decrease with increasing Yb₂O₃ content. The effect of Yb₂O₃ content on upconversion intensity is discussed, and possible effect mechanisms are evaluated. The investigated results were conducing to increase upconversion luminescence efficiency of Tm³⁺.     

The luminescence of Sm in alkaline earth borophosphates

The temperature-dependent luminescence of Sm²⁺ ions in MBPO₅ was studied. At low temperature, Sm²⁺ in this series shows 4f⁶→4f⁶ luminescence with only a single emission line observed for the transition, revealing that only one crystallographic cationic site is available for Sm²⁺ in all the hosts. With increasing temperature, the emission intensity of the transition increases whereas that of the transitions decreases. The transitions of Sm²⁺ were observed in BaBPO₅ and its intensity increases with increasing temperature. At , a broad band of the 4f⁵5d→4f⁶ luminescent transition of Sm²⁺ in SrBPO₅ and BaBPO₅ with maximum at appears due to the thermal population. The lifetime of the transition was recorded at different temperatures, showing a single exponential decay for Sm²⁺ in SrBPO₅ and BaBPO₅ but a non-single-exponential decay in CaBPO₅.     

Spectroscopic properties of Yb ions in La2(WO4)3 crystal

Yb³⁺-doped La₂(WO₄)₃ single crystals were grown by the Czochralski technique. Absorption and fluorescence spectra of the crystal were recorded at the room temperature. The stimulated emission cross-sections of Yb³⁺ ions were calculated using the reciprocity method and Fuchtbauer-Ladenburg formula, respectively. The fluorescence decay curves of ²F_5/2 manifold of Yb³⁺ ions were recorded at room temperature for both crystal and powder samples. The effect of radiation trapping on the spectroscopic properties is discussed. Comparison with other Yb³⁺-doped laser crystals is made. The results show that Yb³⁺:La₂(WO₄)₃ crystal is a promising laser material.     

Spectroscopic characteristic and energy levels of Cr in Cr:KAl(MoO4)2 crystal

This paper reports polarized spectral properties and energy levels of Cr³⁺ in KAl(MoO₄)₂ crystal. The absorption and emission cross sections are estimated as 3.72×10^-20 cm² at 669 nm and 2.74×10^-20 cm^-2 at 823 nm for σ-polarization, respectively. The energy levels of Cr³⁺ ion in KAl(MoO₄)₂ crystal were calculated based on the Tanabe-Sugano theory. It is suggested that Cr³⁺ ions occupy at an intermediate crystal field site in Cr³⁺:KAl(MoO₄)₂.     

Upconversion fluorescence property of Er/Yb-codoped novel bismuth-germanium glass

Infrared-to-visible upconversion fluorescence property of Er³⁺/Yb³⁺-codoped novel bismuth-germanium glass under 975 nm LD excitation has been studied. Intense green and red emissions centered at 525, 546 and 657 nm, corresponding to the transitions ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2, and ⁴F_9/2→⁴I_15/2, respectively, were observed at room temperature. The quadratic dependence of the 525, 546 and 657 nm emissions on excitation power indicates that a two-photon absorption process occurs. The structure of the bismuth-germanium glass has been investigated by peak-deconvolution of FT-Raman spectrum, and the structural information was obtained from the peak wavenumbers. This novel bismuth-germanium glass with low maximum phonon energy (∼750 cm⁻¹) can be used as potential host material for upconversion lasers.