Spectroscopic properties of the 1.8 μm emission of Tm3+/Yb3+ codoped TeO2–ZnO–Bi2O3 glasses with efficient energy transfer

Tm³⁺-doped and Tm³⁺/Yb³⁺-codoped TeO₂–ZnO–Bi₂O₃ (TZB) glasses are prepared by melt-quenching method. The Judd-Ofelt intensity parameters (Ω_t t = 2, 4, 6), radiative transition rate, and radiative lifetime of Tm³⁺ are calculated based on the absorption spectra. The 1.8 μm emission of the samples is investigated under 980 nm laser excitation. The absorption, emission cross-sections, and gain coefficient of Tm³⁺:³F₄ → ³H₆ are calculated. The energy transfer processes of Yb³⁺–Yb³⁺ and Yb³⁺–Tm³⁺ are analyzed, the results show that the Yb³⁺ ions can transfer their energy to Tm³⁺ ions with large energy transfer coefficient, and a maximum efficiency of 79%.     

Growth and downconversion luminescence of Ho3+/Yb3+ codoped α‐NaYF4 single crystals by the Bridgman method using a KF flux

Downconversion (DC) luminescence with emission at about 1000 nm under excitation of 448‐nm light in Ho³⁺/Yb³⁺ codoped α‐NaYF₄ single crystal is realized. The crystal was grown by the Bridgman method using KF as an assisting flux in a NaF‐YF₃ system. The energy‐transfer process and quantum cutting (QC) mechanisms are presented through the analysis of the spectra. The energy‐transfer processes of first‐ and second‐order cooperative DC are responsible for the increase of the emission intensity at 1000 nm, and it is the first‐order cooperative DC that is dominant for the DC process. When the Ho³⁺ concentration is fixed at about 0.8 mol%, the optimal concentration for ∼1000 nm emission is 3.02 mol% Yb³⁺ in the current research. The energy‐transfer efficiency and the total quantum efficiency are analyzed through the luminescence decay curves. The maximum quantum cutting efficiency approaches to 184.4% in α‐NaYF₄ single crystals of 0.799 mol% Ho³⁺ and 15.15 mol% Yb³⁺. However, the emission intensity at 1000 nm decreases while the energy‐transfer efficiency from Ho³⁺ to Yb³⁺ increases, which may result from the fluorescence quenching between Ho³⁺ and Yb³⁺ ions, Yb³⁺ and Yb³⁺ ions.     

Short-wavelength upconversion luminescence of Yb/Tm co-doped glass ceramic containing SrF2 nanocrystals

The preparation process and upconversion luminescence properties of the Yb³⁺ and Tm³⁺ co-doped glass ceramic containing SrF₂ nanocrystals were investigated. In the glass ceramic, the SrF₂ nanocrystals were embedded uniformly in the glass matrix. The Yb³⁺ and Tm³⁺ ions could be enriched into the precipitated SrF₂ nanocrystalline phase, which provide much lower phonon energy than the glass matrix. The glass ceramic exhibited much stronger upconversion luminescence from ultraviolet to visible than the precursor glass. The upconversion luminescence mechanisms were mainly attributed to Yb³⁺-Yb³⁺ cooperative upconversion, Yb³⁺-Tm³⁺ energy transfer and Tm³⁺-Tm³⁺ cross relaxation.     

Investigation of infrared emission and lifetime in Tm-doped 75TeO2:20ZnO:5Na2O (mol%) glasses: Effect of Ho and Yb co-doping

In this paper we describe fabrication and characterization of rare-earth-doped active tellurite glasses to be used as active laser media for fiber lasers emitting in the 2 μm region. The base composition is (mol%): 75TeO₂-20ZnO-5Na₂O with different concentrations of Tm³⁺, Yb³⁺ and Ho³⁺ as dopants or co-dopants. Optical properties of doped glasses were studied and pumping at 800 nm and at 980 nm were tested in order to compare the efficiency of two pumping mechanisms. Optical characterization carried out on glasses containing only Tm³⁺ ions indicated the optimum concentration of Tm₂O₃ in terms of emission efficiency as 1 wt%. The addition of 5 wt% of Yb₂O₃ to Tm³⁺-doped glasses led to the best results in terms of intensity of fluorescence emission and of lifetime values. Yb and Ho co-doped Tm-tellurite glass was measured in emission.     

Growth and spectral properties of Er3+/Tm3+ co‐doped LiYF4 single crystal

High quality Er³⁺/Tm³⁺:LiYF₄ single crystals were grown by a Bridgman method. The absorption spectra and luminescent properties of the crystals were studied to characterize the effect of Tm³⁺ on the spectroscopic properties upon excitation of an 800 nm laser diode. The broaden 1.5 μm and the enhanced 2.7 μm emission were observed in the Er³⁺/Tm³⁺ co‐doped LiYF₄ single crystals. Meanwhile, the up‐conversion and 1.5 μm emission intensities from Er³⁺ decrease with increasing the ratio of Tm³⁺ to Er³⁺. The energy transfer processes between Tm³⁺ and Er³⁺ in the Er³⁺/Tm³⁺ co‐doped samples were analyzed. The energy transfer efficiency η_ETE from Er³⁺ to Tm³⁺ is calculated. The highest η_ETE of 65.30% for the sample with 0.296 mol% of Er³⁺, 0.496 mol% of Tm³⁺ concentration was obtained. The present work indicates that Er³⁺/Tm³⁺ co‐doped LiYF₄ single crystal can be a promising material for the potential application in infrared devices.     

Er3+/Yb3+-codoped silica–germania sputtered films: structural and spectroscopic characterization

We report on the fabrication and spectroscopic characterization of highly photo-refractive Er³⁺/Yb³⁺ coactivated silica–germania slab waveguides, single mode at 1550 nm, deposited by radio-frequency-magnetron-sputtering technique. Details of the sputtering procedure are reported. The structural properties of the films were investigated by Raman spectroscopy. Propagation losses of guided modes were measured at 633 nm and 1320 nm. The emission of the ⁴I_13/2 → ⁴I_15/2 transition of the Er³⁺ ion was analyzed upon excitation of the TE₀ mode at 514 and 981 nm. Back energy transfer from Er³⁺ to Yb³⁺ was observed by measurement of Yb³⁺ emission upon Er³⁺ excitation at 514.5 nm. Photoluminescence excitation spectroscopy was used to obtain information about the Yb³⁺ to Er³⁺ energy transfer process.     

Preparation and upconversion luminescence characterization of 12CaO·7Al2O3:Ho3+/Yb3+ polycrystals

The effect of Yb³⁺ concentration on the fluorescence of 12CaO·7 Al₂O₃:Ho³⁺/Yb³⁺ polycrystals is investigated. The Raman spectra of pure C12A7 under 633‐nm excitation show that the highest photon energy is 787.267 cm⁻¹, which is not much bigger than general fluorides, so it can realize high efficiency upconversion. The upconversion emission spectra suggest that the green upconversion emission centered at 548 nm and the red upconversion emission at 662 nm correspond to the ⁵F₄/⁵S₂→⁵I₈ and ⁵F₅→⁵I₈ transition of Ho³⁺ ions, respectively. The intensity of the upconversion luminescence and the ratio of red to green are changed with Yb³⁺ ion concentration. The pump dependence and luminescence decay dynamics spectra show the green and red upconversion emissions are populated by a two‐photon process, and the upconversion mechanisms are analyzed. The relative luminous efficiencies of green and red emissions are 2.035% and 0.7%, respectively. The normalized efficiency obtained for green emission of Ho³⁺ at RT when the sample is excited by 980‐nm light with an absorbed intensity of 7.5 W/cm² is 0.27 cm²/W. This result is comparable to the values obtained in YF₃ for the Yb³⁺, Er³⁺ green emission. The C12A7 with upconversion red and green light will be a promising luminous material.     

Spectroscopic properties of Tm,Ho:CaYAlO4 single crystal

A Tm,Ho:CaYAlO₄ single crystal was grown using the Czochralski (CZ) method. The absorption spectra, the fluorescence spectrum around 2 μm, the upconversion spectrum and energy‐transfer (ET) schemes between Tm³⁺ ions and Ho³⁺ ions have been studied. The crystal boule was free from cracks, inclusions and scattering centers. The broad FWHM of the 797‐nm absorption band (with E//c polarization is 19 nm and with E//a polarization is 18 nm), the large emission bandwidth (about 600 nm) and the intense ET from Tm³⁺ ions to Ho³⁺ ions make Tm,Ho:CaYAlO₄ a promising media for tunable and ultrashort pulses.     

The influence of Yb3+ concentration on Yb:YAl3(BO3)4

The Yb:YAl₃(BO₃) ₄ crystals with different Yb³⁺ doping concentration have been grown by the flux method. The lattice parameters and decomposition of the Yb:YAl₃(BO₃)₄ crystal with different Yb³⁺ doping concentration were measured by X‐ray and DTA method. The transmission and fluorescence spectra of Yb³⁺:YAl₃(BO₃)₄ crystal have been measured. The growth defects of Yb_xY_1‐xAl₃(BO₃)₄ crystals were also detected by using the chemical etching method. The results show that the ytterbium concentration influences these properties of Yb:YAl₃(BO₃)₄. As the Yb³⁺ concentration increased, the crystal lattice parameter was decreased. At high doping level, the absorption peak concerned at about 980 nm shift to short wavelength. It is also found that the perfection of Yb:YAl₃(BO₃) ₄ crystal with low Yb³⁺ doping concentration is better than that with high Yb³⁺ concentration. (© 2003 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Studies on the growth and optical characterization of Tm3+‐doped BaY2F8 single crystals

The BaY₂F₈ crystals doped with different concentrations of Tm³⁺ ions were prepared by the temperature gradient technique (TGT). X‐ray powder diffraction was applied to analyze the phase. The cracking phenomenon along (010) and (100) planes of the crystals grown by temperature gradient technique was studied on the basis of the structure of BaY₂F₈ crystals. The absorption spectra were measured and investigated in the ultraviolet‐visible and near‐infrared ranges at room temperature. Several characteristic absorption bands of Tm³⁺‐doped BaY₂F₈ crystal were observed. The emission and excitation spectra were obtained and investigated at room temperature and 12 K, showing the characteristic emission peaks of Tm³⁺ ions. The temperature dependence of Photoluminescence curve was also investigated in the range of 12–296 K. The luminescence intensity of emission bands decreased with increasing temperature, while the effective bandwidth increased. The up‐conversion spectrum excited at 650 nm was recorded and up‐conversion mechanism was analyzed in detail. The result showed the purple, green and yellow emissions corresponding to ³P₁→³F₃, ¹D₂→³H₅ and ³P₀→¹G₄ transitions, respectively.     

Down conversion luminescence of Tb3+–Yb3+ codoped SrF2 precipitated glass ceramics

In the Tb³⁺–Yb³⁺ codoped glass ceramics with SrF₂ nanocrystals precipitated, the energy transfer mechanism from Tb³⁺ to Yb³⁺ was investigated. The excitation power dependence of emission intensity study showed that the quantum cutting occurs during the energy transfer from Tb³⁺ to Yb³⁺ with the excitation of Tb³⁺ high energy level. However, the one-photon process is the main reason that is responsible for the Yb³⁺ infrared emission. The external quantum yields of Tb³⁺ and Yb³⁺ were evaluated by using an integrating sphere measurement system with the excitation of 377 and 488 nm lasers, which are much lower than the expected quantum efficiencies calculated from Tb³⁺ lifetimes. The external quantum yields in the glass ceramics and as-made glasses were also compared.     

Enhanced 2.0 μm emission in oxyfluoride glass-ceramics containing nanocrystals MF2 (MF3): Ho3+,Tm3+ (M = Ca,Ba, and La)

Transparent glass-ceramics containing MF₂(MF₃):Ho³⁺,Tm³⁺ (M = Ca, Ba, and La) nanocrystals have been prepared by melt quenching and subsequent thermal treatment. X-ray diffraction and transmission electron microscopy analysis confirmed the precipitation of MF₂ (MF₃) nanocrystals among the glass matrix. Energy-dispersive X-ray spectroscopy results evidenced the incorporation of Tm³⁺ and Ho³⁺ into the MF₂ nanocrystals. Intense 2.0 μm emission originating from the Ho³⁺: ⁵I₇ → ⁵I₈ transition was achieved upon excitation with 808 nm laser diode. A large ratio of the forward Tm³⁺ → Ho³⁺ energy transfer constant to that of the backward process indicated high efficient energy transfer from Tm³⁺ (³F₄) to Ho³⁺ (⁵I₇), and benefited from the reduced ionic distances of Tm³⁺–Tm³⁺ and Tm³⁺–Ho³⁺ pairs and low phonon energy environment with the incorporation of rare earth ions into the precipitated MF₂ nanocrystals. The results indicate that oxyfluoride glass-ceramic is a promising candidate for 2.0 μm laser.     

Visible up-conversion photoluminescence from IR diode-pumped SiO2–TiO2 nano-composite films heavily doped with Er3+–Yb3+ and Nd3+–Yb3+

Efficient infrared-to-visible conversion is reported in thin film nano-composites, with composition 90% SiO₂–10% TiO₂, fabricated by a spin-coating sol–gel route and co-doped with Er³⁺ Yb³⁺ and with Nd³⁺:Yb³⁺ ions. The conversion process is observed under 808 nm laser diode excitation and results in the generation of green (526 and 550 nm) and red (650 nm) emissions: from the former, and blue (478 nm) and green (513 and 580 nm) emissions from the latter. The main mechanism that allows for up-conversion is ascribed to energy transfer among Er³⁺ and Yb³⁺ ions in their excited states. Up-conversion efficiency for red emission predominates in samples doped with Er³⁺:Yb³. The results illustrate the large potential of this class of materials for photonic applications in optoelectronics devices.     

Energy transfer up-conversion in Tm-doped silica fiber

A study of the mechanisms responsible for the infra-red to near infra-red up-conversion in Tm³⁺-doped silica fibers is presented. Up-conversion luminescence was observed from the ³H₄ level of Tm³⁺ under 1586 nm pumping into the ³F₄ level. The quadratic dependence of the up-conversion luminescence at 800 nm on the 1800 nm luminescence from the ³F₄ level confirms that the ³H₄ level is populated by a two photon process. Two possible processes are proposed as mechanisms responsible for the up-conversion: excited state absorption and energy transfer up-conversion. The decay characteristics of the luminescence from the ³H₄ level were studied under direct and indirect pumping at 786 and 1586 nm, respectively. By comparing the decay waveforms to the solution of a simple set of rate equations, the energy transfer up-conversion process (³F₄, ³F₄ → ³H₄, ³H₆) was established at Tm₂O₃ concentrations greater than 200 ppm.     

Effect of the ytterbium concentration on the upconversion luminescence of Yb3+/Er3+ co-doped PbO–GeO2–Ga2O3 glasses

The effect of Yb³⁺ concentration on the frequency upconversion (UPC) of Er³⁺ in PbO–GeO₂–Ga₂O₃ glasses is reported for the first time. Samples were prepared with 0.5 wt% of Er₂O₃ and different concentrations of Yb₂O₃ (1.0–5.0 wt%). The green (523 and 545 nm) and red (657 nm) emissions are observed under 980 nm diode laser excitation. The dependence of the frequency UPC emission intensity upon the excitation power was examined and the UPC mechanisms are discussed. An interesting characteristic of these glasses is the increase of the ratio of red to green emission, through an increase of the Yb³⁺ concentration due to an efficient energy transfer from Yb³⁺ to Er³⁺.     

Spectroscopic investigation and optical characterization of novel highly thulium doped tellurite glasses

In this paper, optical properties of 75TeO₂-20ZnO-5Na₂O host glass doped with concentration of Tm³⁺ up to 10 %mol were studied in order to assess the most suitable rare earth content for short cavity fiber lasers. Raman spectroscopy revealed a change in the glass structure while increasing Tm³⁺ content, similar to the well known addition of alkali ions in a glass. Influence of the fabrication process on the OH⁻ content was determined by FTIR measurements. Refractive index of Tm³⁺ doped tellurite glasses was measured at five different wavelengths ranging from 533 nm to 1533 nm. Lifetime and emission spectra measurements of the Tm³⁺ doped tellurite glasses are reported.     

Dynamics of excited state relaxation and frequency upconversion in Tm3+ and Tm3+/Tb3+ doped ZBLAN glass

The effects of activator concentration on the relaxation of the ¹I₆, ¹D₂, ¹G₄ and ³H₄ levels of Tm³⁺ were investigated by the analysis of the fluorescence decay curves in Tm³⁺ and Tm³⁺/Tb³⁺ doped ZBLAN glasses. UV and blue upconversion luminescence bands around 362 and 450 nm were observed by 655 nm laser excitation in all the samples. The upconversion mechanism was attributed to excited state absorption (ESA) by analyzing the decay profiles and the intensity dependence.     

Compositional dependences on the mechanism of upconversion in Nd3+/Tm3+ co-doped chalcohalide glasses

Mechanisms of the compositional dependence of blue emission from Nd³⁺/Tm³⁺ co-doped Ge–Ga–S–CsBr chalcohalide glasses were investigated. The blue upconversion emissions (centered at 475 nm) due to the Tm³⁺: ¹G₄ → ³H₆ transition decreased as the CsBr/Ga ratio in glasses while the other upconversion emissions from the Nd³⁺ ions increased. Changes in the local environment of rare-earth ions incurred by the CsBr addition significantly increased the excited state absorption within Nd³⁺ ions. This resulted in the decrease in the Nd³⁺ → Tm³⁺ energy transfer rates that led to the large decrease in blue upconversion emission.     

Microscopic studies on imperfections in selected YAB single crystals

The as‐grown surface and inner structures of undoped and Nd³⁺‐, Cr³⁺‐, V³⁺‐, Ce³⁺‐, Er³⁺ and Yb³⁺ – and (Er³⁺ + Yb³⁺) – doped yttrium aluminum borate (YAB) single crystals grown from (K₂Mo₃O₁₀ + B₂O₃) flux by spontaneous crystallization or top seeded solution growth (TSSG) technique, were investigated using optical and scanning electron microscopic and analytic chemical methods. Fine and rough growth hillocks of dislocational origin, growth layers, traces of inner planar defects and foreign phase crystalline debris were found and analyzed on the as‐grown faces of crystals. Irregular grains and regular block structures and foreign phase inclusions were observed and studied in the interior of the crystals. The chemical compositions measured by energy dispersive X‐ray spectrometry on perfect and imperfect micro regions are compared with those obtained by flame atomic absorption spectrometry on bulk crystals. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Electroluminescence from Er and Yb co-doped silicon dioxide layers: The excitation mechanism

The excitation mechanism of photo- (PL) and electroluminescence (EL) of erbium ions co-implanted with ytterbium into the SiO₂ layer of light emitting MOS devices (MOSLED) was investigated. Ytterbium implanted and annealed samples exhibit the blue and near infrared electroluminescence. The blue electroluminescence at 470 nm appears due to cooperative up-conversion emission in the Yb³⁺-Yb³⁺ system, and the near infrared EL at 975 and 1025 nm corresponds to transitions from the multiple state ²F_5/2 to the ²F_7/2 ground state in the Yb³⁺ ions. The Er implanted SiO₂ exhibits the luminescence in the blue-green and infrared region. The green and blue peaks correspond to radiative transitions from the ²H_11/2 or ⁴S_3/2 energy levels and from the ²H_9/2 or ⁴F_5/2 energy levels to the ⁴I_15/2 ground state, respectively. We have found that the energy transfer from Yb³⁺ to Er³⁺ ions exists only during photoluminescence excitation. The electroluminescence investigation shows the cooperative up-conversion in the Er³⁺-Yb³⁺ system.