Increase of the blue upconversion emission in YAG:Tm3+ nanopowders by codoping with Yb3+ ions

The YAG nanopowders were prepared by a co-precipitation method using nitrate and ammonium hydrogen carbonate as raw materials. To obtain homogenous precipitate, reverse-strike (adding salt solutions to the precipitant solution) technique was adopted. Therefore, single (Tm³⁺) and codoped (Tm³⁺–Yb³⁺) YAG nanopowders with a size between 40–90 nm have been obtained.Blue upconversion emission at around 480 nm has been found in YAG: Tm³⁺ nanopowders under excitation to the ³H₄ level of Tm³⁺ at around 800 nm. However, this upconversion emission in nanopowders codoped with Tm³⁺–Yb³⁺ ions is increased by a factor of about 10. The analysis of the temporal evolution of the involved levels and the dependence of the upconversion intensity on the pump power at 800 nm allowed to distinguish the upconversion mechanism. In YAG: Tm³⁺ nanopowders the upconversion mechanism is due to excited state absorption processes. However, in the codoped samples, Yb³⁺ ions acts as the sensitizers; in consequence, the blue upconversion is strongly increased.     

Hydrothermal synthesis and the enhanced blue upconversion luminescence of NaYF4:Nd3+,Tm3+,Yb3+

Nd³⁺, Tm³⁺ and Yb³⁺ co-doped NaYF₄ upconversion (UC) material was synthesized by the hydrothermal method. The structure of the sample was characterized by the X-ray diffraction, and its UC luminescence properties were investigated in detail. Under the 980 nm semiconductor laser excitation, its UC spectra exhibited distinct emission peaks at 451 nm, 475 nm and 646 nm respectively. On the basis of the comparison of UC spectra between NaYF₄:Nd³⁺,Tm³⁺,Yb³⁺ and NaYF₄:Tm³⁺,Yb³⁺, it was indicated that the existence of Nd³⁺ ion enhanced the blue emission intensity. The law of luminescence intensity versus pump power proved that the blue emission at 475 nm, and the red emission at 646 nm were the two-photon processes, while the blue emission at 451 nm was a three-photon process.     

Optical spectroscopy and laser parameters of Zn2+/Er3+/Yb3+-tridoped LiNbO3 crystal

The Zn/Er/Yb:LiNbO₃ and Er/Yb:LiNbO₃ crystals were grown by the Czochralski technique. The laser characteristics of 1.54 μm emission were predicted based on the Judd–Ofelt theory, and the intensity parameters Ω_t (Ω₂=7.23×10^?20 cm², Ω₄=3.15×10^?20 cm² and Ω₆=1.43×10^?20 cm²) were obtained. The stimulated emission cross sections (σ_em) at 1.54 μm emission in Zn/Er/Yb:LiNbO₃ were calculated based on the McCumber theory and the Füchtbauer–Ladenburg theory. The gain cross section spectrum of Zn/Er/Yb:LiNbO₃ crystal was also investigated. Under 980 nm excitation, a lenghthening lifetime of 1.54 μm emission and an enhancement of green upconversion emission were observed for Zn/Er/Yb:LiNbO₃ crystal. The studies on the power pump dependence and the upconversion mechanism suggested that both green and red upconversion emissions were populated via the three-photon process, and Zn²⁺ ion tridoping increases the probability of cross relaxation process between the two neighboring Er³⁺ ions.     

Blue upconversion luminescence in 12 CaO·7 Al2O3:Tm3 +/Yb3 + polycrystals

The effect of Yb^3 + concentration on the fluorescence of 12 CaO·7 Al₂O₃:Tm^3 +/Yb^3 + polycrystals is investigated. Under the excitation of 980 nm laser, the strong blue (477 nm) emission band is observed and attributed to ¹G₄ → ³H₆ of Tm^3 +. The ratio of blue to red emission increases with the increasing of Yb^3 + and remains constant at 10 mol% Yb^3 +. The pump dependence and upconversion mechanisms show that the two-photon cooperative upconversion process is responsible for the enhancement of the blue upconversion emission. The Commission Internationale de l'eclairage chromaticity coordinates (x, y) illustrate that the 12 CaO·7 Al₂O₃:1 mol% Tm^3 +/10 mol% Yb^3 + can emit high-purity blue light.     

Synthesis of TiO2 nanoparticles and spectroscopic upconversion luminescence of Nd3+-doped TiO2–SiO2 composite glass

Nd³⁺-doped TiO₂–SiO₂ composites were prepared by sol–gel method. Optical properties such as radiative life-time (τ), stimulated emission cross-section (σ_p) and branching ratio (β) were calculated using Judd–Ofelt theory. Violet to blue upconversion emissions at 380 nm (⁴D_3/2→⁴I_11/2), 399 nm (²P_3/2→⁴I_11/2), 420 nm (²D_5/2→⁴I_9/2) and 452 nm (²P_3/2→⁴I_13/2) were obtained under 578 nm xenon-lamp excitation. The choice of 578 nm is justified by the absorption spectra of the same samples, which shows a strong absorption peak at 578 nm. This 578 nm excitation pump produces upconversion in Nd³⁺ by a sequential two-photon absorption process.     

Synthesis and luminescence properties of Gd6MoO12:Yb3+, Er3+ phosphor with enhanced photoluminescence by Li+ doping

Yb³⁺/Er³⁺ co-doped Gd₆MoO₁₂ and Yb³⁺/Er³⁺/Li⁺ tri-doped Gd₆MoO₁₂ phosphors were prepared by adjusting the annealing temperature via the high temperature solid-state method. Under the excitation of 980 nm semiconductor, the upconversion luminescence properties were investigated and discussed. In the experimental process, we get the optimum Yb³⁺ concentration and the concentration quench effect will happen while the concentration extends the given region. According to the Yb³⁺ concentration quenching effects, the critical distance between Yb³⁺ ions had been calculated. The measured UC luminescence exhibited a strong red emission near 660 nm and green emission at 530 nm and 550 nm, which are due to the transitions of Er³⁺(⁴F_9/2, ²H_11/2, ⁴S_3/2) → Er³⁺(⁴I_15/2). Then the effect of excitation power density in different regions on the upconversion mechanisms was investigated and the calculated results demonstrate that the green and red upconversion is a two-photon process. A possible mechanism was discussed. After Li⁺ ions mixing, the upconversion emission enhanced largely, and the optimum Li⁺ concentration was obtained while fixed the Yb³⁺ and Er³⁺ on the above optimum concentration. This enhancement owns to the decrease of the local symmetry around Er³⁺ after Li⁺ ions doping into the system. This result indicates that Li⁺ is a promising candidate for improving luminescence in some case.     

Upconversion luminescence in Tm3+/Yb3+co-doped lead tungstate tellurite glasses

This work reports the upconversion luminescence properties of Tm³⁺/Yb³⁺ ions in lead tungstate tellurite (LTT) glasses. Judd–Oflet intensity parameters have been obtained from the absorption band intensities of Tm³⁺ singly-doped and Tm³⁺/Yb³⁺ co-doped LTT glasses. The spontaneous emission probabilities, radiative lifetimes and branching ratios for ¹G₄ and ³H₄ emission levels of Tm³⁺ have been determined. Upconversion luminescence has been observed by exciting the samples at 980 nm (Yb³⁺:²F_7/2→²F_5/2) at room temperature. Four upconversion emission bands corresponding to the ¹G₄→³H₆ (477 nm), ¹G₄→³F₄ (651 nm), ¹G₄→³H₅ (702 nm) and ³H₄→³H₆ (810 nm) transitions have been identified. The relative variation in the intensities of upconversion bands, the different channels responsible for upconversion spectra and the effect of Yb³⁺ ions concentration on the upconversion luminescence of Tm³⁺ ions have also been discussed.     

High index contrast Er:Ta2O5 waveguide amplifier on oxidised silicon

We report a high index contrast erbium doped tantalum pentoxide waveguide amplifier. 2.3 cm long waveguides with erbium concentration of 2.7 × 10²⁰ cm^? 3 were fabricated by magnetron sputtering of Er-doped tantalum pentoxide on oxidised silicon substrates and Ar-ion milling with photolithographically defined mask. A net on-chip optical gain of ~ 2.25 dB/cm at 1531.5 nm was achieved with 20 mW of pump power at 977 nm launched into the waveguide. The pump threshold for transparency was 4.5 mW.     

Photophysical parameters and laser performance of 3-(4′-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP): A new laser dye

The spectral properties such as singlet absorption, molar absorptivity, emission spectra, fluorescence quantum yield and excited state lifetime of 3-(4′-dimethylaminophenyl)-1-(2-furanyl)prop-2-en-1-one (DMAFP) have been determined in different solvents. DMAFP dye exhibits a large red shift in both electronic absorption and emission spectra as the solvent polarity increases, indicating a large change in the dipole moment of molecules upon excitation. A crystalline solid of DMAFP gives an excimer like emission at 566 nm due to the excitation of molecular aggregates. This is expected from the idealized crystal structure of the dye that belongs to the B-type class of Steven's classification. The ground and excited state protonation constants of DMAFP are calculated and amounted to 1.71 and 8.3, respectively. DMAFP acts as a good laser dye upon pumping with nitrogen laser (λ_ex=337.1 nm) in chloroform, methylene chloride and dioxane and gives laser emission in the range 460–590 nm. The laser parameters such as the tuning range, gain coefficient (α), emission cross section (σ_e) and half-life energy (E_1/2) are calculated. The photoreactivity and net photochemical quantum yield of DMAFP in chloromethane solvents are also studied.     

Optical temperature sensor through infrared excited blue upconversion emission in Tm3 +/Yb3 + codoped Y2O3

An analysis of the intense blue upconversion emission at 476 and 488 nm in Tm^3 +/Yb^3 + codoped Y₂O₃ under excitation power density of 86.7 W/cm² available from a diode laser emitting at 976 nm, has been undertaken. Fluorescence intensity ratio (FIR) variation of temperature-sensitive blue upconversion emission at 476 and 488 nm in this material was recorded in the temperature range from 303 to 753 K. The maximum sensitivity derived from the FIR technique of the blue upconversion emission is approximately 0.0035 K^? 1. The results imply that Tm^3 +/Yb^3 + codoped Y₂O₃ is a potential candidate for the optical temperature sensor.     

Crystal growth and optical properties of Cr3+, Er3+, RE3+: Gd3Ga5O12 (RE=Tm,Ho, Eu) for mid-IR laser applications

In this paper we report on the optical properties of triply Cr³⁺, Er³⁺, and RE³⁺ (RE=Tm, Ho, Eu) doped Gd₃Ga₅O₁₂ crystals that were grown by the Czochralski method. Optical absorption, near-infrared (NIR), and mid-infrared (mid-IR) fluorescence spectra were characterized for the fabricated crystals and corresponding luminescence decay measurements under 654 nm excitation were also carried out. Based on the analysis of energy transfer process between Er and RE (RE=Tm, Ho, Eu) ions, the energy transfer efficiency (ETE) values were evaluated, correspondingly. From the spectral data of all the studied crystals, it is observed that the co-doped Cr³⁺ ion highly increases the absorption pump power and the three kinds of co-doped RE³⁺ ions depopulate the Er:⁴I_13/2 energy level effectively. The spectral analysis shows that titled rare earth doped crystals are promising materials for ～3.0 μm mid-IR laser applications and among them Cr,Er,Eu:GGG is relatively more suitable due to its excellent optical properties compared with others.     

Absorption,luminescence, and electron paramagnetic resonance of molybdenum ions in CdWO4

A single crystal of cadmium tungstate (CdWO₄) containing approximately 200 ppm of molybdenum was grown by the Czochralski method and then characterized in a series of optical absorption, photoluminescence (PL), photoluminescence excitation (PLE), and electron paramagnetic resonance (EPR) experiments. The Mo⁶⁺ ions substitute for W⁶⁺ ions and serve as recombination sites for electrons and holes when the crystal is exposed to ionizing radiation. A charge-transfer absorption band for the Mo⁶⁺ ions was observed near 320 nm at 10 K. The PL experiments, performed at low temperature with 325 nm excitation, showed a Mo-associated emission peaking near 680 nm. A direct correlation of the 680 nm emission and the 320 nm absorption band was established by the PLE data. When these doped CdWO₄ crystals are exposed at low temperature either to light that is near or above the band gap or to X-rays, the Mo⁶⁺ ions can trap an electron and form stable Mo⁵⁺ ions. The EPR spectrum of the Mo⁵⁺ ions was observed at temperatures near 15 K, and a complete set of parameters describing the g matrix was obtained from an angular dependence study.     

Luminescence study of self-trapped excitons in CdMoO4

Luminescence properties of CdMoO₄ crystals have been investigated in a wide temperature range of T=5–300 K. The luminescence-excitation spectra are examined by using synchrotron radiation as a light source. A broad structureless emission band appears with a maximum at nearly 550 nm when excited with photons in the fundamental absorption region (<350 nm) at T=5 K. This luminescence is ascribed to a radiative transition from the triplet state of a self-trapped exciton (STE) located on a (MoO₄)^2? complex anion. Time-resolved luminescence spectra are also measured under the excitation with 266 nm light from a Nd:YAG laser. It is confirmed that triplet luminescence consists of three emission bands with different decay times. Such composite nature is explained in terms of a Jahn–Teller splitting of the triplet STE state. The triplet luminescence at 550 nm is found to be greatly polarized in the direction along the crystallographic c axis at low temperatures, but change the degree of polarization from positive to negative at T>180 K. This remarkable polarization is accounted for by introducing further symmetry lowering of tetrahedral (MoO₄)^2? ions due to a uniaxial crystal field, in addition to the Jahn–Teller distortion. Furthermore, weak luminescence from a singlet state locating above the triplet state is time-resolved just after the pulse excitation, with a polarization parallel to the c axis. The excited sublevels of STEs responsible for CdMoO₄ luminescence are assigned on the basis of these experimental results and a group-theoretical consideration.     

Effect of Yb3+ concentration on photoluminescence properties of cubic Gd2O3 phosphor

Yb³⁺ doped phosphor of Gd₂O₃ (Gd₂O₃:Yb³⁺) have been prepared by solid state reaction method. The structure and the particle size have been determined by X-ray powder diffraction measurements. The average particle size of the phosphor is in between 35 and 50 nm. The particle size and structure of the phosphor was further confirmed by TEM analysis. The visible and NIR luminescence spectra were recorded under the 980 nm laser excitation. The visible upconversion luminescence of Yb³⁺ ion was due to cooperative luminescence and the presence of rare earth impurity ions. The cooperative upconversion and NIR luminescence spectra as a function of Yb³⁺ ion concentration were measured and the emission intensity variation with Yb³⁺ ion concentration was discussed. Yb³⁺ energy migration quenched the cooperative luminescence of Gd₂O₃:Yb³⁺ phosphor with doping level over 5%, while the NIR emission luminescence continuously increases with increasing Yb³⁺ ion concentration.     

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.     

Optical characterization of Er3+and Yb3+ co-doped barium fluorotellurite glass

Spectroscopic studies of Er³⁺/Yb³⁺ co-doped (Ba,La)-fluorotellurite glass composition have been carried out using standard experimental and theoretical methods. Quantitative analyses of the room temperature absorption and emission spectra as well as the emission lifetimes yield various important spectroscopic parameters such as the radiative decay rates, fluorescence branching ratios, and emission/absorption cross sections. In addition, internal radiative quantum yields have been determined for the infrared emission at 1571 nm and for the upconversion emission at 547 nm. The influence of various non-radiative properties such as multiphonon relaxation, concentration quenching, and quenching by hydroxyl radicals have also been quantitatively estimated and correlated with the observed spectral properties. The comparative studies with the other composition of tellurite and different glasses showed that present glass composition could be a potential candidate for the broadband amplifier.     

Radioluminescence of synthetic quartz related to alkali ions

The radioluminescence (RL) of synthetic quartzes (GEMMA Quartz & Crystal Company) has been measured at room temperature. Some samples were treated by electrodiffusion (“sweeping”) in order to change the concentrations of alkali ions, mainly Li⁺ and Na⁺, which in quartz are known to be linked to Al ions, substitutional for Si ions.The RL emission spectra show evidence of a role of alkali ions in affecting some specific emissions. All the spectra could be analysed as composed of four bands in the blue and UV region. Specifically, the well known blue emission at around 470 nm was seen to be composed by two bands at 430 nm (2.86 eV) and at 485 nm (2.53 eV). Effects of irradiation, during the RL measurements, were clearly seen only in the “Li swept in” sample, namely an increase in the 485 nm band intensity and a decrease in the 430 nm band one. The previously reported UV emission was detected at 355 nm (3.44 eV) in all the samples, being the most intense band in the “swept out” sample. A further UV emission was detected at 315 nm (3.94 eV), more intense in untreated samples.Possible assignments of the detected emission bands are discussed in relation to the defects of quartz, specifically focusing on the Al centres that are most affected by sweeping procedures.     

56 dB Gain EYDFA with improved noise figure with dual-stage partial double pass configuration

An efficient erbium/ytterbium co-doped fiber amplifier (EYDFA) is demonstrated by using a dual-stage partial double pass structure with a band pass filter (BPF). The amplifier achieves the maximum small signal gain of 56 dB and the corresponding noise figure of 4.66 dB at 1536 nm with an input signal power and total pump power of ?50 dBm and 140 mW, respectively. Compared with a conventional single-stage amplifier, the maximum gain enhancement of 16.99 dB is obtained at 1544 nm with the corresponding noise figure is improved by 2 dB. The proposed amplifier structure only uses a single pump source with a partial double pass scheme to provide a high gain and dual-stage structure to provide the low noise figure.     

Infrared to visible upconversion luminescence in Er3+/Yb3+ doped titanate glass prepared by containerless processing

Er³⁺ doped TiO₂–La₂O₃ glasses modified by ZrO₂ have been successfully fabricated by the containerless method with incorporated Yb³⁺ ions as sensitizers. Under the excitation of 980 and 808 nm diode lasers, visible emissions centered at 534, 554 and 674 nm are observed, which are assigned to the Er³⁺ transitions of ²H_11/2→⁴I_15/2, ⁴S_3/2→⁴I_15/2 and ⁴F_9/2→⁴I_15/2, respectively. The emission signals are so strong that they can be observed by naked eyes even at pumping power as low as 20 mW. Measurements of pump-power dependent intensity and time-resolved decay behavior of upconversion luminescence show that two-photon excited state absorption (ESA) and energy transfer (ET) between rare earth ions are the predominant mechanisms for upconversion emissions. Besides, the intensity of upconversion luminescence has been enhanced by increasing the concentration of ZrO₂ in these rare earth doped bulk titanate glasses.     

Photoluminescence of Er-doped Si-SiO2 and Al–Si-SiO2 sputtered thin films

Er-doped Si-SiO₂ and Al–Si-SiO₂ films have been deposited by rf-sputtering being annealed afterwards. Annealing behavior of the Er³⁺: ⁴I_13/2→⁴I_15/2 emission of Er-doped Si-SiO₂ yields a maximum intensity for annealing at 700–800 °C. ⁴I_13/2→⁴I_15/2 peak emission for Er-doped Al–Si-SiO₂ at 1525 nm is shifted from that for Er-doped Si-SiO₂ at 1530 nm and the bandwidth increases from 29 to 42 nm. ⁴I_13/2→⁴I_15/2 emission decays present a fast decaying component related to Er ions coupled to Si nanoparticles, defects, or other ions, and a slow decaying component related to isolated Er ions. Excitation wavelength dependence and excitation power dependence for the ⁴I_13/2→⁴I_15/2 emission correspond with energy transfer from Si nanoparticles. Populating of the ⁴I_11/2 level in Er-doped Si-SiO₂ involves branching and energy transfer upconversion involving two or more Er ions. Addition of Al reduces the populating of this level to an energy transfer upconversion involving two ions.