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.     

Additive colouring of CaF2:Yb crystals: determination of Yb2+ concentration in CaF2:Yb crystals and ceramics

When growing CaF₂ crystal doped with rare-earth ions, most of these ions are present in a trivalent state. However, due to contact with graphite crucible, a small proportion of a number of ions (Eu, Sm, Yb and Tm) are reduced to a bivalent state. A similar situation takes place during fabrication of CaF₂ ceramics doped with rare-earth metals. This fact is of particular importance for laser CaF₂:Yb crystals (ceramics), a promising material for short-pulse, high-power, high-energy diode-pumped solid state lasers since the presence of bivalent Yb ions can be a source of thermal losses. To date, there has been no technique to determine Yb²⁺ concentration in as-grown crystals. The proposed technique is based on a total reduction of Yb³⁺ ions via the heating of as-grown CaF₂ crystals with known concentration of Yb in the reducing atmosphere of metal vapour and determining the cross section of absorption bands of Yb²⁺ ions. The knowledge of these parameters allows estimation of the Yb²⁺ content in CaF₂:Yb crystals or ceramics by analysing their absorption spectra. Examples of using this technique are given. The technology of CdF₂ crystals reduction (an “additive colouring”) and features of colouring of crystals doped with rare-earth ions are considered.     

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.     

镱、钠共掺的氟化钙晶体在1050nm的可饱和吸收作用

通过在稳定连续波运转的Yb:YAG 激光器中插入不同掺杂浓度的新型钠、镱共掺的氟化钙晶体的对比性实验,证明了镱、钠共掺的氟化钙晶体在1050nm具有明显的可饱和吸收作用,从而解释了该晶体作为增益介质在该波段总是趋于自调Q运转的原因.Yb³⁺离子是该晶体可饱和吸收作用的主要因素,但是共掺入适当的Na离子可以明显改善晶体的调Q效果.优化共掺镱、钠离子的浓度和比例后的氟化钙晶体能够作为1050nm波段激光器的被动Q开关. 关键词： 镱、钠共掺氟化钙 可饱和吸收体 调Q     

Cooperative emission study in ytterbium doped NdVO4

The infrared and visible cooperative emissions of ytterbium ions are studied in Yb-doped NdVO₄ single crystals. The absorption of optical phonons allows the emissions at room temperature when a Nd:YAG laser is used. Low temperature emissions are observed due to the Nd³⁺→Yb³⁺ energy transfer following an argon ion laser excitation of the Nd³⁺ ions. Analysis of the cooperative emission at low doping concentration (1%) indicates that it is generated by distant pair forming Yb³⁺ ions while at high doping concentration (≥ 5%) close ions magnetically coupled and superexchange mechanisms prevail in the emitting process.     

Continuous wave and tunable laser operation of Yb3+ in disordered NaLa(MoO4)2

Continuous-wave Yb³⁺ laser operation is studied in single crystals of disordered NaLa(MoO₄)₂ at room temperature. The sample used was grown by the Czochralski technique and incorporates an Yb ion density of 3.1×10²⁰ cm^-3. The effect of the Yb concentration on some of the crystal properties is described as well as the spectroscopic Yb³⁺ properties at 5 K. Maximum slope efficiencies of about 40% for π and 38% for σ polarization were obtained under Ti:sapphire laser pumping near 976 nm, respectively. The maximum output power for the π polarization was 400 mW at 1039.5 nm, the threshold in this case amounted to 240 mW (absorbed pump power). The laser emission was tunable between 1016 and 1064 nm with a Lyot filter. Lasing was also realized by pumping with a fiber-coupled diode laser module. Maximum output power of 900 mW at 1035 nm was achieved in this case for the π polarization and the threshold was 280 mW. The results, in terms of output power and tunability, are superior in comparison to all previous reports on Yb-doped disordered double tungstate or molybdate crystals and represent a significant improvement in comparison to earlier experiments with low-doped Yb:NaLa(MoO₄)₂.PACS 42.55.Rz; 42.55.Xi; 42.70.Hj     

ANALYSIS OF LASING PERFORMANCES OF Yb3+-DOPED YAG, FAP, KYW CRYSTALS

The Yb³⁺ laser emitting at a 1.03μm wavelength has been evoking strong interest recently due to its advantages of long fluorescence lifetime, broad absorption band and the fact that it never shows concentration quenching. On the other hand, as a laser of three-level system it has, in general, a relatively high threshold power, which makes it important to seek some suitable host crystals to reduce this. Here, we present a comparison of the lasing performances of Yb³⁺-doped YAG [Y₃Al₅O₁₂], FAP [Ca₅(PO₄)₃F] and KYW [KY(WO₄)₂] crystals, including threshold power and slope efficiency, with those of the Nd:YAG laser based on the threshold formula of three-and four-level systems deduced by the authors. The results show that the Yb³⁺ laser can output a power larger than the Nd:YAG laser does in the case of comparably higher pumping power, if the length of the lasing rod and the concentration of the active ions satisfy some conditions. The theoretical results are also close to the experimental results reported.     

The fluorescence properties of Yb3+ and Er3+ Co-doped YAl3(BO3)4 powders prepared by sol-gel method

Yb³⁺ and Er³⁺ co-doped YAB powders were prepared by sol-gel method. The structure and fluorescence properties were investigated. XRD pattern indicated that the single phase was obtained at 1150°C and the structure belonged to rhombohedral. Under 379 nm excitation, two emissions around 983 nm and 1531 nm were observed and the effect of Yb³⁺ ion concentration on the emission intensity was discussed. The energy transfer was observed under 930 nm excitation and the energy transfer efficiencies for all samples were calculated. The lifetimes of ² F _5/2 level of Yb³⁺ ion and ⁴ I _13/2 level of Er³⁺ ion were measured and the effect of Yb³⁺ ion concentration on the lifetime was also discussed. The results indicated that there was an additional mechanism for the decay of ⁴ I _13/2 level in powder samples. The Yb³⁺ and Er³⁺ co-doped YAB powders should be a potential candidate for ceramic laser materials.     

Yb effect on the spectroscopic properties of Er-Yb codoped SnO2 thin films

We have investigated the optical properties of sol-gel thin films of tin dioxide (SnO₂) codoped with Er³⁺-Yb³⁺ as a function of Yb³⁺ concentration. The Judd-Ofelt model has been applied to absorption intensities of Er³⁺ (4f¹¹) transitions to establish the so-called Judd-Ofelt intensity parameters: Ω₂, Ω₄, Ω₆. Various spectroscopic parameters were obtained to evaluate their dependence and the potential of the samples as a laser material in the eye-safe laser wavelength (1.53 μm) as a function of Yb³⁺ concentration. An amelioration of the quality factor Ω₄/Ω₆ was found with Yb content. Both the IR photoluminescence (PL) intensity and the up-conversion emission, from Er³⁺ ion in SnO₂, were found to increase with Yb concentration. We show that the Yb³⁺ ion acts as sensitizer for Er³⁺ ion and contributes largely to the improvement of the spectroscopic properties of SnO₂:Er. The mechanism of up-conversion emission is discussed and a model is proposed. The results showed that sol-gel SnO₂ is promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable up-conversion lasers.     

Nonlinear refractive properties of Yb3+-doped KY(WO4)2 and YVO4 laser crystals

The nonlinear refractive indices of Yb³⁺: KY(WO₄)₂ and Yb³⁺:YVO₄ laser crystals are characterized using a Z-scan technique at a wavelength of 1.08 μm for different polarizations. The results reveal that the nonlinear refractive index of Yb³⁺:YVO₄ is superior to that of Yb³⁺:KY(WO₄)₂ and is found to be 1.9×10^-15 cm²/W and 1.5×10^-15 cm²/W for E⊥c and E∥c polarizations, respectively. PACS 42.65.Hw; 42.55.Rz; 42.65.Re; 42.70.Hj     

Variation in luminescence behavior of Yb3+ doped GdAlO3 phosphor with gradual increase in Yb3+ concentration

The present paper reports the combustion synthesis of Yb³⁺ doped GdAlO₃ phosphors. The structural characterization and luminescence spectra of Yb³⁺ doped GdAlO₃ phosphors have been discussed. The effects of variable concentration of Yb³⁺ on Photoluminescence (PL) behavior were studied. The structural characterization was done by X-ray diffraction (XRD) and Transmission electron microscope technique (TEM). The good connectivity with grains and the semi-sphere line structure was found by TEM. The functional group analysis was carried out by Fourier transform infrared (FTIR) spectroscopic techniques. The prepared phosphor gives emission spectra in visible as well as NIR region. Both emissions were studied as a function of Yb³⁺ concentration. The emission intensity variation with Yb³⁺ ion concentration for visible and NIR region were discussed separately. The NIR emission luminescence of GdAlO₃:Yb³⁺ phosphor luminescence continuously increases with increasing Yb³⁺ ion concentration.     

Hydrothermal synthesis of BaYF5:Yb/Er upconversion luminescence submicrospheres by a surfactant-free aqueous solution route

BaYF₅:Yb³⁺/Er³⁺ upconversion (UC) luminescence submicrospheres have been synthesized by the hydrothermal synthesis method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), scanning probe microscope (SPM), transmission electron microscope (TEM), laser diffraction particle analyzer (LDPA) and UC emission spectra. The as-prepared highly crystalline BaYF₅:Yb³⁺/Er³⁺ submicrospheres are of uniform size depending on different reaction temperatures and reaction times. It is found that the usage of fluoride source NaBF₄ plays the crucial key in the formation of submicrosphere. Under the 980 nm excitation, the UC emission transitions for ⁴F_9/2→⁴I_15/2 (red), ²H_11/2, ⁴S_3/2→⁴I_15/2 (green) in the BaYF₅:Yb³⁺/Er³⁺ submicrospheres came from two-, two-, and two-photon UC processes, respectively. Further, the effects of Yb³⁺ ion concentration, size and surface of as-prepared submicrospheres, and pumping power on the UC luminescence properties of BaYF₅:Yb³⁺/Er³⁺ have also been discussed.     

Luminescence spectroscopy of Er-doped and Er, Yb-codoped LaPO4 single crystals

LaPO₄ single crystals lightly doped with Er³⁺, and codoped with Er³⁺ and Yb³⁺ have been grown by spontaneous nucleation in a lead phosphate flux. Absorption and luminescence spectra have been measured in the visible and near-IR regions and the excited state dynamics has been studied upon pulsed laser excitation. The obtained results have allowed the evaluation of the effective emission cross-sections around 1.5 μm, that have been found to be similar to important oxide laser crystals doped with Er³⁺. Efficient visible upconversion has been observed upon excitation at 980 nm in the codoped crystals. This behaviour is attributed to Yb³⁺-Er³⁺ energy transfer processes.     

Visible upconversion emission and non-radiative direct Yb to Er energy transfer processes in nanocrystalline ZrO2:Yb,Er

Wide band gap Yb³⁺ and Er³⁺ codoped ZrO₂ nanocrystals have been synthesized by a modified sol-gel method. Under 967 nm excitation strong green and red upconversion emission is observed for several Er³⁺ to Yb³⁺ ions concentration ratios. A simple microscopic rate equation model is used to study the effects of non-radiative direct Yb³⁺ to Er³⁺ energy transfer processes on the visible and near infrared fluorescence decay trends of both Er³⁺ and Yb³⁺ ions. The microscopic rate equation model takes into account the crystalline phase as well as the size of nanocrystals. Nanocrystals phase and size were estimated from XRD patterns. The rate equation model succeeds to fit simultaneously all visible and near infrared fluorescence decay profiles. The dipole-dipole interaction parameters that drive the non-radiative energy transfer processes depend on doping concentration due to crystallite phase changes. In addition the non-radiative relaxation rate (⁴I_11/2→⁴I_13/2) is found to be greater than that estimated by the Judd-Ofelt parameters due to the action of surface impurities. Results suggest that non-radiative direct Yb³⁺ to Er³⁺ energy transfer processes in ZrO₂:Yb,Er are extremely efficient.     

从头计算法计算Yb3+掺杂钽酸盐的晶体场参数和能级结构

介绍了一种基于从头计算的DV-Xα方法和有效哈密顿量模型,它可以计算晶体中掺杂离子的晶体场参数和旋轨耦合参数,尤其适合计算低对称性的晶体.对于低对称性的晶体,参数的数目比能级的数目多,因此通过实验能级拟合确定所有的参数不太准确,而从头计算法可以准确地确定所有的晶体场参数和旋轨耦合参数.首先用这种模型计算了Yb3+掺杂GdTaO4晶体中的晶体场参数和旋轨耦合参数,然后给出了Yb3+在GdTaO4中的能级结构,并分析了Yb3+:GdTaO4的发射谱形成一个连续的发射带.这有利于激光的调谐和锁模激光输出,预言了Yb3+:GdTaO4有望成为新型全固态超短脉冲激光工作物质.同样用这种模型分别计算了Yb3+掺杂YTaO4和ScTaO4中的晶体场参数和旋轨耦合参数,并给出了Yb3+在YTaO4和ScTaO4中的能级结构,得到了与Yb3+:GdTaO4晶体类似的结论.     

Up-conversion emission in triply-doped Ho/Yb/Tm KGd(WO4)2 single crystals

Detailed spectroscopic studies of the triply doped KGd(WO₄)₂:Ho³⁺/Yb³⁺/Tm³⁺ single crystals (which exhibit multicolor up-conversion fluorescence) are reported for the first time. The absorption spectra of crystals were measured at 10 and 300 K; the room temperature luminescence spectra were excited at 980 nm wavelength. The dependence of the intensity of luminescence on the excitation power for three different concentration of Ho³⁺, Yb³⁺ and Tm³⁺ ions was investigated. Efficient green and red up-converted luminescence of Ho³⁺ ions and weak blue up-conversion luminescence of Tm³⁺ ions were observed in spectra. The red emission of Ho³⁺ ions is more intensive than their green emission. Dependence of the up-conversion luminescence intensity on the excitation power and impurities concentration was also studied; the number of phonon needed for efficient up-conversion was determined for each case. All possible energy transfer processes between different pairs of the impurity ions' energy levels are also discussed.     

Spectral kinetic properties of Yb3+:Na4Y6F22 and Yb3+:LiLuF4 crystals

Yb³⁺ (2.7 at.%):Na₄Y₆F₂₂ and Yb³⁺(1 at.%):LiLuF₄ crystals were grown by the Bridgman-Stockbarger method. We measured the temperature dependences of the thermal conductivity of the crystals and the absorption spectra. We determined the radiative lifetime of the Yb³⁺ ion in these crystals (1.94 msec and 2.13 msec) and calculated the stimulated emission cross section spectra. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 6, pp. 761–766, November–December, 2007.     

Conversion of infrared radiation into visible emission in NaGd(WO4)2:Yb3+, Ho3+ crystals

NaGd(WO₄)₂:Yb³⁺, Ho³⁺ single crystals have been grown by the Czochralski technique along the (0 0 1) orientation. Conversion of the infrared (IR) radiation at 980 nm into the visible emission in NaGd(WO₄)₂ crystals containing several different concentrations of Yb³⁺ and Ho³⁺ has been investigated. The NaGd(WO₄)₂: 8 at. % Yb³⁺, 4 at. % Ho³⁺ system exhibits intense red upconverted emission originating from the ⁵F₅ level. The upconversion mechanism in a Ho³⁺-Yb³⁺ system under near infrared excitation is discussed. It is concluded that the green emission is excited by energy transfers from Yb³⁺ to Ho³⁺, whereas excited state absorption is involved in the excitation of red emission. The emission cross-section of the ⁵F₅→⁵I₈ transition at about 660 nm was estimated by using the Füchtbauer–Ladengurg formula. PACS 78.55.Hx; 78.20.-e     

Cooperative down-conversion of UV light in disordered scheelitelike Yb-doped NaGd(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> and NaLa(MoO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

Concentration series of disordered scheelitelike Yb:NaGd(MoO₄)₂ and Yb:NaLa(MoO₄)₂ single crystals are grown by the Czochralski method. The actual concentrations of Yb³⁺ ions in the crystals are determined by optical-absorption spectroscopy. The luminescence of Yb³⁺ ions in these crystals in the region of 1 μm is studied under UV and IR excitation. In the case of UV excitation, this luminescence appears as a result of nonradiative excited state energy transfer from donor centers of unknown nature to ytterbium. The character of the concentration dependence of Yb³⁺ luminescence indicates that the energy transfer at high Yb concentrations occurs with active participation of a cooperative mechanism, according to which the excitation energy of one donor center is transferred simultaneously to two Yb³⁺ ions. In other words, the quantum yield of this transfer exceeds unity, which can be used to increase the efficiency of crystalline silicon (c-Si) solar cells.     

Theoretical and comparative investigations of Yb ion in MWO4 and M′MoO4 scheelites crystals (M=Sr, Pb, Ca, Ba) and (M′=Sr, Pb, Ca, Cd)

The crystal-field model is applied to a series of scheelites crystals (CaWO₄, SrWO₄, PbWO₄, BaWO₄, CdMoO₄, CaMoO₄, SrMoO₄ and PbMoO₄) doped with the Yb³⁺ ion. The calculated crystal-field parameters present a general trend of variation with M²⁺ ionic radius of the host cation. The maximum splitting ΔE of the ²F_7/2 manifold of the Yb³⁺ ion is then obtained as a function of N_V crystal-field strength parameters. The agreement between experimental results and theoretical predictions for all investigated systems is very satisfactory. The crystal-field effects are very important for the prediction of emission energies of the Yb³⁺ ion in different scheelites.