Yb:YAG晶体的晶胞参量及Yb3+分凝系数的研究

应用提拉法生长出不同Yb³⁺浓度的Yb_3xY_3(1-x)Al₅O₁₂晶体.Yb³⁺离子的分凝系数1是1.08±0.01,与Yb³⁺离子掺杂浓度无关.研究了Yb_3xY_3(1-x)Al₅O₁₂晶体的晶胞参量,推导出联系晶胞参量、密度与Yb³⁺离子掺杂浓度的关系方程式.     

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     

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.     

Size-effect on concentration quenching in Yb-doped Y3Al5O12 nano-crystals

Ytterbium-doped nano-crystalline powders of yttrium aluminium garnet Y₃Al₅O₁₂ (Yb³⁺-doped YAG) have been prepared by a modified Pechini method. The morphology of the annealed materials was observed using SEM images. The structure and the average crystalline grain sizes have been determined by X-ray powder diffraction measurements. An increase in elongated spherical crystalline grain sizes from 20 to 50 nm has been observed with annealing temperature increasing from 800 to 1200 °C. The Yb³⁺ concentration dependence of both luminescence spectra and lifetimes has been investigated and compared with those reported for the Yb³⁺-doped YAG bulk single crystals. Decays of Yb^{3+ 2}F_5/2 excited state, much longer in nano-crystalline powders than in bulk single crystals, are discussed in terms of refractive index dependence.     

Effect of ytterbium concentration on cw Yb:YAG microchip laser performance at ambient temperature – Part I: Experiments

The microchip laser performance of Yb:YAG crystals doped with different ytterbium concentrations (C_Yb=10, 15, and 20 at. %) has been investigated at ambient temperature without active cooling of the gain media. Efficient laser oscillation for a 1-mm-thick YAG doped with 10 at. % Yb³⁺ ions was achieved at 1030 and 1049 nm with slope efficiencies of 85% and 81%, correspondingly. The laser performance of heavy-doped Yb:YAG crystals was limited by the thermal population at terminated lasing level and thermal lens effect at room temperature without sufficient cooling of the samples. The laser emitting spectra of Yb:YAG microchip lasers with different Yb concentrations and output couplings are addressed with the local temperature rise, due to the absorption of the pump power inside the gain media under different pump levels. PACS 42.55.Xi; 42.70.Hj; 42.55.Rz     

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.     

Nonlinear losses in photoconductive Yb:YAG laser materials: identification of photocarrier properties by non-steady-state photoEMF

We report on the investigation of the photocarrier sign and the excitation dynamics in Yb:YAG at 940 nm excitation wavelength by non-steady-state photo-electromotive force technique. Annealed crystals with 25 and 50 % Yb-doping concentration demonstrate hole conductivity, while 80 % Yb:YAG displays electron conductivity. A characteristic relaxation time of approximately 1 ms is observed, which corresponds to the lifetime of the excited Yb³⁺-ion.     

Laser-diode-pumped Yb:YAG laser as a new pump source and its application to an Er3+, Yb3+-codoped high-output-power fiber amplifier

We propose a laser-diode-pumped Yb:YAG laser as a novel practical fiber amplifier pump source, and describe its first successful application of high-output-power operation (850 mW=+29.3 dB m) of an Er³⁺,Yb³⁺-codoped fiber amplifier at 1.55 μm. We have developed both bulk and microchip Yb:YAG lasers and obtained 3.3 W and 2.7 W, respectively, in the cw mode at room temperature. Laser-diode-pumped Yb:YAG lasers are shown to have a potential applicability to such fiber amplifiers as Pr³⁺-doped amplifiers and Tm³⁺-doped amplifiers, as well as Er³⁺,Yb³⁺-codoped fiber amplifiers. Received: 12 August 1999 / Revised version: 3 September 1999 / Published online: 20 October 1999     

Yb：YAG晶体中的荧光浓度猝灭现象

在Ｙｂ：ＹＡＧ晶体中发现浓度猝灭现象,对猝灭机制进行了分析研究,指出退火前晶体的荧光浓度猝灭现象主要由Ｙｂ＾２＋、色心和由此产生的晶格畸变所致高掺杂浓度时痕量稀土杂质离子的存在也将导致浓度猝灭,确定了Ｙｂ：ＹＡＧ晶体中Ｙｂ＾３＋的理想掺杂浓度。     

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.     

Spectral-kinetic studies of SrAlF5 doped by trivalent rare-earth ions

Photochemical properties of Ce³⁺:SrAlF₅ and Ce³⁺,Yb³⁺:SrAlF₅ single crystals together with spectroscopic and kinetic characteristics of several optically nonequivalent impurity centers and energy transfer between them are described. It is shown that co-activation by Yb³⁺ ions effectively suppresses color centers in Ce,Yb:SAF crystals. It was found out that in Ce,Yb:SAF crystals Yb ions exist simultaneously in 2+ and 3+ valent state. Three types of optically nonequivalent luminescent centers corresponding to the doublets in luminescence spectrum centered at 290, 305 and 370 nm (Ce^I, Ce^II, Ce^III, respectively) have been observed. Analysis of luminescence spectra and decays leads to the conclusion that there is no energy transfer between either cerium centers or from Ce³⁺ to Yb²⁺ apart from the Ce^III center which luminescence is slightly quenched by Yb²⁺.     

Tm3+/Yb3+ ∶LaF3纳米体系中掺杂Yb3+ 离子对Tm3+ 离子荧光发射的影响

为研究Yb³⁺离子浓度变化对Tm³⁺离子在蓝色波段荧光强度的影响,以NaF和La(NO₃)₃为原料,采用水热法制备了Tm³⁺和Yb³⁺共掺的Tm³⁺/ Yb³⁺∶LaF₃纳米颗粒.用X射线衍射对LaF₃纳米颗粒进行表征的结果显示,纳米晶体结构呈六方相.透射电镜的观测结果显示,纳米颗粒样品大小均匀、分散性良好.在波长为800 nm的激光激发下,观测到了上转换蓝光发射,其中包括波长为474 nm和479 nm的较强的荧光辐射(相应的跃迁为1G4→3H6)和波长位于450 nm的强度较弱的荧光发射(相应的跃迁为1D2→3F4).通过观测不同Yb³⁺离子浓度条件下共掺Tm³⁺/Yb³⁺∶LaF₃样品的荧光光谱,研究了Yb³⁺离子掺杂浓度对于Tm³⁺离子的荧光发射的影响,并探讨了产生这种现象的原因.研究结果显示,对于1G4→3H6跃迁产生的荧光发射(474 nm),当Yb³⁺离子浓度增大时,反向能量传递速率的增加导致了荧光强度的增大.然而,当Yb³⁺离子浓度增大到一定程度时,Yb³⁺离子激发态能级寿命的减少将引发荧光强度的下降.相比较而言,Yb³⁺离子的浓度的变化对于1D2→3F4跃迁产生的位于450 nm处荧光强度的影响较弱.     

Preparation and upconversion properties of Ba2ErF7 and Ba2ErF7:Yb powders

Novel Ba₂ErF₇ and Yb³⁺-doped Ba₂ErF₇ powders were synthesized by a coprecipitation method. In Ba₂ErF₇ sample, abundant upconverted emission bands from violet to infrared region are observed under 980 nm excitation, whereas only green and red emissions are observed under 812 nm excitation. Under the two excitations, the luminescence decay curves of the green and red emissions are measured and the quenching behaviors of Yb³⁺ doping are also explored. It is found that a suitable Yb³⁺ concentration can efficiently enhance the intensity ratio of the blue and violet emissions to the green and red ones, which may be due to the competition between the energy transfer process from Er³⁺ to Yb³⁺ and the sensitizing process from Yb³⁺ to Er³⁺ in Ba₂ErF₇:Yb³⁺. This indicates that the Yb³⁺-doped Ba₂ErF₇ might be a good candidate for blue and violet upconversion phosphor.     

Crystal growth, optical properties, and laser operation of Yb3+-doped NYW single crystal

Laser crystal Yb³⁺-doped NaY(WO₄)₂ (Yb:NYW) with excellent quality has been grown by Czochralski technique. The rocking curves from (400) plane of as-grown Yb:NYW crystal was measured and the full-width value at half-maximum was 19.92″. The effective segregation coefficients were measured by the X-ray fluorescence method. The polarized absorption spectra and the fluorescence spectra of Yb:NYW crystal were measured at room temperature. The fluorescence decay lifetime of Yb³⁺ ion in NYW crystal has been investigated. The spectroscopic parameters of Yb:NYW crystal are calculated and compared with those of Yb:YAG crystal. A continuous wave output power of 3.06 W at 1031 nm was obtained with a slope efficiency of 42% by use of diode pumping.     

Cooperative Processes in Nd3☎/Yb3☎ Co-Doped Yag Nanocrystallites

The cooperative processes in doubly doped Nd/Yb:YAG nanocrystallites were investigated. An extremely strong Yb^3? ion concentration influence on absorption intensities was noticed. An efficient Nd^3? → Yb^3? energy transfer in YAG nanocrystallites was found. However no back transfer was observed. The mechanisms of cooperative interactions between Nd^3? and Yb^3? ions in YAG nanocrystallites responsible for the observed concentration effects are discussed.     

Optical study of single crystals grown by the Czochralski method from Yb3+-doped (Gd1−x Y x )2SiO5 solid solution

Yb³⁺-doped crystals were grown by the Czochralski method from (Gd_0.5Y_0.5)₂SiO₅ melt employing seeds cut out from single crystals of Yb:Gd₂SiO₅ and Yb:Y₂SiO₅. XRD analysis revealed that the structure of the solid solution crystals grown is consistent with the structure of the seed, namely C2/c for the Yb:Y₂SiO₅ seed and P2₁/c for the Yb:Gd₂SiO₅ seed. Optical absorption spectra, emission spectra and luminescence decay curves were recorded at 10 K and 300 K. Analysis of gathered spectroscopic data made it possible to evaluate radiative transition rates for the ²F_5/2 multiplet of Yb³⁺ in the two structures and to assess crystal field splitting of multiplets involved in radiative transitions. It has been concluded that transition intensities and relaxation dynamics of Yb³⁺ in the systems studied are similar but the intensity distribution of emission and absorption bands depends significantly on the system structure. This feature, combined with inhomogeneous broadening of spectral lines, may be advantageous for the tailoring of lasers employing ytterbium-doped crystals for specific applications.     

Structural and optical characterization of Er3+/Yb3+-doped LiNbO3

We report the dependence of the unit-cell parameters and the extraordinary and ordinary refractive indices of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals. Both properties depend in a non-monotonic manner on the Er³⁺/Yb³⁺ content. A singularity was observed at concentrations of 1.1-1.2 mol. % in the crystal (0.6-0.7 mol. % in the melt). In the same way the Er and Yb concentration influences the periodically poled lithium niobate formation. The observed behavior of refractive indices and unit-cell parameters of Er³⁺/Yb³⁺-codoped LiNbO₃ crystals could be explained in terms of the RE³⁺-ion concentration affecting the Li-vacancy concentration and the RE³⁺-ion positions in the crystal. Received: 21 May 2001 / Revised version: 22 August 2001 / Published online: 23 October 2001     

Optical and EPR study of BaY2F8 single crystals doped with Yb

Optical and electron paramagnetic resonance study have been carried out on BaY₂F₈ single crystals doped with Yb ions at 0.5 and 10 mol%. The crystals have been obtained using the Czochralski method modified for fluoride crystal growth. Optical transmission measurements in the range of 190-3200 nm and photoluminescence measurements were carried out at room temperature. Absorption spectra of BaY₂F₈ single crystals doped with Yb due to the ²F_7/2→²F_5/2 transitions have been observed in the 930-980 nm range. To analyze the possible presence of Yb²⁺ ions in the investigated crystals, irradiation with γ-quanta with a dose of 10⁵ Gy have been performed. The observed photoluminescence bands show usual emission in IR and other one in VIS, being an effect of cooperative emission of Yb³⁺ ions and energy up-conversion transitions of photons from IR to UV-vis(visible) due to hoping process between energy levels of paired Yb³⁺ and Er³⁺, where Er³⁺ ions are unintentional dopants. The EPR spectra of BaY₂F₈:Yb 10 mol% consist of many overlapping lines. They have been analyzed in terms of spin monomers, pairs, and clusters. The angular dependence of the resonance lines positions have been studied also to find the location of coupled ytterbium ions in the crystal structure.     

不同Yb掺杂量的Yb:Y3Al5O12晶体的光谱分析

采用提拉法生长了Yb³⁺掺杂量分别为5.4at%, 16.3at%, 27.1at%, 53.6at%和100at%的Yb:Y₃Al₅O₁₂晶体.系统地表征和分析了Yb³⁺掺杂量对晶体吸收光谱和荧光光谱的影响.随着Yb³⁺掺杂量的增加，各峰值吸收系数呈线性增加的趋势.应用Smakula公式计算了各吸收峰对应的振荡强度，并分析了Yb³⁺掺杂量对振荡强度的影 关键词： 吸收光谱 荧光光谱 自吸收 浓度猝灭     

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.