Nonlinear theory of multiphonon relaxation of excited rare-earth ions in laser crystals

A general formula for the probability of multiphonon nonradiative transitions between J multiplets of 4f ^N states of trivalent rare-earth ions in a crystal derived for a nonlinear mechanism is discussed. A technique is developed for calculating the quantities involved in this formula. Particular attention is given to calculating the spectral density J ^(p)(Ω), which is the Fourier transform (at the transition frequency) of the pth power of the correlation function K(t) of host ion displacements. Based on the central limit theorem from probability theory, an approximation is proposed for the spectral density J ^(p)(Ω). The theoretical values of nonradiative multiphonon transition rates are compared with experimental ones.     

Spectroscopic properties of Sm3+ ions in lead fluorophosphate glasses

The Sm³⁺-doped lead fluorophosphate glasses of composition 44P₂O₅–17K₂O–9Al₂O₃–(24?x)PbF₂–6Na₂O–xSm₂O₃, where x=0.01, 0.05, 0.1, 0.5, 1.0 and 2.0 mol%, have been prepared by conventional melt quenching technique and are characterized through differential thermal analysis, Raman, absorption and emission spectra and decay rate measurements. Free-ion Hamiltonian model for energy level analysis and Judd–Ofelt theory for spectral intensities have been used to analyze the spectroscopic properties of Sm³⁺ ions in lead fluorophosphate glasses. The decay rates for the ⁴G_5/2 level of Sm³⁺ ions have been measured and are found to be single exponential at lower concentration (≤0.1 mol% Sm₂O₃) and turn into non-exponential at higher concentrations (≥0.5 mol% Sm₂O₃) due to energy transfer through cross-relaxation. The experimental lifetimes for ⁴G_5/2 level of Sm³⁺ ions are found to decrease from 2.54 to 0.92 ms when the concentration increased from 0.01 to 2.0 mol% Sm₂O₃ due to energy transfer. In order to know the nature of the energy transfer mechanism, the non-exponential decay rates are well fitted to Inokuti–Hirayama model for S=6, which indicates that the energy transfer process is of dipole–dipole type.     

Intensity and bandwidth of multiphonon vibronic transitions of rare-earth ions in crystals

The theory of multiphonon vibronic coupling to electronic transitions is applied in analysing fluorescence spectra of Eu²⁺ in BaFCI, which consist of the 4f⁷(⁶P_7/2,) → 4f⁷(⁸S_7/2) and 4f⁶5d → 4f⁷ transitions, and the 4f⁷-4f⁶5d excitation spectrum of Ce³⁺ in YPO⁴. The 4f electrons are weakly coupled to lattice vibration modes so that only weak one- and two-phonon sidebands are observable in the 4f-4f optical transitions, whereas the electron-phonon coupling is significantly stronger for a 5d electron. Accordingly, intensive multiphonon vibronic transitions overwhelmingly dominate the 4f⁶5d → 4f⁷ spectrum. It is shown that the extended Judd-Ofelt theory for weak vibronic coupling in the framework of the M-process is equivalent to the Huang-Rhys theory for the δ-process. In the analysis of experimental data, contributions from local ligand modes and lattice acoustic modes are separated, and the coupling strength is evaluated, in terms of the Huang-Rhys parameter S, for the 4f-4f and 5d-4f vibronic transitions.     

Influence of borate content on the radiative properties of Nd ions in fluorophosphate glasses

Spectroscopic properties of Nd³⁺ in barium fluoroborophosphate glassy matrix have been analyzed by fitting the experimental data with the standard Judd-Ofelt theory. Various spectroscopic parameters viz. radiative transition probabilities, radiative decay time, fluorescence branching ratios, electric dipole line strengths, stimulated emission cross-sections and optical gain of the principal fluorescence transitions from the ⁴F_3/2 metastable level are obtained. Results show that addition of borate content to the fluorophosphate matrix will reduce the fluorescence spectral properties of Nd³⁺. However, the radiative properties of the present fluoroborophosphate glassy matrix are found to be well improved over that of pure borate and phosphate matrix and is attributed to the influence of fluorine content in the glassy matrix. The changes in the position and the Judd-Ofelt intensity parameters are correlated with the structural changes in the host glass matrix. The shift of the hypersensitive band shows that the covalency of the rare earth to oxygen bond increases with the increase of Na₂O content. This covalency effect and the formation of the BO₄ groups with the addition of Na₂O content are responsible for the increase in the radiative properties of the present system. Quantitative estimation of the non-radiative processes such as multiphonon relaxation and quenching by water content was carried out and the results show that both are below the critical level for optimum laser performance.     

Er3+掺杂Ge-Ga-S-KBr硫卤玻璃的中红外发光和多声子弛豫的研究

用熔融急冷法制备了系列不同Er3+离子掺杂浓度的Ge-Ga-S-KBr硫卤玻璃,测试了样品折射率、吸收光谱、中红外荧光光谱。通过吸收光谱计算了Er3+离子吸收谱线的振子强度,应用Judd-Ofelt理论计算分析了Er3+离子在Ge-Ga-S-KBr硫卤玻璃中的强度参数Ωi ( i = 2 , 4 , 6) 、自发辐射跃迁几率A、荧光分支比?和辐射寿命?rad等光谱参数。研究了808nm激光泵浦下样品中红外荧光特性与掺杂浓度之间变化关系,并用Futchbauer-Ladenburg公式分别计算了2.8?m处的受激发射截面。结果表明,在808nm 激光泵浦下观察到了2.8?m中红外荧光,分别对应于Er3+： 4I11/2?4I13/2跃迁,当Er3+离子掺杂浓度从0.4wt%增加到1.0wt%时,中红外荧光强度都随相应增加,计算的Er3+:4I11/2?4I13/2跃迁多声子驰豫速率分别为37 s-1。     

Luminescent properties of cadmium selenide quantum dots in fluorophosphate glasses

The optical properties of fluorophosphate glasses with CdSe quantum dots are studied. Secondary heat treatment at a temperature exceeding the glass transition temperature resulted in the formation of quantum dots with sizes of 3.7–6.2 nm. The influence of the semiconductor component concentration on the spectral-luminescent characteristics of glasses is shown. It is experimentally demonstrated that glasses with a lower CdSe concentration have a higher absolute luminescence quantum yield.     

Spectroscopic properties of Er/Yb codoped fluorophosphate glasses

Er³⁺/Yb³⁺ codoped fluorophosphate glasses were prepared and their thermal stabilities, Raman spectra, absorption spectra, and fluorescence spectra were measured. It is found that proper content of NaF or PbF₂ is helpful for the increase of stability against crystallization. The variation of Al(PO₃)₃ or NaF content in the composition affects not the maximum phonon energy but the phonon density. The introduction of PbF₂ decreases the phonon energy slightly. Intense green and red upconversion luminescence was observed for the fluorophosphate glass with low phosphate content. A glass matrix for upconversion luminescence requiring neither expensive raw material nor special atmospheric conditioned preparation is provided. Infrared luminescence around 1530 nm was researched. Fluorophosphate glasses with bandwidth properties and stimulated-emission cross sections better than tellurite, germanate and silicate glasses are obtained. Through the introduction of NaF, the bandwidth properties are decreased. Through the introduction of PbF₂ the gain properties are increased. On the whole, it is difficult to obtain a material with the best gain properties and bandwidth properties simultaneously. There should be a compromise between them according to the demand.     

Multiphonon relaxation of excited rare-earth ions in ionic matrices

A theory of multiphonon relaxation for rare-earth ions in ionic crystals and glasses is presented. The theory is based on the static coupling scheme. The electron-vibration coupling is assumed to be linear with respect to the relative ligand rare-earth displacements. The multiphonon transitions are induced by higher-order lattice anharmonicity of the ionic motions. The probability of multiphonon transitions could be related to the IR multiphonon spectrum. A simplified form of the general expression is proposed. The electronic part of the expression obtained has exactly the same form as the corresponding expression in the well-known Judd-Ofelt theory for optical transitions between multiplets of rare-earth ions in solids.     

EPR and spin-lattice relaxation of rare-earth ions in LiLuF4 monocrystals

The EPR and spin-lattice relaxation are studied of impurity rare-earth ions in LiLuF₄ crystals at liquid-helium temperatures. It is detected that paramagnetic relaxation of Er³⁺ ions is retarded by the effect of the phonon throat. The effect of resonance attenuation of the phonon throat is clarified in two-phonon resonance relaxation processes of Er³⁺ ions. The Debye temperature of the crystal is determined from an analysis of experimental results.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 24–27, February, 1988.     

Identification of the relative distribution of rare-earth ions in phosphate glasses

The relative distribution of rare-earth ions R3+ (Dy3+ or Ho3+) in the phosphate glass RAl(0.30)P(3.05)O(9.62) was measured by employing the method of isomorphic substitution in neutron diffraction. It is found that 7.9(7) R-R nearest neighbors reside at 5.62(6) A in a network made from interlinked PO4 tetrahedra. Provided that the role of Al is explicitly considered, a self-consistent account of the local matrix atom correlations can be developed in which there are 1.68(9) bridging and 2.32(9) terminal oxygen atoms per phosphorus.     

Influence of rare-earth ions on the dielectric response in stillwellite glasses

Experimental spectra of the dielectric response function of glasses with the composition LnBGeO₅ (Ln = La, Pr, Tb, Er, Sm) in the terahertz and infrared ranges are compared. Absorption bands are identified, and their contributions to permittivity are established. The contours of the boson peak and the dependence of the dielectric loss on the type of rare-earth ion are determined.     

Yb3+掺杂铝氟磷酸盐玻璃的光谱和激光性能

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Spectroscopic properties of Tm3+ in fluorophosphate glasses

The optical properties of Tm³⁺ in a fluorophosphate glass are reported. Oscillator strengths and Ω_λ parameters for Tm³⁺ were calculated. Energy transfer between Ce³⁺ → Tm³⁺, Tm³⁺ → Eu³⁺ and Tm³⁺ → Tb³⁺ were measured at room temperature and interpreted using the Yokota and Tanimoto scheme. Fluorescence decay results seem to support the hypothesis of a diffusion-limited relaxation. The effects of Tb³⁺ and especially Ce³⁺ on the fluorescence of Tm³⁺ ion point out the interest of these dopants for obtaining a better emission of Tm³⁺ at 450 nm.     

Yb3+掺杂铝氟磷酸盐玻璃的光谱和激光性能

研究了Yb3 掺杂铝氟磷酸盐 (AFP)玻璃的吸收光谱、荧光光谱 ,测量了Yb3 离子的荧光有效线宽 (Δλeff>5 5nm)以及2 F5 2 能级的荧光寿命 (τmax=2ms)及随掺杂浓度的变化 .应用倒易法计算了Yb3 的发射截面 ,其发射截面可达 0 6 6 82 3pm2 ,且激光增益系数τfσemi达 1 2 89ms.pm2 .评估了Yb3 在AFP玻璃中的激光性能 ,发现其具有较理想的激发态最小粒子数 (0 15 )、饱和抽运强度 (8 3kW cm2 )和最小抽运强度 (1 2 4 5kW cm2 )值及良好的热稳定性 .研究结果表明掺Yb3 氟磷酸盐玻璃是实现高功率超短可调谐激光器的理想增益介质 .     

Multiphonon relaxation of rare earth ions in borate,phosphate, germanate and tellurite glasses

Non-radiative multiphonon relaxation rates were obtained for excited electronic states in borate, phosphate, germanate and tellurite glasses. The rates were calculated from the intensities of fluorescence in the visible range of the spectrum, the measured radiative transitions and the decay times of fluorescence. A functional dependence was found between the relaxation rates and the energy gaps of the rare earth ion. It was shown that by changing the glass host from the borate to tellurite matrix, an increase in visible fluorescence was achieved. This was especially notable in Er³⁺, where the increase of fluorescence from germanate to tellurite was by a factor of 15.     

Spectroscopic properties of Mn2+ in new bismuth and lead contained fluorophosphate glasses

Luminescence, absorption and electron spin resonance (ESR) spectroscopic measurements are performed for two new Mn²⁺ doped fluorophosphate glass systems containing bismuth and lead, respectively, i.e., 45Ba(PO₃)₂-55BiF₃ and 45Ba(PO₃)₂-55PbF₂, to elucidate the nature of dopant–ligand bonding in terms of the covalence degree of dopant–ligand bond and ligand field strength. It is found from luminescence measurements that an octahedral coordination of dopant is formed, which is the same as that in fluoroberyllate and phosphate glasses. The optical absorption measurements indicate that the position of the absorption band, ⁴T_1g(G), is red shifted, and the Racah coefficient B is 625 and 627 cm^-1, respectively, as ligand field strength increases in the order of fluoroberyllate, fluorophosphate, phosphate and silicate glasses. The hfs constant A determined from the ESR measurements is 94.33 and 92.12 Oe for two glass systems, respectively. The hfs constant A is also found to be 89.98 Oe and 94.33 Oe in the respective crystalline and glasses 45P₂O₅-55BiF₃. In addition, the resonance observed at g2 in both glass systems is mainly attributed to Mn²⁺ ions in an environment close to octahedral symmetry distorted by agglomeration of resonant centers. It, thus, can be concluded from the ESR measurements that the degree of covalency of dopant–ligand bond in both fluoroberyllate and phosphate glasses is similar, and the crystallization of the glass leads to an increase in the degree of covalency of the ligand–Mn²⁺ bond. PACS 42.70.Ce; 61.43.Fs; 78.55.Qr