Role of monovalent alkali ions in the Yb3+ centers of CaF2 laser crystals

Yb³⁺ and M⁺ monovalent alkali ions (M⁺ = Li⁺, Na⁺, K⁺)-co-doped CaF₂ cubic laser crystals were grown by the micro-pulling-down method (μ-PD) under CF₄ atmosphere. Structural and spectroscopic characterizations of Yb³⁺ in substitution of Ca²⁺ (absorption, emission and decay curves) were carried out to study the effect of M⁺ ions as charge compensators.     

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     

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.     

Study of Nd2+ absorption in x-irradiated CaF2, SrF2, BaF2 crystals

The absorption spectra of x-irradiated alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals doped with Nd³⁺ ions have been investigated. X-irradiation results in creating the absorption bands of inter-configuration 4fⁿ–4fⁿ⁻¹–5 d¹ transitions of Nd²⁺. The charge reduction of the neodymium by irradiation is not temperature-stable and the ions reoxidation (Nd²⁺ → Nd³⁺) occurs under heating to 570 K in CaF₂, 520 K in SrF₂ and 470 K in BaF₂.     

Clusters of group-III ions in activated fluorite-type crystals

The data on optically detected EPR in absorption bands of alkaline-earth fluoride crystals doped with rare-earth (Er, Tm, Yb, Lu) or yttrium activators indicate that these crystals contain clusters similar to Y₆F₃₇, which is the structural unit of the naturally occurring mineral tveitite and of synthetic yttrofluorite [(CaF₂)_1?y(YF₃)_y] crystals, whose lattices exhibit a superstructure at certain values of y. Starting from a rare-earth ion concentration of the order of 0.1%, the greater part of the rare-earth impurity is concentrated in these clusters, having a tendency to aggregate into larger clusters. Clusters are also of considerable importance in semiconducting fluorite-structure crystals of CdF₂ doped with Ga, In, or Y ions, as indicated by microwave and IR radiation absorption in these crystals.     

Divalent rare-earth ions in LaF<Subscript>3</Subscript> crystals

The optical spectra and electric conductivity of LaF₃ crystals doped with 0.01, 0.1, and 0.3 mol % YbF₃, where Yb was partly or completely recharged to the divalent state, are studied. The long-wavelength absorption band of 370 nm is caused by electrons transitioning from state 4f ¹⁴ to the level of anion vacancies. The remaining bands at 300–190 nm are caused by 4f ¹⁴–5d ¹4f ¹³ transitions in Yb²⁺. The bulk electric conductivity and peaks of the dielectric losses of LaF₃–Yb²⁺ crystals are caused by Yb²⁺–anion vacancy dipoles. The activation energy of the reorientation of Yb dipoles is 0.58 eV. The optical and dielectric properties of Yb²⁺ centers are compared to those of Sm²⁺ and Eu²⁺ centers studied earlier in LaF₃ crystals.     

Lead fluorosilicate glass ceramics doped with Nd3+, Er3+, and Yb3+

Glasses in the PbF₂-PbO-SiO₂ system doped with 1 mol % of rare-earth elements (Nd³⁺, Er³⁺, or Yb³⁺) are synthesized and studied. The glasses were heat-treated in order to obtain glass ceramics with a fluoride crystalline phase. The changes in the structure and spectral optical properties of glass ceramics with respect to initial glasses were determined by using X-ray diffraction analysis and by studying the luminescent characteristics of dopant ions.     

Growth and optical properties of Yb and Tb codoped BaB2O4 crystals

Optical absorption and luminescence spectra of ytterbium and terbium codoped BaB₂O₄ (β-BBO and α-BBO) crystals grown in different conditions have been studied. Low-temperature absorption peaks were observed in all samples. Features related to rare earth ions were observed in absorption and luminescence spectra. Absorption and emission in the range 860-1000 nm are caused by ²F_5/2↔²F_7/2 transitions in Yb³⁺ ions. Emission peaks at 500, 550, 590 and 630 nm correspond to ⁵D₄→⁷F₆, ⁷F₅, ⁷F₄, and ⁷F₃ transitions of Tb³⁺ ions, respectively. The probable reasons of variations in spectroscopic features related to Yb in BBO host are discussed. It has been shown that the replacement of Ва²⁺ by Yb³⁺ in the lattice of ВаВ₂О₄ results in the decrease in the symmetry of oxygen surrounding of Yb³⁺.     

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     

Gamma-ray induced color centers in Yb:YAG crystals grown by Czochralski method

20 at.% Yb:YAG single crystals have been grown by the CZ method and gamma-ray irradiation induced color centers and valence change of Fe³⁺ and Y b³⁺ ions in Yb:YAG have been studied. One significant 255 nm absorption band was observed in as-grown crystals and was attributed to Fe³⁺ ions. Two additional absorption (AA) bands located at 255 nm and 345 nm, respectively, were produced after gamma irradiation. The changes in the AA spectra after gamma irradiation and air annealing are mainly related to the charge exchange of the Fe³⁺, Fe²⁺, oxygen vacancies and F-type color centers. Analysis shows that the broad AA band is associated with Fe²⁺ ions and F-type color centers. The transition Y b³⁺→Y b²⁺ takes place as an effect of recharging of one of the Y b³⁺ ions from a pair in the process of gamma irradiation.     

From optical spectroscopy to a concentration quenching model and a theoretical approach to laser optimization for Yb3+-doped YLiF4 crystals

A spectroscopic characterization was carried out to identify crystal-field levels for magnetic-dipole transitions of Yb³⁺ ions located in the Y³⁺ dodecahedral S₄ crystallographic site in YLiF₄ (YLF) crystals which were grown either by the Czochralski technique or by the laser heated pedestal growth (LHPG) technique. The concentration dependence of the measured decay time of the ²F_5/2 excited level of Yb³⁺ was analysed in order to understand relevant concentration quenching mechanisms. Under Yb³⁺ ion infrared pumping, self-trapping and up-conversion non-radiative energy transfer to trace rare-earth impurities (Er³⁺, Tm³⁺) has been observed over the visible region and interpreted by a limited-diffusion process within the Yb³⁺ doping ion subsystem to the impurities. The principal parameters useful for a theoretical approach for potential laser applications of Yb³⁺-doped YLiF₄ crystals have also been given.     

The optical properties of Yb3+ ions in LiTaO3:Nd,Yb crystals

3+ ions excited by energy transfer from Nd³⁺ ions in LiTaO₃:Nd, Yb crystals are presented. The emission band of Yb³⁺ ions is broad, due to the strong phonon-coupling and to the relative large Stark-splitting of the ground ²F_7/2 multiplet. The emission cross-section was evaluated by the reciprocity method, and a value of 0.53×10^-20 cm² was obtained. The gain coefficients derived for the inversion parameters in the range 0.05 to 0.5 indicate positive gain in the 985–1070 nm range. Received: 17 March 1997/Revised version: 10 June 1997     

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.     

Concentration quenching in Yb:YAG

The concentration quenching in Yb:YAG crystals with high Yb³⁺ doping level was demonstrated, and studied as well. The color center and lattice distortion which originated from Yb²⁺ are the main reasons for producing the concentration quenching in unannealed Yb:YAG crystals, and after annealing at 1600°C in oxygen atmosphere for 24 h, Yb²⁺ vanished. Trace impurity ions are also responsible for this phenomenon.     

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.     

Magneto-optical spectroscopy and optical detection of EPR spectra of Yb<Superscript>3+</Superscript> paramagnetic centers with cubic symmetry in single crystals <Emphasis Type="Italic">Me</Emphasis>F<Subscript>2</Subscript> (<Emphasis Type="Italic">Me</Emphasis> = Cd,Ca, Pb)

Cubic paramagnetic centers formed by Yb³⁺ impurity ions in fluorite-type crystals MeF₂ (Me = Cd, Ca, Pb) have been investigated using electron paramagnetic resonance, magnetic circular dichroism, magnetic circular polarization of luminescence, Zeeman splitting of optical absorption and luminescence lines, and optical detection of electron paramagnetic resonance. The g factors of the ²Γ₇ state in the excited multiplet ² F _5/2 of Yb³⁺ ions in Me F₂ crystals, the hyperfine interaction constant ¹⁷¹ A (¹⁷¹Yb) for the excited multiplet ² F _5/2 in the CaF₂ crystal, and the energies and symmetry properties of all energy levels of Yb³⁺ ions in MeF₂ crystals are determined. The crystal-field parameters for the crystals under investigation are calculated.     

EPR of Yb3+ ions in Ba1−xLaxF2+x mixed crystals

Electron paramagnetic resonance (EPR) spectra of impurity Yb³⁺ ions (about 0.1 at.%) in mixed crystals BaF₂(1-x) plus LaF₃(x) have been investigated for different values of the concentrationx at a frequency of about 9.5 GHz by both continuous-wave (CW) EPR and electron spin echo methods. A spectrum of trigonal symmetry with a complex hyperfine structure is observed in “pure” BaF₂:Yb³⁺ (x=0). Upon admixture of small amounts of LaF₃ (x=0.001), additional EPR lines arise with intensities increasing with the increase ofx up to 0.005. These lines are attributed to trigonal centers including two rare-earth ions and two compensating fluorine ions. A further increase ofx results in a decrease of the total EPR spectrum intensity, and atx≥0.05 the CW resonance becomes practically unobservable. This may be due to the formation of rare-earth ion clusters with paramagnetic Yb³⁺ ions occurring in domains with a disordered structure of surroundings resulting in very broad EPR lines, which cannot be registered by CW EPR. Indeed, very broad (not less than 1 KG) EPR lines were observed by the electron spin echo method for concentrationsx<-0.02.     

Divalent rare-earth ions Pr,Sm, Нo,Er, Tm,and Yb in crystals of alkaline-earth fluorides

The absorption spectra of radiation-colored CaF₂, SrF₂, and BaF₂ crystals activated by trivalent Pr, Sm, Нo, Er, Tm, and Yb (rare-earth, RE) ions are studied. It is shown that ionizing radiation reduces the impurity ions to the divalent state. The temperature resistance of divalent RE ions of radiation-colored CaF₂ crystals correlates with the chemical stability of the compounds with divalent RE ions. The photochromic centers are produced in CaF₂-Pr crystal colored by radiation at room temperature and heated to 200°C.     

Spectral properties and energy transfer of Yb,Er:GdVO4 crystal

GdVO₄ single crystal co-doped with Yb³⁺ and Er³⁺ was grown by the Czochralski method. The X-ray powder diffraction pattern of Yb,Er:GdVO₄ crystal confirms that the as-grown crystal is isostructural with pure GdVO₄ crystal. Its polarized absorption spectra and non-polarized fluorescence spectra were measured at room temperature. The absorption band at 984 nm for π-polarization has an FWHM of about 36 nm, which is favorable for InGaAs LD laser pumping. The spectrum properties of Er³⁺ in Yb,Er:GdVO₄ crystal were investigated based on Judd–Ofelt theory. There is strong energy transfer from Yb³⁺ to Er³⁺ in this crystal. When excited with 980 nm radiation, this crystal emitted strong fluorescence at about 1529 nm and 552.5 nm. The total energy transfer rate and efficiency from Yb³⁺ to Er³⁺ is 3.33 ms^-1 and 67%, respectively. The energy transfer between Er³⁺ and Yb³⁺ ions is a multistep transfer process, and was investigated based on a random-walk model. The investigation result shows that there is strong cooperative-sensitization effect from Yb³⁺ to Er³⁺, which is the main upconversion energy-transfer process in this crystal. PACS 42.70.Hj; 81.10.Fq; 42.55.Rz     

Up and down conversion fluorescence studies on combustion synthesized Yb/Yb: MO-Al2O3 (M=Ca, Sr and Ba) phosphors

The ytterbium ions doped MO-Al₂O₃ (M=Ca, Sr and Ba) phosphors have been synthesized through combustion technique and their up and down conversion fluorescence properties have been studied and compared. The samples were calcinated at different temperatures and their FTIR and XRD spectra have shown a close relationship. With 976 nm excitation all these phosphors show cooperative upconversion emission at 488 nm from the pairs of two Yb³⁺ ions along with an unexpected broad upconversion band in the blue green region and has been assigned to arise from the defect centers. Contrary to this upconversion emission, calcium aluminate phosphor exhibits bright and very broad down-conversion fluorescence (FWHM≈160 nm) upon UV (266 nm) excitation due to Yb²⁺ ions. The inter-conversion between the 3+ and 2+ valence states of Yb ion has been observed on calcinations of samples in open atmosphere and has been correlated to the emission properties. The Yb²⁺ ions containing calcium aluminate phosphor has been found suitable for producing broad band light in the visible region (white light). Lifetime of the emitting states of Yb³⁺ and Yb²⁺ ions have also been measured and discussed.