Spectral properties of Yb ions in LiNbO3 single crystals: influences of other rare-earth ions, OH ions, and γ-irradiation

Absorption spectra have been studied in 190–3100 nm region at various temperatures from 16 to 292 K for Yb³⁺-doped and Yb³⁺/Nd³⁺-, Yb³⁺/Er³⁺- and Yb³⁺/Pr³⁺-co-doped LiNbO₃ single crystals before and after γ-irradiation with a dose of 10⁵–10⁷ Gy. Intense 400 and 500 nm absorption bands were observed after γ-irradiation, which are due to the creation of oxygen vacancy (F-type color center) and Nb⁴⁺ polaron, respectively. Different change was observed in the 2870 nm OH⁻ absorption band intensity among the various rare-earth doped crystals. These are interpreted by discrepancy of ionic radii between substituting rare earth dopant ion and Li⁺ or Nb⁵⁺ host ion. The observed temperature dependence of the hot bands is understood by electronic transition from the thermally populated ²F_7/2 Stark levels to the excited ²F_5/2 level. The position of the Yb^{3+ 2}F_7/2→²F_5/2 first resonant line was observed to be slightly different among the co-doped crystals. This is due to the perturbation of Yb³⁺ by co-doped rare earth ion which is located at the neighborhood of theYb³⁺.     

Luminescence from tungstate functionalized sodalites involving NIR states

Sodalite derivatives of the Hauyne and Noseane type containing rare earth ions and the tungstate group have been prepared via zeolite precursors and in direct syntheses and studied with respect to luminescence phenomena involving low energy states. While the parent zeolites do not yield noticable luminescence of Nd³⁺, Er³⁺, Yb³⁺, Ho³⁺, their conversion to sodalites allows the observation of near infrared emission and even of upconversion processes.     

Properties of ytterbium and neodymium doped alkali metal yttrium double phosphates of the M3Y1−xLnx(PO4)2 type

The spectroscopic behaviour of the Nd³⁺ and Yb³⁺ doped alkaline metal yttrium double phosphates, M₃Y_1−xLn_x(PO₄)₂ (M=Na, Rb; x=0.01–0.3) were studied for both powder and single crystal samples. The high resolution absorption and emission spectra were measured in the visible and IR regions. Spectral changes with the Nd³⁺ and Yb³⁺ concentration were interpreted. The absorption strengths of the 4f–4f transitions were analysed and used to assess the structural modifications of the two double phosphates. Based on the 4 K absorption spectra the number of metal sites occupied by the dopants was investigated.

Strong emission from Na₃Y_1−xNd_x(PO₄)₂ involving the ⁴F_3/2→⁴I_9/2,⁴I_11/2,⁴I_13/2,⁴I_15/2 transitions were observed whereas the corresponding emission from the rubidium phosphate was presumably quenched by multiphonon processes due to the water molecules absorbed in the channel-like structure.

The IR spectra were used to assign the vibronic components of the electronic transitions. The Yb³⁺ emission bands were broadened depending on the Yb³⁺ concentration (1–10 mol%). The tentative energy level scheme of the ground and excited ²F_J (J=7/2, 5/2) levels was described.     

Low temperature thermoluminescence properties of Eu and R doped CaAl2O4

The thermoluminescence (TL) of calcium aluminate persistent luminescence materials doped with Eu²⁺ and co-doped with R³⁺ ions were studied between 20 and 325 K. The basic material, CaAl₂O₄:Eu²⁺, showed three TL bands between 150 and 300 K in the glow curve. Changing the R³⁺ co-dopant to the adjacent element in the rare earth series affects significantly the thermoluminescence intensity and even the position of the glow curve maximum. The physical effect seemed to be removing the traps since the La³⁺, Y³⁺, and Lu³⁺ ions suppressed the thermoluminescence. The Pr³⁺, Ho³⁺, and Dy³⁺ co-doping enhanced mainly the low temperature traps and these materials have an intense but relatively short persistent luminescence at room temperature. The Nd³⁺ and Tm³⁺ ions enhanced the TL bands close to room temperature and are thus the most suitable co-dopants to induce intense and long persistent luminescence. The quenching of the thermoluminescence by Sm³⁺ was concluded to be due to the presence of Sm²⁺ that removes totally the traps.     

Spectroscopic properties of Yb: LuLiF4 crystal grown by the Czochralski method for laser applications and evaluation of quenching processes: a comparison with Yb: YLiF4

Spectroscopic properties of Yb³⁺ ion in LuLiF₄ (LLF) laser host are presented here for the first time. Czochralski technique was used to grow undoped and Yb³⁺-doped LLF single crystals under CF₄ atmosphere. Detailed analysis of Yb³⁺-doped LLF spectroscopy were made to contribute to the determination of energy levels in this host and a comparison with the isomorphic YLiF₄ (YLF) laser host is done. We are dealing with temperature and concentration dependences of both π and σ polarizations of the infrared (IR) absorption and emission spectra. Raman spectra were also used to give an attempt for the interpretation of electronic and vibronic levels. Concentration dependence of fluorescence lifetimes allows the measurement of the high radiative lifetime in the range of 2–3 ms and shows a strong self-trapping process. Self-quenching was not seen by the reduction of the decay times but observed by non-radiative up-conversion energy transfer due to the presence of Er³⁺ and Tm³⁺ ions as unexpected impurities. Contrary to oxide crystals this process still remains lower than the self-trapping process. Yb³⁺ pairing and clustering were investigated as well. Evaluation of the laser potentiality of this host by the evaluation of figure-of-merit developed by our group is presented.     

Optical properties of single doped Cr and co-doped Cr–Nd aluminum tantalum tellurite glasses

The optical properties for single doped Cr³⁺ and co-doped Cr³⁺–Nd³⁺ aluminum tantalum tellurite glasses have been studied as a function of temperature. For the single doped glass, the existence of two bands in the emission spectra at low temperature indicates the presence of two different sites for the Cr³⁺ ions, labelled as usual as low- and high-field sites. The broad band centred in the Near Infrared region, corresponds to low-field sites transition ⁴T₂→⁴A₂, and the narrow band centred at approximately 715 nm to the high-field sites transition ²E→⁴A₂. The emission intensity for both high- and low-field sites shows a strong decrease with increasing temperature, with the emission for the former sites vanishing at RT. In both cases the quenching observed with the increase of temperature can be ascribed to the presence of non-radiative relaxation mechanisms. Experimental observations for the co-doped glass show that both radiative and non-radiative energy transfer processes from Cr³⁺ to Nd³⁺ are present.     

The level structures of Yb in AYbI3 (A=K, Rb, Cs) in the solid state

The absorption and fluorescence spectra of Yb²⁺ in AYbI₃ (A=K, Rb, Cs) in the solid state were measured. The level structures of the Yb²⁺ ion in iodide crystals were obtained, according to the J₁Γ_n hypothesis on the lower crystal symmetry. At the same time, the spectra were assigned by the level diagrams.     

Ionic conductivities of polymeric solid electrolyte films containing rare earth ions

Polymeric solid electrolyte films containing rare earth metal ions (Ce³⁺, La³⁺, Yb³⁺) have been prepared, with a view to applying them to solid-state electrochemical devices. The films, composed of poly(ethylene oxide)-grafted poly(methylmethacrylate) (PEO–PMMA), were prepared by dissolving rare earth salts with appropriate amounts of poly(ethylene glycol) dimethyl ether (PEG) that have ethylene oxide units in their structure. The ionic conductance behaviour of the polymeric composite electrolyte systems was investigated by AC impedance and DC polarization methods in an ambient temperature range. The oligo(ethylene oxide) units in the polymer matrix and the PEG components ensured the dissociation of the salts and the high mobility of the resulting ionic species in the solid films. About 10⁻⁵ S cm⁻¹ or above of ionic conductivity was obtained for a PEO–PMMA/PEG/Ce(ClO₄)₃ system at room temperature. The addition of a liquid plasticizer in the composite improved the conductivity by about two orders of magnitude.     

Net-value of the relativistic crystal field effect

The relativistic crystal field theory in case of the rare earth ion doped materials is presented. In order to calculate the values of crystal field splittings the effectively relativistic crystal field operators are applied. The matrix elements of these operators are evaluated in the non-relativistic LS scheme, whereas the radial integrals include one electron Dirac–Fock wavefunctions. The numerical results of the relativistic crystal field effects are presented for Ce³⁺ or Yb³⁺ ions in LuPO₄, and they are compared with those obtained from the non-relativistic approach.     

Near-infrared to visible photon upconversion in Mn and Yb containing materials

Near-infrared (NIR) laser excitation into the Yb^{3+ 2}F_7/2→²F_5/2 absorption transition around 10,200 cm⁻¹ at 12 K leads to red and green upconversion emission in MnCl₂:Yb³⁺ and Zn₂SiO₄:Yb³⁺; Mn²⁺, respectively. The photon upconversion (UC) emission is centered around 15,240 cm⁻¹ for MnCl₂:Yb³⁺ and around 19,050 cm⁻¹ for Zn₂SiO₄:Yb³⁺; Mn²⁺ and is ascribed to the Mn^{2+ 4}T₁→⁶A₁ transition in both compounds. The shift of the Mn²⁺ emission energy is due to the different coordination and ligand strength. Pulsed measurements indicate a sequence of ground-state absorption (GSA) and excited-state absorption (ESA) steps for the upconversion process. We conclude that the UC process in both compounds occurs by an exchange mechanism involving Yb³⁺ and Mn²⁺ ions. Zn₂SiO₄:Yb³⁺; Mn²⁺ is the first example of a Yb³⁺–Mn²⁺ upconversion system to show visible (VIS) by eye Mn²⁺ UC emission at room temperature.     

Comparative analyses of Nd (4f) energy level structures in various crystalline hosts

We have performed an in-depth analysis of the energy-level structures in seven Nd³⁺ (4f³) crystal systems: [Nd(H₂O)₉]·3CF₃SO₃, Nd³⁺: Cs₂NaGdCl₈, four Nd³⁺-doped garnets (Nd³⁺: A₃B₅O₁₂), and Nd³⁺: CsCdBr₃. A model Hamiltonian employing 20 free-ion operators and the appropriate one-electron crystal-field interaction operators (plus selected two-particle correlation crystal-field (CFF) operators) was diagonalized within the full 364 SLJM, basis of the f³ electronic configuration. Ample spectroscopic experimental data allowed us to use least-squares fitting routines to produce a crystal-field energy-level structure for Nd³⁺ in each host. Particular attention is given in this report to some differences in the values of the free-ion parameters among the seven hosts and to the strong effects of three CCF operators on crystal-field level ordering in specific multiplets. The effects of CCF operators (originally proposed by Judd and later studies by Reid) are shown to be a small perturbation on the standard one-particle (B_q^k) formalism.     

Notorious discrepancies in crystal field analyses eliminated by configuration interaction

Some conspicuous discrepancies between experimental and calculated energy levels in the crystal field analysis of rare earth (4f²) or actinide (5f²) electronic spectra are eliminated if one includes in the interaction matrix the participation of the 4f¹6p¹ (or 5f¹7p¹) excited configuration in addition to the ground configuration. This was recently demonstrated in the case of the tetravalent uranium U⁴⁺ in the cubic centrosymmetric sites of elpasolithe compounds (M.D. Faucher, O.K. Moune, D. Garcia and P. Tanner, Phys. Rev. B, 53 (1996) 9501). The present work is a step towards the generalization of the proposition. It is shown that it is also true for the 4f³ configuration of Nd³⁺ in non-centrosymmetric sites. Indeed, the notorious discrepancy between experimental and calculated sublevels of the ²H(2)_11/2 group at 15000 cm⁻¹ is eliminated by configuration interaction with the excited 4f²6p¹ configuration. The experimental case of Nd₂O₂S is quoted as an example, but the demonstration is valid for all neodymium compounds.     

Excitons and rare-earth ions in CsCdBr3

CsCdBr₃ has a quasi-linear crystal structure. It consists of covalently bound [CdBr]₆⁴⁻ chains separated by chains of Cs⁺ ions. The trivalent rare-earth (RE) ions substitute for divalent Cd ions forming predominantly pair centers of the type RE³⁺-(Cd vacancy)-RE³⁺. A minority of RE ions forms “single-ion” centers with more distant charge compensation. The electronic structure around the band gap is determined by the [CdBr]₆⁴⁻ octahedra. The lowest excitonic states of the lattice are charge-transfer states of these octahedra. At low temperatures they form self-trapped excitons which become mobile around 80 K.

In addition we find defect-localized excitons at the RE pairs and single ions with slightly modified spectra. There is a strong energy transfer between the RE ions and the defect-localized excitons in both directions with transition times below 10⁻⁸ s. For the cooperative fluorescence transition ¹D₂×¹G₄→³H₄׳H₄ in Pr³⁺: CsCdBr₃ a frequency-modulated vibronic sideband spectrum was found with up to four repetitions of the frequency of the localized optical phonon mode at the ion pair.     

Energy transfer and frequency upconversion in Yb, Er co-doped sodium–lead–germanate glasses

Yb³⁺–Er³⁺ co-doped Na₂O–GeO₂–PbO glass suitable for developing optical fiber lasers and amplifiers has been fabricated. The energy transfer efficiency from Yb³⁺ to Er³⁺ was investigated in the glasses with different Yb³⁺–Er³⁺ concentration ratios and the maximum energy transfer efficiency was found to be 67% at the Yb³⁺–Er³⁺ concentration ratio of 5:1. Subsequently, the studies of upconversion emissions in the visible range were performed. An intense green together with a relatively weak red emission was observed under the excitation of 976 nm diode laser. The quadratic dependence of the green emission on excitation power indicates that a two-photon absorption process occurs under a 976 nm excitation. However, for the red emission the slope of log-log plot of integrated intensity versus pump power declines with increasing Yb³⁺ ions content, indicating it is not resulted from a biphotonic process.     

Measurements and simulation of the Zeeman splitting in NdOCl and Nd:LiYF4

Measurements and simulation of the Zeeman effect in NdOCl and 1%Nd³⁺:LiYF₄ single crystals were carried out. The rare earth local symmetries are C_4v and D_2d respectively. The isomorphism between these two point groups allows discussion on symmetry assignments. The attention is focused on the ⁴G_7/2 and ⁴G_9/2 levels for NdOCl and on the ⁴F_9/2 and ⁴G_7/2 levels for Nd³⁺:LiYF₄, because they are representative of the general splitting behaviour. We have first revisited the simulation of the energy level scheme in order to deal with confident wave functions. In the experimental magnetic field (B) range we have obtained linear, non-linear, undefined crossing and anti-crossing levels. These features depend on the orientation of B, on the irreducible representation and on the transition closeness. The line widths have been the main obstacle in confirming crossing regions, because their average values are 8 cm⁻¹ at liquid helium temperature. The simulations were performed by introducing the magnetic Hamiltonian μ_BB(L+g_eS) in the secular determinant before diagonalisation. A magnetic accidental degeneracy is considered when discussing the crossing and anti-crossing cases.     

Optical absorption spectrum, oscillator strengths, magnetic splitting factors and paramagnetic susceptibility of Nd doped into LiYO2

Optical and magnetic measurements are performed on Nd³⁺ doped into LiYO₂. 105 energy levels of Nd³⁺ are assigned from the optical absorption and excitation spectra for the powdered sample at 20 K. The crystal field analysis is performed in the C₁ site symmetry of Nd³⁺. Theoretical values of the crystal field parameters, oscillator strengths, magnetic susceptibility and ground state splitting factors are calculated and compared with experimentally measured quantities.     

Optical dephasing of rare earth ions in mixed crystalline and size-restricted systems

High resolution optical hole burning spectroscopy of rare earth ions is used to study the dynamics of disordered and size-restricted crystalline solids. Materials described include mixed crystals of Ca_1−xLa_xF_2+x and Ca_1−xY_xF_2+x with Eu³⁺ and Pr³⁺ and sol–gel prepared nanometer scale γ-Al₂O₃:Eu³⁺. All of these materials exhibit, at low temperatures, a temperature dependence of their spectral hole linewidth which is similar to the dynamical properties observed in amorphous materials such as glasses. This suggests an enhanced density of states at low frequencies. The results on the mixed alkaline earth fluorides are discussed in relation to far infrared and heat capacity measurements, which indicate the presence of low frequency excitations. It appears that while the density of defect states responsible for the dynamics increases with Y or La content, the density saturates at low concentrations. Both persistent and transient hole burning mechanisms occur for nanoscale sol–gel γ-Al₂O₃:Eu³⁺. The nearly exponential increase in the linewidth above 7 K is explained by Raman scattering by phonons associated with the size-restricted nature of the phonon density of states.     

Spectroscopic and 1.06 μm laser properties of Nd-doped K–Sr–Al phosphate and fluorophosphate glasses

Optical absorption, fluorescence and decay curves for the ⁴F_3/2 level of Nd³⁺ ions in phosphate (P₂O₅–K₂O–SrO–Al₂O₃) and fluorophosphate (P₂O₅–K₂O–SrO–Al₂O₃–AlF₃ and P₂O₅–K₂O–SrO–Al₂O₃–BaF₂) glasses doped with three concentrations (0.1, 1.0 and 2.0 mol%) of Nd³⁺ ions have been investigated. The Judd–Ofelt (JO) theory has been applied to the absorption spectra of 1.0 mol% Nd³⁺-doped glasses to derive JO intensity parameters which are in turn used to calculate the radiative properties of the Nd³⁺ ion fluorescent levels. The assigned energy level data of Nd³⁺ (4f³) ions are analysed in terms of a parametrized free-ion Hamiltonian model that consists of 20 interaction parameters of atomic nature. The stimulated emission cross section and branching ratios have been calculated using the emission spectra. The relatively higher branching ratio for ⁴F_3/2 → ⁴I_11/2 transition shows the suitability of these glasses for laser application. It is interesting to note that the measured decay curves of the ⁴F_3/2 level remain nearly single exponential even for higher Nd³⁺ ion concentration but with shortening of lifetime.     

Crystal field splitting and anomalous thermal expansion in YbVO4

We have investigated the Yb³⁺ crystal-field-level structure in YbVO₄ using inelastic magnetic neutron-scattering measurements and crystal-field model calculations. We determined the temperature dependence of the a and c lattice parameters of the tetragonal unit cell by neutron diffraction and observed a minimum at ca. 120 K in the c lattice parameter. This anomaly in the thermal expansion is interpreted as arising from a coupling of the anisotropic low-lying crystal-field states of the Yb³⁺ ions and the crystal lattice at low temperatures.     

Ultraviolet and visible emissions of Er in monoclinic KYb(WO4)2 single crystals

We observed ultraviolet and visible up-conversion signals with maximum intensity at 383 nm, 407 nm, 457 nm, 474 nm, 525 nm, 552 nm, 652 nm, 696 nm, 772 nm, 801 nm, 814 nm and 848 nm from the 4f states of erbium-doped KYb(WO₄)₂ single crystals after pumping at 981 nm (10 194 cm⁻¹) at room temperature (RT) and low temperature (10 K). These emissions were generated after simultaneous excitation of erbium and ytterbium at 981 nm (Yb³⁺ acts as a sensitizer of Er³⁺). We discuss the up-conversion mechanism in which three and two photons were involved in the generation of the ultraviolet and visible emissions.