Photoconductivity and photodielectric effect in LiY1 − x Lu x F4 crystals doped with Ce3+ and Yb3+ ions

Time and spectral dependences of the dielectric permittivity of the LiY_{1 ? x} Lu _x F₄ (x = 0, 0.5, and 1) crystals doped with Ce³⁺ and co-doped with Yb³⁺ ions under UV laser excitation were studied by the 8-mm microwave resonant technique at room temperature. The obtained photoconductivity spectrum in 240–310 nm spectral range was interpreted as a stepwise photoionization spectrum of the Ce³⁺ ions due to sequential 4f–5d and 5d–6s transitions. Average lifetimes of free and defect trapped (color centers) charge carriers were estimated.     

Optical and gain properties of series of crystals LiF-YF3-LuF3 doped with Ce and Yb ions

Here, we present first results of systematic studies of host cation variation impact on spectral-kinetic, photochemical and gain properties of Ce³⁺-doped LiYF₄ (YLF), LiLuF₄ (LLF) and LiY_1−xLu_xF₄ family crystals. 5d-4f luminescence decay of Ce³⁺ ions studies, together with pump-probe experiments, indicate that previously reported twice higher luminescence quantum yield in LLF compared with that of YLF crystals is provided by more efficient upper lasing level feeding due to recombination and higher color center destruction rate in LLF against YLF crystals. Namely, it is responsible for higher energetic characteristics of laser based on Ce³⁺:LLF crystals. Strong and wide pump-induced absorption band centered at 310 nm is observed in Ce³⁺:YLF. This band is shifted to blue and its intensity goes down with Lu content. We have evaluated free charges recombination rate, excited state absorption cross-section for Ce³⁺ ions and some other photodynamic processes related microparameters. Fitting results indicate that pump-induced color centers lifetime decreases with Lu-content in LiYF₄-LiLuF₄ mixture and it can be associated with more efficient color center bleaching by Ce³⁺ ions 5d-4f fluorescence.     

New all-solid-state tunable UV Ce3+, Yb3+:LiY0.4Lu0.6F4 laser

Here we report laser test results of the new UV solid-state active medium based on a Ce³⁺, Yb³⁺:LiY_0.4Lu_0.6F₄ mixed crystal pumped by radiation from Ce:LiCAF laser. The 10-Hz pulse repetition rate Ce³⁺, Yb³⁺:LiY_0.4Lu_0.6F₄ laser yielded a maximum output power of 0.25 mJ at 311nm in non-optimized non-selective resonator with a maximal slope efficiency of 13%. Tunability from 304 to 332 nm was achieved in the selective single-prism resonator.     

Two-step photoconductivity in LiY<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Lu<Subscript>1 – <Emphasis Type="Italic">x</Emphasis> </Subscript>F<Subscript>4</Subscript>:Ce,Yb crystals

Photoconductivity of LiY_xLu_1–xF₄:Ce,Yb (x = 0–1) crystals is measured under one- and two-step excitation. It is established that the photoconductivity is due to intra-center transitions from excited states of Ce³⁺ ions. The position of the ground 4 f-state of Ce³⁺ ion relative to the bottom of the conduction band is determined. The choice of pumping conditions to obtain the lasing on the 5d–4f transitions of trivalent cerium in these active media is substantiated.     

On the distribution coefficient of Ce<Superscript>3+</Superscript> ions in LiF-LuF<Subscript>3</Subscript>-YF<Subscript>3</Subscript> solid-solution crystals

The optimum ratio of the numbers of the Y³⁺ and Lu³⁺ ions in LiF-LuF₃-YF₃ solid solutions at which the distribution (introduction) coefficient of Ce³⁺ ions is three to five times larger than that in LiYF₄ and LiLuF₄ crystals has been determined by the EPR and optical spectroscopy methods.     

EPR of Ce<Superscript>3+</Superscript> ions in mixed yttrium-lutecium orthoaluminates

The results of EPR studies of Ce³⁺ ions incorporated into single crystals of mixed yttrium-lutecium orthoaluminates Y_1?xLu_xAlO₃ (YLuAP, x=0.1, 0.3) are reported. In compositionally disordered YLuAP compounds, in comparison to YAlO₃, new paramagnetic Ce³⁺ centers are found. These centers are caused by the changes in symmetry and in the crystal field magnitude due to the isomorphic substitution of Y³⁺ ions by Lu³⁺ in the yttrium sublattice of orthoaluminates. It is shown that the formation of 27 different types of centers is possible in YLuAP with variation of the Lu content. The probabilities of formation of new paramagnetic centers are calculated.     

Highly efficient cooperative energy transfer from Ho3+ and Tm3+ ions to Ce3+ ions in crystals

A phenomenon of highly efficient cooperative energy transfer from Ho³⁺ and Tm³⁺ ions to two-particle (2Ce³⁺) cooperative acceptors in crystals of solid solutions of La_1?x Ce _x F₃ is revealed. The rates of cooperative energy transfer in Ho³⁺→2Ce³⁺, Tm³⁺→2Ce³⁺, and Tb³⁺→ 2Yb³⁺ systems are measured, as well as their dependence on the magnitude of the matrix elements of donor transition.     

Low-temperature high-resolution VUV spectroscopy of Ce doped LiYF4, LiLuF4 and LuF3 crystals

For LiYF₄:Ce³⁺, LiLuF₄:Ce³⁺ and LuF₃:Ce³⁺ crystals UV/visible emission and time-resolved VUV/UV excitation spectra were recorded at liquid helium temperature with spectral resolution of 0.1 nm for excitation spectra and better than 0.3 nm for emission spectra. Well resolved fine structures due to zero-phonon lines were clearly observed in both excitation and emission spectra for LiYF₄:Ce³⁺ and LiLuF₄:Ce³⁺. For LuF₃:Ce³⁺ crystal no fine structure was detected in the spectra even at the highest spectral resolution. Under the host excitation, the fine structure for high-energy emission band of Ce³⁺ (5d-²F_5/2) in LiLuF₄:Ce³⁺ becomes well pronounced because of weaker reabsorption effect, as compared to Ce³⁺ 4f-5d absorption, due to small penetration depth for exciting radiation. As a result the crystal-field splitting for ²F_7/2 and ²F_5/2 levels of Ce³⁺ in LiLuF₄ crystal was measured. First observation of zero-phonon lines at ∼81,550 and ∼82,900 cm⁻¹ as well as vibronic side bands due to interconfigurational 4f¹⁴-4f¹³5d transitions in Lu³⁺ is reported for excitation spectrum of LiLuF₄:Ce³⁺.     

Luminescence properties of solid solutions of borates doped with rare-earth ions

The structural and luminescence properties of Lu _x Y_{1 ? x} BO₃ solid solutions doped with Ce³⁺ or Eu⁺³ have been investigated. It has been found that the solid solutions crystallize in the vaterite phase with a lutetium concentration x < 0.5. For a higher lutetium concentration x, the solid solutions contain an additional calcite phase with a content less than 5 wt %. The luminescence spectra are characterized by intensive impurity emission under excitation with the synchrotron radiation in the X-ray and ultraviolet spectral ranges. It has been shown that, as the lutetium concentration x in the Lu _x Y_{1 ? x} BO₃: Ce³⁺ solid solutions increases, the emission intensity smoothly decreases, which is associated with a gradual shift of the Ce³⁺ 5d(1) level toward the bottom of the conduction band, as well as with a decrease in the band gap. It has been established that, in the Lu _x Y_{1 ? x} BO₃: Eu³⁺ solid solutions with intermediate concentrations x, the efficiency of energy transfer to luminescence centers increases. This effect is explained by the limited spatial separation of electrons and holes in the solid solutions. It has been demonstrated that the calcite phase adversely affects the luminescence properties of the solid solutions.     

UV and VUV spectroscopic study of Na0.4Y0.6F2.2 crystals doped with rare-earth ions

The short-wavelength transmission spectra of Na_0.4 R _0.6F_2.2 crystals with R = Y, Yb, or Lu have been investigated. For these crystals, the VUV transmission cutoffs are 78750, 58820, and 75200 cm^?1, respectively. The 4f ⁿ–4f ^n?15d absorption and excitation spectra of Na_0.4Y_0.6F_2.2 crystals activated with Ce³⁺, Pr³⁺, Nd³⁺, Er³⁺, Tm³⁺, and Yb³⁺ ions have been analyzed in the range 30000–80000 cm^?1. The energy positions of the lowest levels of the 4f ^n?15d configurations of these ions in the fluorite crystal matrix Na_0.4Y_0.6F_2.2 are determined. The absorption band in the spectral range 60600–70000 cm^?1 in Na_0.4(Y, Yb)_0.6F_2.2 crystals is due to the charge transfer from F^? to Yb³⁺. It is shown that the environmental symmetry of Ce³⁺ ions in Na_0.4R_0.6F_2.2 (R = Y, Yb, Lu) crystals is almost identical.     

Spectroscopic investigations of wide-band fluoride crystals doped with ions of some rare-earth elements under X-ray excitation

Results of studying the properties of luminescence spectra of wide-band fluoride CaF₂, Ca_0.9Y_0.1F_2.1, Na_0.4Y_0.6F_2.2, Na_0.4Yb_0.6F_2.2, and BaYb₂F₈ crystals (pure and doped with Ce³⁺ and Yb³⁺ ions) under synchrotron radiation excitation with energies ranging from 3 to 60 keV are presented. Luminescence spectra are obtained in the region 200–600 nm. Transmission spectra are studied, and the effect of intense short-wavelength radiation on the optical properties of these crystals is studied.     

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.     

Spectral kinetics of Ce<Superscript>3+</Superscript> ions in double-fluoride crystals with a scheelite structure

The influence of the cation composition on the spectral kinetics of Ce³⁺ ions in double-fluoride crystals with a scheelite structure is studied. The importance of the photodynamic processes induced in these crystals by the exciting radiation is demonstrated. The difference in luminescence quantum efficiency between Ce³⁺ ions in LiYF₄ and LiLuF₄ crystals is found to be due to the different lifetimes of color centers produced in the samples by the exciting radiation and to the different efficiency of the free-carrier recombination at cerium impurity centers. It is shown that Yb³⁺ ions can increase the carrier recombination rate in the crystals.     

Engineering lattice matching, doping level, and optical properties of KY(WO4)2:Gd, Lu, Yb layers for a cladding-side-pumped channel waveguide laser

Single-crystalline KY_1?x?y?z Gd_xLu_yYb_z(WO₄)₂ layers are grown onto undoped KY(WO₄)₂ substrates by liquid-phase epitaxy. The purpose of co-doping the KY(WO₄)₂ layer with suitable fractions of Gd³⁺ and Lu³⁺ is to achieve lattice-matched layers that allow us to engineer a high refractive-index contrast between waveguiding layer and substrate for obtaining tight optical mode confinement and simultaneously accommodate a large range of Yb³⁺ doping concentrations by replacing Lu³⁺ ions of similar ionic radius for a variety of optical amplifier or laser applications. Crack-free layers, up to a maximum lattice mismatch of ~0.08 %, are grown with systematic variations of Y³⁺, Gd³⁺, Lu³⁺, and Yb³⁺ concentrations, their refractive indices are measured at several wavelengths, and Sellmeier dispersion curves are derived. The influence of co-doping on the spectroscopy of Yb³⁺ is investigated. As evidenced by the experimental results, the lattice constants, refractive indices, and transition cross-sections of Yb³⁺ in these co-doped layers can be approximated with good accuracy by weighted averages of data from the pure compounds. The obtained information is exploited to fabricate a twofold refractive-index-engineered sample consisting of a highly Yb³⁺-doped tapered channel waveguide embedded in a passive planar waveguide, and a cladding-side-pumped channel waveguide laser is demonstrated.     

Spectral characteristics of solid solutions LiY1−x Lu x F4 doped by Ce3+ ions

During crystallization, the LiF-YF₃-LuF₃ compounds form a continuous series of solid solutions having a scheelite structure at any proportion of the YF₃ and LuF₃ components. It is established that the crystals conform to the Vegard law and the Rutgers rule. The spectral characteristics of the 5d–4f transitions of Ce³⁺ ions and color centers induced by ultraviolet radiation in these crystals were studied as a function of the Lu³⁺ concentration. It is shown that the active media based on LiF-YF₃-LuF₃: Ce³⁺ solid solutions are more efficient as compared to LiYF₄: Ce³⁺ and LiLuF₄: Ce³⁺ crystals in possible directional changes in the spectral-kinetic characteristics of impurity ions and the parameters of the losses induced by pumping radiation and in enhancement of energetic and spectral characteristics of lasers on their basis.     

Spectral-kinetic characteristics of Pr luminescence in LiLuF4 host upon excitation in the UV-VUV range

Spectral-kinetic study of Pr³⁺ luminescence has been performed for LiLuF₄:Pr(0.1 mol%) single crystal upon the excitation within 5-12 eV range at T=8 K. The fine-structure of Pr³⁺ 4f ²→4f 5d excitation spectra is shown for LiLuF₄:Pr(0.1 mol%) to be affected by the efficient absorption transitions of Pr³⁺ ions into 4f 5d involving 4f ¹ core in the ground state. Favourable conditions have been revealed in LiLuF₄:Pr(0.1 mol%) for the transformation of UV-VUV excitation quanta into the visible range. Lightly doped LiLuF₄:Pr crystals are considered as the promising luminescent materials possessing the efficient Pr³⁺³P₀ visible emission upon UV-VUV excitation. The mechanism of energy transfer between Lu³⁺ host ion and Pr³⁺ impurity is discussed.     

Superhyperfine structure in the EPR spectra and optical spectra of impurity f ions in dielectric crystals: A review

The results of observation and simulation of the superhyperfine (ligand hyperfine) structure (SHFS) of the electron paramagnetic resonance (EPR) spectra of rare-earth and uranium impurity ions in dielectric crystals have been systematized. The resolved SHFS of the EPR spectra of doped cubic crystals (with the fluorite and perovskite structures) has been observed for orientations of a constant magnetic field along the crystallographic axes. Most attention has been paid to tetragonal double fluorides LiRF₄ (R = Y, Lu, Tm), in which the SHFS of the EPR spectra has also been found for intermediate orientations of the magnetic field. For the LiYF₄: Nd³⁺ single crystal, the splitting of optical spectral lines due to the interaction of Nd³⁺ ions with nuclear magnetic moments of the nearest neighbor fluorine ions has been observed for the first time. The Van Vleck paramagnet LiTmF₄: U³⁺ is characterized by the SHFS with clearly distinguishable components due to the interaction of uranium ions both with nuclei of the fluorine ions and with enhanced magnetic moments of the thulium nuclei. The SHFS envelopes of the EPR spectra of Yb³⁺, Ce³⁺, Nd³⁺, and U³⁺ ions in LiYF₄ and LiLuF₄ crystals are well reproduced by numerical calculations based on the microscopic model using only three fitting parameters: the width of transitions between the electron-nuclear sublevels of the complex containing the paramagnetic ion and nuclei of the ligands and two constants of covalent bonding of the f electrons with 2s and 2p electrons of the nearest neighbor fluorine ions.     

Comparative spectroscopic investigation of Yb3x Y3(1 ? x )Al5O12 (x = 3, 5, 10 and 15%) transparent ceramics

Transparent Yb_3x Y_{3(1 ? x} )Al₅O₁₂ (x = 3, 5, 10 and 15%) ceramics were fabricated by solid-state reaction and vacuum sintering method with a mixture of commercial Al₂O₃, Y₂O₃ and Yb₂O₃ powders. Ethanol and 0.5 wt % tetraethoxysilane were doped. Transparent fully dense samples with grain sizes of several micrometers were obtained by sintered at 1750 K. The absorption spectra, emission spectra and fluorescence life time of these transparent Yb_3x Y_{3(1 ? x} )Al₅O₁₂ ceramics at room temperature have been measured and calculated. The air annealed effects of Yb_3x Y_{3(1 ? x} )Al₅O₁₂ ceramics have been compared with un-annealed Yb:YAG transparent ceramics which shows the difference of the Yb³⁺ ions between annealed and unannealed Yb:YAG ceramics is less than 1%.     

Synthesis and characterization of new LnxSb2−xSe3 (Ln: Yb, Er) nanoflowers and their physical properties

New Ln_xSb_2−xSe₃ (Ln: Yb³⁺, Er³) based nanomaterials were synthesized by a co-reduction method. Powder XRD patterns indicate that the Ln_xSb_2−xSe₃crystals (Ln=Yb³⁺, Er³⁺, x=0.00-0.12) are isostructural with Sb₂Se₃. The cell parameters b and c decrease for Ln=Er³⁺ and Yb³⁺ upon increasing the dopant content (x), while a increases. SEM images show that doping of the lanthanide ions in the lattice of Sb₂Se₃ generally results in nanoflowers. UV-vis absorption and emission spectroscopy reveals mainly electronic transitions of the Ln³⁺ ions in case of Yb³⁺ doped nanomaterials. Emission spectra of doped materials, in addition to the characteristic red emission peaks of Sb₂Se₃, show additional emission bands centered at 955 nm, originating from the ²F_7/2→²F_5/2 transition (f-f transitions) of the Yb³⁺ ions. DSC curves indicate that Sb₂Se₃ has the highest thermal stability. The temperature dependence of the electrical resistivity of doped-Sb₂Se₃ with Yb³⁺ and Er³⁺ was studied.     

Growth and characterization of cerium-substituted yttrium iron garnet single crystals for magneto-optical applications

This paper is concerned with the preparation and characterization of cerium-substituted yttrium iron garnets, which are known to be oxides having a large Faraday rotation effect. Using the improved flux method we successfully grew bulk single crystals of iron garnet doped with Ce³⁺ ions with maximum substitutions up to 0.349. Here we investigate different solution compositions for maximum Ce³⁺ substitution. The Faraday rotation and optical absorption spectra were measured in the near infrared region for different Ce³⁺ ion substituted iron garnets. The specific Faraday rotation of Ce_0.349Eu_0.195Y_2.456Fe₅O₁₂ was found to be 1430 deg/cm at a wavelength of 780 nm and –1280 deg/cm at 1150 nm. The Ce substitution prominently enhances the Faraday rotation effect, and Yb³⁺ and Eu³⁺ ions substituted for Y³⁺ in the dodecahedral sites of YIG can increase the concentration of Ce³⁺ ions, depressing the formation of nonmagnetic Ce⁴⁺ ions by charge compensation. Received: 24 January 2001 / Accepted: 2 March 2001 / Published online: 27 June 2001