Comparative spectroscopic and laser properties of Yb<Superscript>3+</Superscript>-doped CaF<Subscript>2</Subscript>, SrF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript> single crystals

We present the spectroscopic properties and room-temperature cw tunable laser operation of Yb³⁺-doped CaF₂, SrF₂ and BaF₂ single crystals grown and studied in the same conditions. Emission cross sections, lifetimes, laser thresholds, laser slope efficiencies and laser wavelength tuning ranges are compared. It appears that Yb³⁺-doped BaF₂ might be more promising for diode-pumped high power laser operation. PACS 42.55.Rz; 42.70.Hj     

Thermal decay of photochromic color centers in CaF2, SrF2, and BaF2 crystals doped by La and Y impurities

The absorption spectra of photochromic centers in CaF₂, SrF₂, and BaF₂ crystals doped by La and Y impurities and thermal decay of the centers in the temperature range 80–600 K are investigated. Under low-temperature x-ray irradiation, ionized photochromic color (PC⁺) centers are generated in La- and Y-doped CaF₂ crystals and in a La-doped SrF₂ crystal. It is revealed that, upon heating of the CaF₂-LaF₃ crystal, PC⁺ centers are transformed into photochromic color (PC) centers. In the SrF₂-YF₃ crystal irradiated at room temperature, photochromic color centers are generated as well. All color centers decay at a temperature of approximately 600 K. After irradiation of the BaF₂-YF₃ crystal at a temperature of 80 K, absorption bands are observed at energies of 2.25 and 3.60 eV, which are related to neither PC centers nor PC⁺ centers.     

Charge-transfer bands in crystals of alkaline earth fluorides with Eu<Superscript>3+</Superscript> and Yb<Superscript>3 + 1</Superscript>

The absorption, luminescence, and excitation spectra of CaF₂, SrF₂, and BaF₂ crystals with EuF₃ or YbF₃ impurity have been investigated in the range 1–12 eV. In all cases, strong wide absorption bands (denoted as CT₁) were observed at energies below the 4f ⁿ -4f ^{n ? 1}5d absorption threshold of impurity ions. Weaker absorption bands (denoted as CT₂) with energies 1.5–2 eV lower than those of the CT₁ bands have been found in the spectra of CaF₂ and SrF₂ crystals with EuF₃ or YbF₃ impurities. The fine structure of the luminescence spectra of CaF₂ crystals with EuF₃ impurities has been investigated under excitation in the CT bands. Under excitation in the CT₁ band, several Eu centers were observed in the following luminescence spectra: C _4v , O _h , and R aggregates. Excitation in the CT₂ bands revealed luminescence of only C _4v defects.     

F<Stack> <Subscript>3</Subscript> <Superscript>−</Superscript> </Stack> molecular ions in fluoride crystals

The UV absorption spectra of F₃^? molecular ions in LaF₃, SrF₂, CaF₂, and BaF₂ crystals doped with rare-earth elements are studied. Comparison of radiation-colored and additively colored crystals reveals the absorption bands of F₃^? hole centers in the region near 6 eV. Nonempirical calculations of optical transitions agree well with experimental results.     

Ligand hyperfine interaction in Gd<Superscript>3+</Superscript> tetragonal centers in CaF<Subscript>2</Subscript> and SrF<Subscript>2</Subscript> and the structure of the impurity nearest surroundings

ENDOR experimental spectra of Gd³⁺ tetragonal impurity centers in CaF₂ and SrF₂ crystals were used to determine the superhyperfine interaction (SHFI) constants of the impurity with ¹⁹F nuclear spins of its first coordination sphere and the compensator ion. The distances in the Cd³⁺F₉ complex were estimated within the model of isotropic SHFI constants suggested in [1]. An analysis of the data on the SHFI and spin-Hamiltonian constants [2] in terms of the superposition model indicates significant changes in the contributions (due to the Gd³⁺ mixed states) to these parameters for the tetragonal centers in comparison with the corresponding contributions for the cubic and trigonal centers in the same crystals.     

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.     

Additive coloring of CaF<Subscript>2</Subscript> optical ceramic

The specificity of additive coloring of CaF₂ optical ceramic (formation of color centers in it and photothermochemical transformation of these centers in colored ceramic samples) has been considered. Under the same coloring conditions, this process occurs more slowly in ceramics rather than in crystals; at the same time, the limiting concentration of color centers that can be introduced into ceramics is much higher. The photothermochemical transformations of color centers in crystals and ceramics, which occur under illumination at different wavelengths and upon heating, have been studied. The specific features of introduction of color centers into ceramic and their transformation under illumination and heating are likely to be related to the mass twinning of ceramic grains.     

Instability of monovalent ions of transitional metals in alkaline earth fluorides: CaF<Subscript>2</Subscript> and SrF<Subscript>2</Subscript> activated by Co<Superscript>2+</Superscript> or Ni<Superscript>2+</Superscript>

The ground state of monovalent ions of transitional metals in alkaline-earth fluorides is localized in the conduction band. It is shown by the example of monovalent Co and Ni ions in CaF₂ and SrF₂ crystals that this is responsible for the unstable state of these ions in cubic environments. A stable state is achieved by the monovalent ions shifting to off-center positions.     

Photodynamic processes in CaF<Subscript>2</Subscript> crystals activated by Ce<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> ions

The photochemical properties of CaF₂ crystals activated by Ce³⁺ and Yb³⁺ ions are studied. A model of the photodynamic processes induced by pumping UV or VUV radiation in active media is suggested and experimentally verified. This model explains both the presence of color centers of electronic and hole nature in crystals activated by cerium and the mechanism of suppressing of solarization processes after additional activation of the samples by Yb³⁺ ions. The cross sections of the processes of free-carrier capture by various ytterbium impurity centers are estimated. These impurity centers are established to be effective centers of recombination of free carriers of both signs.     

5d-4f luminescence of Nd3+, Gd3+, Er3+, Tm3+, and Ho3+ ions in crystals of alkaline earth fluorides

The vacuum ultraviolet emission spectra of alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals with rare earth impurity ions (Nd, Gd, Er, Tm, Ho) have been investigated. The main luminescence bands are described well by the transitions from the lowest excited 5d state to different 4f levels of rare earth ions.     

A high-stability medium based on CaF<Subscript>2</Subscript>:Na crystals with colloidal color centers: I. Photothermochemical conversion of colloidal color centers in CaF<Subscript>2</Subscript>:Na crystals

Photothermochemical conversion of simple color centers (which include from one to four anionic vacancies) and highly aggregated ones in additively colored crystals of calcium fluoride doped by sodium is studied. The annealing of crystals with a low sodium content in a reducing atmosphere (additive coloration) leads to the predominant formation of simple color centers, which convert into highly aggregated centers under the joint action of heating and irradiation in absorption bands of simple centers. The irradiation of highly aggregated centers into their absorption bands and simultaneous heating causes these centers to convert into simple centers. The additive coloration of crystals with a relatively high sodium content leads to the predominant formation of highly aggregated centers. The heating of these crystals along with the irradiation in absorption bands of highly aggregated centers causes these centers to convert into simple centers. The formation of different color centers in the course of additive coloration of crystals with different impurity content and different results of photothermochemical conversion of centers in these crystals are connected with the dual action of impurities. Anion vacancies, which compensate the charge of the impurity alkali metal, facilitate the aggregation of color centers. At the same time, the alkali impurity stabilizes simple color centers.     

A Study of Mechanochemical Doping of Fluoride Crystals with a Fluorite Structure by Er3+ Ions via Electron Paramagnetic Resonance Spectra

Using electron paramagnetic resonance (EPR) spectroscopy, we have shown that, upon mecha- noactivated doping of powders of compounds CaF₂, SrF₂, and BaF₂ with Er³⁺ ions, impurity centers of single erbium ions with cubic symmetry are formed. Investigations of dependences of EPR spectra intensities on the particle size show that the process of mechanochemical doping with Er³⁺ ions proceeds differently for CaF₂, SrF₂, and BaF₂ host matrices. In the case of CaF₂, impurity centers are localized in a very thin near-surface layer of CaF₂ particles, in SrF₂, the impurity is distributed over the volume of particles, while, in BaF₂, there is a layer of a finite thickness for which the probability of doping in the course of mechanosynthesis is very small and the impurity of the rare-earth element is localized in the core of large particles. These data can be explained assuming that the result of mechanosynthesis of particles of fluorides with a fluorite structure doped with Er3+ ions at room temperature is governed by two processes—mechanoactivated diffusion of rare-earth ions into particles and segregation of impurity ions at grain boundaries. In this case, the typical scales for compounds CaF₂, SrF₂, and BaF₂ considerably differ from each other.     

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.     

High energy implantation of buried insulating layers

Single crystals of CaF₂, SrF₂, and BaF₂ were irradiated at room temperature with alpha particles emitted from an effectively semi-infinite ²³⁸PuO₂ source to a cumulative dose of 3 × 10²⁰ alpha particles/m². Although no change in the lattice parameter of CaF₂ was observed, the crystals exhibited increasing coloration with dose. The lattice parameters of both SrF2 and BaF2 increased exponentially with dose, in agreement with a previously developed model, but the crystals developed no apparent coloration. Isochronal and isothermal-step annealing were used to study the recovery behavior of the lattice defects in both the SrF₂ and BaF₂ single crystals. A single recovery stage, tentatively associated with cation defects, was observed in each and the activation energy determined.     

Time-resolved spectroscopy of self-trapped excitons in fluorides of alkaline-earth metals under pulsed electron irradiation

Nanosecond-resolution absorption spectroscopy at room temperature was used to study the laws governing the creation and evolution of the primary defect structure in CaF₂, SrF₂, and BaF₂ crystals exposed to an accelerated electron pulse. It is shown that the spectral-kinetic characteristics of self-trapped excitons created in undamaged parts of the crystal lattice are qualitatively similar. Partial polarization of the absorption of self-trapped excitons is observed in CaF₂. The structure of the transient absorption spectra becomes more complex in the sequence CaF₂, SrF₂, BaF₂ because of the formation of excitons trapped in phase inclusions of homologous cationic impurities. The spectral characteristics of excitons trapped in undamaged parts of the CaF₂ and SrF₂ lattice and in their phase inclusions in BaF₂ are the same although the latter have a considerably shorter relaxation time. Short-lived (τ⩽100 ns) absorption of unknown defects was observed in the spectral range ⩾5 eV. Fiz. Tverd. Tela (St. Petersburg) 40, 1228–1234 (July 1998)     

Concentration of Cr<Superscript>4+</Superscript> impurity ions and color centers as an indicator of saturation of forsterite crystals Mg<Subscript>2</Subscript>SiO<Subscript>4</Subscript> with oxygen

The influence of the oxygen partial pressure P _{O 2} in the growth atmosphere on the coefficient of chromium distribution between the crystal and the melt of forsterite, the Cr³⁺ and Cr⁴⁺ ion contents in crystals, and the concentration of color centers induced by irradiation has been investigated. It has been established that the crystals grown at low oxygen partial pressures P _{O 2} (0.01–0.05 kPa) are characterized by low concentrations of Cr⁴⁺ ions and color centers. A change in the oxygen partial pressure to P _{O 2} ∼ 0.85 kPa leads to an increase in the Cr⁴⁺ center concentration by a factor of ∼10 and in the color center concentration by a factor of ∼5. A further increase in the oxygen partial pressure to P _{O 2} to 12 kPa remains the concentration of these centers almost unchanged. A model has been proposed according to which the intrinsic defects formed under conditions of a relative excess of oxygen leads to both the self-oxidation of chromium and the formation of color centers in the forsterite crystals under irradiation.     

Diode-pumped regenerative Yb:SrF<Subscript>2</Subscript> amplifier

We report what we believe to be the first Yb:SrF₂ regenerative femtosecond amplifier. The regenerative amplifier produces 325-fs pulses at 100-Hz repetition rate with an energy before compression of 1.4 mJ. The interest of Yb:SrF₂ in such regenerative amplifiers and its complementarity to its well-known isotype Yb:CaF₂ is also discussed.     

Lifetimes of anion vacancies and <Emphasis Type="Italic">F</Emphasis><Stack><Subscript>2</Subscript><Superscript>+</Superscript></Stack> color centers in lithium fluoride following irradiation

A method is developed for determining the mean lifetime of anion vacancies in dielectric crystals following irradiation. This method is used to study the temporal kinetics of the concentrations of vacancies and F ₂⁺ color centers in lithium fluoride crystals irradiated by ionizing radiation. It is then possible to study the dependences of the vacancy lifetime on the concentration of F ₁ color centers, temperature, and impurity content of a crystal. Vacancy and F ₂⁺ color lifetimes determined by this method are given.     

Exciton states in CaF2, SrF2, and BaF2

We have applied the pseudopotential theory to the calculation of exciton states and binding energies for the ionic crystals CaF₂, SrF₂, and BaF₂, using a transfer model. Because of the satisfying agreement between our results and the experimental data in each case, we assume this pseudopotential formalism will describe transfer exciton states and energies in other ionic crystals.     

Spectral-kinetic characteristics of crystals and nanoceramics based on BaF<Subscript>2</Subscript> and BaF<Subscript>2</Subscript>: Ce

Optical characteristics of BaF₂ and BaF₂: Ce single crystals and nanoceramic materials prepared from these single crystals by uniaxial hot pressing have been studied. It has been shown that the introduction of Ce₃₊ ions into BaF₂ hardly affects the ultrafast (∼0.9 ns) luminescence component. The integrated luminescence intensity of the BaF₂: Ce nanoceramics is higher than that of the corresponding single crystal and considerably higher that the intensity of the undoped BaF₂ crystal. It has been demonstrated that the slow (several hundred nanoseconds) component of the luminescence decay of BaF₂: Ce is due to the energy transfer from excitons to Ce³⁺ ions.