BaY2F8 Crystals Research Articles

Growth and optical properties of high-concentration Ho3+ ion-doped Ba(Y1−xLux)2F8 crystal for mid-infrared laser application

30mol% Ho:BaY2F8 (Ho:BYF) and 30mol% Ho, 20mol% Lu:BaY2F8 (Ho:BYLF) crystals with monoclinic phase were prepared by the Bridgman technique. After introducing Lu3+ ions, the thermal property of Ho:BYLF crystal was improved without significantly shortening the 5I5 level lifetime of Ho3+ ion compared with Ho:BYF crystal. Under laser diode pumping at 889 nm, the emission peaks of both Ho:BYF and Ho:BYLF crystals in the range of 3–5 μm could be detected. The spectral parameters of the crystal are calculated in detail, and the cross sections of absorption, emission and gain are given. During laser pumping, the two crystals showed low pumping threshold, and Ho:BYF crystal obtained a maximum energy output of 5.22 mJ, while Ho:BYLF crystal tended to be saturated at 1 mJ due to the high concentration of Lu3+ ion. These two crystals show great application potential for laser operating in the mid-infrared range.

Investigation of growth and polarized spectra of Ho3+ ions heavily doped BaY2F8 crystal

A BaY2F8 (BYF) single crystal heavily doped with Ho3+ ion was grown successfully using the Czochralski (Cz) method. The infrared fluorescence spectra (2–4 μm) and fluorescence lifetime at 3.9 μm were investigated. The detailed polarization absorption properties of Ho:BYF, especially the absorption spectrum in 850–950 nm range, were reported for the first time. The maximum absorption cross-section was at 888.15 nm, σabs = 1.26✕10−21 cm2 (E//c). In comparison, the peak positions of the absorption cross sections in the other two polarization directions were at 885.80 nm and 890.45 nm, respectively, indicating that incident light's polarization direction greatly influences absorption. The spectroscopic parameters were calculated based on the Judd–Ofelt theory.

Concentration effects on the local structures and electronic properties of Er x BaY2− x F8: a first-principles study

Er3+ doped barium yttrium fluoride (BaY2F8) crystal has gained long-term attention due to its great potential in laser and medical device applications. However, the local structures of Er3+ doped BaY2F8 system (Er:BYF) remain uncertain, and the effect of doping concentration on structures and properties is unknown. Therefore, in this study, the first-principles study of the structural evolution of Er x BaY2−x F8 (x = 0.125, 0.25) crystals was carried out. By means of density functional theory and particle swarm optimization algorithm, the stable structures of Er:BYF crystals with two different concentrations are shown as standard monoclinic structures with P2 symmetry for the first time. The impurity Er3+ ions successfully enter the main lattice, replacing the Y3+ ions, and forming a [ErF8]5− polyhedron with C 2 point group symmetry. By calculating the electronic properties, the band gap values of the two structures are significantly reduced compared with that of pure BaY2F8 crystal. However, the conduction band does not break through the Fermi level, and the crystals still maintain the insulation characteristic. According to the calculation of the electron local density function, we conclude that Er–F and Y–F in Er:BYF are connected by ionic bonds. These results fill a theoretical gap in the study of Er:BYF crystals and provide inspiration for structural evolution and material design at different doping concentrations.

Excited-state absorption in thulium-doped materials in the near-infrared.

Excited-state absorption (ESA) is a key process for upconversion pumping schemes of thulium (Tm3+) doped laser materials. We have systematically studied two ESA transitions in the near-infrared spectral range, namely 3F4 → 3F2,3 (at ∼1 µm) and 3F4 → 3H4 (at ∼1.5 µm), in various Tm3+-doped fluoride (ZBLAN glass, cubic KY3F10 and CaF2, tetragonal LiYF4 and LiLuF4, monoclinic BaY2F8 crystals) and oxide (cubic Y3Al5O12, orthorhombic YAlO3 crystals) laser materials, using a pump-probe method with a polarized light. An approach to calculate the constants of energy-transfer upconversion (ETU) is also presented.

Fabrication of femtosecond laser written depressed-cladding waveguides in Tm3+:BaY2F8 crystal and laser operation near 2 μm

To our knowledge, we report the first waveguide laser operation of Tm3+:BaY2F8 crystal near the wavelength of 2 μm. Femtosecond laser inscription was employed to fabricate two waveguides, each in the form of a circular depressed cladding, with diameters of 30 μm and 50 μm. For the 50-μm-diameter waveguide, the propagation loss was determined as 0.22 dB/cm at the wavelength of 731 nm. A two-mirror resonator consisting of a butt-coupled input high reflector and a 10-cm-radius curved output coupler housed the Tm3+:BaY2F8 crystal waveguide during lasing experiments. By using a continuous-wave Ti3+:sapphire laser at 781 nm, waveguide lasing was obtained at 1.86 μm. By using a 6% output coupler, as high as 73 mW of output power was obtained with the 50-μm-diameter waveguide at an incident pump power of 680 mW, corresponding to a slope efficiency of 13.1%. Lasing threshold pump powers as low as 23 mW were further achieved with a 2% output coupler by using the 30-μm-diameter waveguide. These results show that the Tm3+:BaY2F8crystal possesses an important potential in the development of compact, efficient, femtosecond laser written waveguide lasers near 2 μm.

Crystal growth, spectral properties and Judd–Ofelt analysis of Pr: CaF2-YF3 * *Project supported by the National Natural Science Foundation of China (Grant Nos. 61805177, 61861136007, and 61621001).

The 0.6 at.% Pr3+-doped CaF2-YF3 crystal was successfully grown by the temperature gradient technique (TGT). X-ray diffraction analysis showed that the grown crystal still had cubic structure. The absorption spectrum, emission spectrum, Judd–Ofelt analysis and fluorescence decay curve at room temperature were discussed. The fluorescence lifetime of Pr: CaF2-YF3 crystal was 45.46 μs, and the σ em ⋅ τ of 3P0 → 3H6 and 3P0 → 3F2 transitions were calculated to be 80.92 × 10−20 cm2 ⋅ μs and 388.7 × 10−20 cm2 ⋅ μs, respectively. The FWHMs are 20.1 nm and 6.8 nm, which are higher than those of Pr: LiYF4, Pr: LiLuF4, Pr: LiGdF4 and Pr: BaY2F8 crystals. The results show that the Pr: CaF2-YF3 crystal is expected to achieve 605 nm orange light and 642 nm red light laser operation.

Deactivation effect of Pr ions on the emission performance of Ho: BaY2F8 crystals in the range of 1–4 μm

In this paper, with the modified Bridgeman method, 1 mol% Ho, x mol% Pr: BaY2F8 (x = 0, 0.2, 0.5, 0.8, 1.2) crystals were successfully prepared and their emission properties were studied. The X-ray diffraction data of the crystals showed that the crystals still maintained the monoclinic structure of the host. The infrared emission spectra showed that the doping of Pr3+ ions weakened the intensity of the emission peak at 2.0 μm and enhanced the intensity of the emission peak at 2.9 μm of the crystals. The optimal doping concentration of Pr3+ ions was 0.5%. As the doping concentration of Pr3+ ions increased, the lifetime of the 5I7 level of Ho3+ ions decreased from 2.03 ms to 0.23 ms, thus promoting particle number reversal and laser operation. As for Ho, Pr: BaY2F8 crystal, the energy transfer efficiency from Ho: 5I7 level to Pr: 3F2 level was calculated to be 88.7%.

Influence of Yb ions concentration on Ho: BaY2F8 crystals emission in the range of 1–3 μm

Abstract 1mol% Ho, x mol% Yb: BaY2F8 crystals (x = 6, 7, 8, 9, 10) were grown by using the micro pull-down method and studied. X-ray diffraction data reveal that Ho3+, Yb3+: BaY2F8 crystals are a pure monoclinic crystal system and the space group is C2/m. By analyzing the fluorescence spectrum, BaY2F8 crystal with doped concentrations of 8 mol% Yb3+ and 1 mol% Ho3+ ions obtained the strongest radiation at 1.2 μm and 2.0 μm. The near-middle infrared radiation characteristics and fluorescence decay curves of the 1%Ho, x%Yb: BaY2F8 crystals are studied in detail at Yb3+: 2F5/2 and Ho3+: 5I7.

Emission properties of Tm3+-doped CaF2, KY3F10, LiYF4, LiLuF 4 and BaY2F8 crystals at 1.5 μm and 2.3 μm

Thulium ions (Tm3+) are known for their emissions in the near- and mid-infrared spectral ranges suitable for efficient laser operation. We systematically study the stimulated-emission cross-sections σSE for the 3H4 → 3F4 (at ~1.5 μm) and 3H4 → 3H5 (at ~2.3 μm) Tm3+ transitions in five low-phonon energy fluoride single-crystals, namely, cubic Tm:CaF2 and Tm:KY3F10, tetragonal Tm:LiYF4 and Tm:LiLuF4 and monoclinic Tm:BaY2F8. A promising material is Tm:BaY2F8 which offers broad and intense polarized emission spectra in the mid-infrared (2251–2452 nm); the maximum σSE is 0.39 × 10−20 cm2 at 2289 nm (for E ||Z) and the emission bandwidth is exceeding 100 nm (for E ||X). We also revise the transition probabilities for Tm3+ ions in LiLuF4 and BaY2F8 crystals using the Judd-Ofelt formalism and accounting for an intermediate configuration interaction (ICI).

Effects of Nd ions on the fluorescence properties of Ho: BaY2F8 crystals in the wavelength range of 1–2.5 μm

In the study, with the traditional vertical Bridgman method, 1 at.% Ho, x at.% Nd: BaY2F8 crystals (x = 0, 1, 1.5, 2, 2.5, and 3) were grown and investigated. The monoclinic phase of Ho3+, Nd3+: BaY2F8 crystals was determined as C2/m space group with an X-ray diffractometer. In the fluorescence spectra, near-infrared radiation at 1.3 μm and 2.0 μm was obtained with optimal doping concentrations (1% Ho3+ and 2% Nd3+). In addition, the near infrareds radiation properties of Ho3+, Nd3+: BaY2F8 crystals and the fluorescence lifetime under the radiation at 1.3 μm and 2.0 μm were explored.

Generation and Nonlinear Self-Addition of Laser Frequencies Upon High-Energy Diode Pumping of Er:BaY2F8 Crystals

Special features of laser line generation upon high-energy diode pumping of Er:BaY2F8 crystals are investigated. The UV and visible laser lines are effectively formed via nonlinear self-addition of frequencies of induced Er3+ lines at 541 and 553 nm with the corresponding frequencies of IR Er3+ lines upon pumping of the Er:BaY2F8 crystals with a diode laser beam having a wavelength of 457 nm.

A general way of analyzing EPR spectroscopy for a pair of magnetically equivalent lanthanide ions in crystal: A case study of BaY2F8:Yb3+ crystal

Electron paramagnetic resonance (EPR) is an important tool to study the complex interactions (e.g., exchange and magnetic dipole-dipole interactions) for a pair of lanthanide (Ln) ions in crystals. How to analyze these EPR spectra and obtain the strength of each interaction is a challenge for experimentalists. In this work, a general way of calculating the EPR lines for two magnetically equivalent Ln ions is given by us to solve this problem. In order to explain their EPR spectra and obtain exchange interaction parameters Ji (i = x, y, z) between them, we deduce the analytic formulas for computing the angular dependent EPR lines for such Ln pairs under the condition of weak coupling (|Ji| ≪ hv, where v is the microwave frequency in the EPR experiment) and set up the spin-Hamiltonian energy matrix that should be diagonalized to obtain these lines if intermediate (|Ji| ∼ hv) and strong (|Ji| > hv) couplings are encountered. To verify our method, the experimental EPR spectra for the Yb3+ doped BaY2F8 crystal are considered by us and the EPR lines from the isolated Yb3+ ion and Yb3+-Yb3+ pair with distance R equal to 0.371 nm are identified clearly. Moreover, exchange interaction parameters (Jx ≈ –0.04 cm–1, Jy ≈ –0.24 cm–1, and Jz ≈ –0.1 cm–1) for such a pair are also determined by our calculations. This case study demonstrates that the theoretical method given in this work would be useful and could be applied to understand interactions between Ln ions in crystals.

Efficiency of the generation and self-addition of laser line frequencies upon the diode pumping of Er:BaY2F8 crystals

Features of the generation of laser lines upon the diode pumping of Er:BaY2F8 crystals are studied. Blue, violet, and UV laser lines are most efficiently generated via the nonlinear frequency self-addition of 541 and 553 nm lines of Er3+ radiation with the corresponding IR line of Er3+ for Er:BaY2F8 crystals pumped by a laser diode with a wavelength of 972 nm.

Thermal quenching of luminescence of BaY2F8 crystals activated with Er3+ and Tm3+ ions

Thermal quenching of interconfigurational 5d-4f luminescence of Er3+ and Tm3+ ions in BaY2F8 crystals is studied in the temperature range of 330–790 K. The quenching temperatures are ~575 and ~550 K for Er3+ and Tm3+, respectively. It is shown that quenching of 5d-4f luminescence of Tm3+ ions is caused by thermally stimulated ionization of 5d electrons to the conduction band.

Growth and spectral properties of Tm:BaY2F8 crystals with different Tm3+ concentration

Tm3+:BaY2F8 (Tm:BYF) laser crystals with different doping concentrations were successfully grown by Czochralski method. The optimal growth parameters obtained are as follows: the pulling rate is 0.5 mm/h; the rotation speed is 5 rpm; the cooling rate is 10°C/h. Phase composition, absorption spectra, and fluorescence properties of crystals were studied by XRD and spectral methods. XRD analysis indicates that the crystal belongs to monoclinic system with the C2/m space group. The lattice parameters were calculated and the anisotropy of the crystals was studied, confirming that the a axis is the best growth direction. The absorption peaks around 790 nm became larger with increase of Tm3+ concentration. The cross section of 15% Tm:BYF crystal around 791 nm is 9.47 × 10–21 cm2. The 10% Tm:BYF crystal has the strongest emission peak around 1879.6 nm with the FWHM of 79 nm and the emission cross-section of 2.13 × 10–21 cm2, which is favorable for the 1.88 μm laser output.

Spectroscopic properties of heavily Ho3+-doped barium yttrium fluoride crystals**Project supported by the National Natural Science Foundation of China (Grant No. 61275146), the Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120002110066), and the Special Program of the Co-construction with Beijing Municipal Government of China (Grant No. 20121000302).

The 30 at.% Ho: BaY2F8 crystals were grown by the Czochralski method, and their spectroscopic properties are analyzed systematically by standard Judd–Ofelt theory. The Judd–Ofelt intensity parameters are estimated to be Ω2 = 6.74 × 10−20 cm2, Ω4 = 1.20 × 10−20 cm2, and Ω6 = 0.66 × 10−20 cm2, and the fluorescence branching ratios and radiative lifetimes for a series of excited state manifolds are also determined. The emission cross sections with our measured infrared luminescence spectra, especially important for 4.1 μm, are calculated to be about 4.37 × 10−21 cm2. The crystal quality is preliminarily tested through a mid-infrared laser emission experiment.

Investigation on growth and defects of Ho3+:BaY2F8 crystals grown by Czochralski method

Large and heavily Ho3+-doped BaY2F8 single crystals were grown by the Czochralski method. X-ray powder diffraction was applied to analyze the phase of the crystal samples. Simultaneously, metallographic microscope, scanning electron microscopy and energy dispersive spectrometer were employed to observe and investigate defects in the as grown crystals. Two significant kinds of defects, namely cracking and impurities were discovered in the samples of Ho3+:BaY2F8 single crystals. Theoretical analyses suggested that mechanisms concerning the formation of the impurities such as bubbles and inclusions were considered to be closely related to the growth temperature and atmosphere while the former defect was primarily brought by the lattice distortion relating to the thermal stress and the impurities. Based on the results of experiments and theoretical analyses, the parameters of growth process were optimized and a crack free 20 mol% Ho3+:BaY2F8single crystal has been successfully obtained. Furthermore, the UV–Vis-IR (0.2–10 μm) absorption spectra of BaY2F8 single crystal and the crystal heavily doped with Ho3+ ions (20 mol%) have been investigated at room temperature.

Optical spectroscopy of low-phonon Ho3+ doped BaY2F8 single crystal

The Ho:BaY2F8 crystal was grown by Czochralski method. The crystal phase structure and absorption spectra were tested, the absorption peak exists near 899 nm, the absorption cross section was 1.27 × 10−21 cm2. The emission spectra of crystals in the vicinity of 2 and 3.9 μm were measured, the 2 μm near infrared light induced by 5I7 → 5I8 transition of Ho3+ ions was observed, as well as the fluorescence output at 3.9 μm (5I5 → 5I6), emission cross section at 3.9 μm was calculated to be 0.86 × 10−21 cm2. We suppose that the Ho:BaY2F8 crystal has a large application prospect for the 2–4 μm wavelength near infrared laser.

Multiple polarization orange and red laser emissions with Pr:BaY₂F₈.

We investigated the polarization of continuous-wave laser emission in the orange region, at 607 nm, and in the red region, at 639 nm and 643 nm, from a Pr:BaY2F8 (Pr:BYF) crystal, pumped by a 445 nm laser diode. We achieved linearly polarized emission along two optic axes of the crystal by changing its orientation with respect to the pump. Simultaneous emission of two orthogonal linear polarizations was observed in the orange region, at the same wavelength, and in the red region, with concurrent emission from the two separate lines.

Temperature dependence of luminescence behavior in Er3+-doped BaY2F8 single crystal

BaY2F8 single crystals doped with Er3+ ions have been grown by the temperature gradient method. The absorption, excitation and emission spectra for Er3+-doped BaY2F8 crystals were measured at room temperature (297K) and 12K. The effect of temperature on the luminescence intensity and effective bandwidth was investigated in the range of 12–297K. The temperature dependence of the fluorescence intensity ratio (FIR) for the 522nm emission (2H11/2→4I15/2 transition) and the 552nm emission (4S3/2→4I15/2 transition) was also studied in the range of 12–297K. Based on the fitting FIR curve, the value of the constant term B (2.25) was obtained. The fitting FIR curve and FIR equation may have a potential application in the temperature measurement. In addition, the up-conversion spectrum at room temperature was recorded under excitation of 980nm and the up-conversion mechanism was analyzed in detail.