Spectroscopic properties of Nd<Superscript>3+</Superscript>:CaNb<Subscript>2</Subscript>O<Subscript>6</Subscript> crystal

The absorption spectra, fluorescence spectrum and fluorescence decay curve of Nd³⁺ ions in CaNb₂O₆ crystal were measured at room temperature. The peak absorption cross section was calculated to be 6.202×10⁻²⁰ cm² with a broad FWHM of 7 nm at 808 nm for E//a light polarization. The spectroscopic parameters of Nd³⁺ ions in CaNb₂O₆ crystal have been investigated based on Judd-Ofelt theory. The parameters of the line strengths Ω _t are Ω ₂=5.321×10⁻²⁰ cm²,Ω ₄=1.734×10⁻²⁰ cm²,Ω ₆=2.889×10⁻²⁰ cm². The radiative lifetime, the fluorescence lifetime and the quantum efficiency are 167 μs, 152 μs and 91%, respectively. The fluorescence branch ratios are calculated to be β ₁=36.03%,β ₂=52.29%,β ₃=11.15%,β ₄=0.533%. The emission cross section at 1062 nm is 9.87×10⁻²⁰ cm².     

Growth and laser properties of Nd<Superscript>3+</Superscript>:Sr<Subscript>6</Subscript>YSc(BO<Subscript>3</Subscript>)<Subscript>6</Subscript> crystal

The crystal of Nd³⁺:Sr₆YSc(BO₃)₆ with dimensions of O 19×42 mm³ was grown by the Czochralski method. It’s spectral and laser properties have been investigated. The absorption cross section is 1.47×10^-20 cm² with a FWHM 12.0 nm at 807 nm, the emission cross section is 1.57×10^-19 cm² at 1060 nm, and the fluorescence lifetime is 76 μs at room temperature. The maximum laser output is 25.7 mJ at 1.06 μm pumped by a single Xenon flash lamp and the overall and average slope efficiencies are 0.12% and 0.09%, respectively. The laser energy threshold value is 1.28 J. PACS 42.55.Rz; 42.70.Hj; 78.20.-e     

Luminescence and selective heat radiation of Yb<Subscript>2</Subscript>O<Subscript>3</Subscript> upon the resonant and thermal laser excitation

Yb₂O₃ polycrystals with a size of up to 10 mm are synthesized using the sintering and melting of the ultrapure Yb₂O₃ powders by the CO₂-laser radiation with the power P _L ≤ 100 W at the wavelength λ = 10.6 μm at the melting point T _m = 2703 K, forming due to surface tension in melt, and crystallization in air. The analysis of the polycrystal microstructure using the methods of optical and electron microscopy and X- ray diffractometry shows that perfect oxide crystallites are formed in the course of crystallization after melting-through. The transformation of the luminescence and selective heat radiation (SHR) spectra of the Yb₂O₃ polycrystals is studied under the resonant excitation at λ ≈ 975 nm using a laser diode and the laser heating at the wavelength λ = 10.6 μm. When the resonant excitation power of the Yb³⁺ ions increases from 0.15 to 4.5 W, the Stokes luminescence of the Yb₂O₃ polycrystals is sequentially transformed into SHR and the thermal radiation of the crystal lattice. The transformation of the emission spectra of the Yb₂O₃ polycrystals with an increase in the laser heating intensity by about four orders of magnitude can be represented as the low-temperature heat radiation, spectral burst of the thermodynamically nonequilibrium SHR of the Yb³⁺ ions, and the high-temperature radiation of the crystal lattice. The temperature dependence of the luminescence spectra and SHR of the Yb₂O₃ polycrystals on the intensity of the laser and laser-thermal excitation and the concentration quenching of the Yb³⁺ luminescence in oxides indicate the key role of the interaction of the f-electron shell of the Yb³⁺ ions with the natural oscillations of the crystal lattice in the processes of the multiphonon excitation and nonradiative (multiphonon) and radiative (vibronic) relaxation.     

Thermal and optical properties of Tm<Superscript>3+</Superscript>:NaLa(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

A Tm³⁺-doped NaLa(WO₄)₂ single crystal with a dimension of Φ20 mm×40 mm was grown by the Czochralski method. Anisotropic thermal expansion coefficients of this crystal were investigated. Polarized absorption spectra, emission spectra and decay curve were recorded at room temperature. The absorption and emission cross-section were presented. Based on the Judd–Ofelt analysis, we obtained the three intensity parameters: Ω₂=10.21×10^-20, Ω₄=2.66×10^-20, and Ω₆=1.46×10^-20 cm². The radiative probabilities, radiative lifetimes, and branch ratios of Tm³⁺:NaLa(WO₄)₂ were calculated, too. Luminescence lifetime of the ³ H ₄ level was measured to be 220 μs. The stimulated emission cross-section for the ³ F ₄→³ H ₆ transition were determined using the reciprocity method, potential laser gain for this transition were also investigated, the gain curves implied that the tunable range is up to 200 nm. PACS 42.70.Hj; 78.20.-e     

Absorption saturation properties and laser Q-switch performance of Cr<Superscript>5+</Superscript>-doped YVO<Subscript>4</Subscript> crystal

The ground-state and excited-state absorption cross sections of a Cr⁵⁺:YVO₄ single crystal were determined by the intensity-dependant transmission measurements to be(2.0±0.4)×10^-18 cm² and less than 0.1×10^-18 cm² at 1.08 μm, respectively. The bleaching relaxation time of the Cr⁵⁺:YVO₄ crystal was estimated to be 3.5±1.5 ns. Passive Q-switching of a diode-pumped Yb³⁺:YVO₄ laser with the Cr⁵⁺:YVO₄ saturable absorber was realized. PACS 42.55.Xi; 42.60.Gd; 42.70.Hj     

Continuous wave laser operation of Yb<Superscript>3+</Superscript>:YVO<Subscript>4</Subscript>

We have demonstrated the continuous wave laser operation of Yb³⁺:YVO₄. For Ti:Al₂O₃ laser pumping at 985 nm, a maximum slope efficiency of 41.1% and a threshold pump power of 76 mW were obtained. The maximum output power was 433 mW at a laser wavelength of 1037 nm.Using a cw diode laser around 974 nm as a pump source, a slope efficiency of 10.9% and a maximum output power of 152 mW were achieved at a laser wavelength of 1039 nm. The laser threshold pump power was 608 mW with respect to the absorbed pump power. The effective emission cross-sections for the ²F_5/2²F_7/2 transition were determined using the Füchtbauer–Ladenburg equation. The maxima of the effective absorption and emission cross-sections were found at 984.5 nm (6.74×10^-20 cm²) in -po larization and 985.5 nm (4.28×10^-20 cm²) also in -p olarization. The upper laser level lifetime was measured with suppression of radiation trapping and is around 318 s. PACS 42.55.Rz; 42.55.Xi; 42.70.Hj     

A cobalt-doped transparent glass ceramic saturable absorber Q-switch for a LD pumped Yb<Superscript>3+</Superscript>/Er<Superscript>3+</Superscript> glass microchip laser

We fabricate a transparent glass ceramic contains magnesium-aluminum spinel nanocrystallites doped with Co²⁺ ions. The ground-state and excited-state absorption cross section of this glass ceramic at 1.54 μm are estimated to be (2.8 ± 0.3) × 10⁻¹⁹ cm² and (4.8 ± 0.5) × 10⁻²⁰ cm², respectively. For the first time, the passively Q-swithched operation of LD pumped 1.54 μm microchip Yb³⁺/Er³⁺ glass laser is realized with transparent glass ceramic as saturable absorber. The Q-switched pulses of 3.846 kHz in repetition rate, 6.2 ns in duration and 6.3 μJ in energy are obtained. At last, the dependences of pulse width and repetition rate on pump power are also investigated.     

Anisotropy of nonlinear absorption in Co<Superscript>2+</Superscript>:MgAl<Subscript>2</Subscript>O<Subscript>4</Subscript> crystal

Anisotropy of the nonlinear absorption of Co²⁺ ions in MgAl₂O₄ single crystal at the wavelengths of 1.35 and 1.54 μm has been experimentally demonstrated. The experimental data are analyzed in the framework of a phenomenological model when the Co²⁺ ions are described as three sets of linear dipoles oriented along the crystallographic axes. Ground-state and excited state absorption cross-sections at 1.35 and 1.54 μm are evaluated to be σ_gsa=(4.0±0.3)×10^-19, σ_esa=(3.6±0.4)×10^-20 cm² and σ_gsa=(5.1±0.3)×10^-19, σ_esa=(4.6±0.4)×10^-20 cm², respectively. PACS 42.55.Rz; 71.20.Be     

Lasing Yb<Superscript>3+</Superscript> in crystals with a wavelength dependence anisotropy displayed from La<Subscript>2</Subscript>CaB<Subscript>10</Subscript>O<Subscript>19</Subscript>

We report spectroscopic and laser properties for propagation directions outside the principal axes of Yb³⁺-doped low symmetry laser crystals with a special devotion to the wavelength dependence anisotropy. We illustrate our report with experimental data in the 900–1075 nm range of wavelengths from the Yb³⁺:La₂CaB₁₀O₁₉ monoclinic crystal excited under laser diode pumping at 975 nm. This study, which makes easier the realization of Yb³⁺ lasers with an efficient free-running operation at the wavelength having the highest emission intensity or at a specified wavelength, or emitting two frequencies with a specified frequency difference, is of promising interest for applications.     

Spectroscopic properties of Er<Superscript>3+</Superscript> ions in LiNbO<Subscript>3</Subscript> crystals codoped with HfO<Subscript>2</Subscript>

The results of the spectroscopic analysis of transition strengths for Er³⁺ ions in a series of Hf:Er:LiNbO₃ crystals with variable Hf content and fixed Er content are reported. Unpolarized UV-VIS-NIR absorption spectra, upconversion fluorescence spectra excited at 800 nm, and microsecond time-resolved spectra excited at 400 nm and 800 nm by 800 nm femtosecond laser were measured at room temperature. The HfO₂ incorporation has influence on Er³⁺ radiative lifetimes, and fluorescence branching ratios. For Hf(4 mol %):Er(1 mol %):LiNbO₃, Ω₂=2.63×10^-20 cm², Ω₄=2.86×10^-20 cm², and Ω₆=0.72×10^-20 cm². Ω₂<Ω₄ is contrary to the Er³⁺ general trend of Ω₂>Ω₄>Ω₆ when the Hf content is below its threshold concentration. In addition, the sum of Ω increases with the Hf content when the HfO₂ content below 6 mol % is unfamiliar. The upconversion mechanism is discussed in this work. PACS 71.20.Eh; 77.84.Dy; 42.62.Fi; 42.65.Ky     

CW and tunable laser operation of Yb<Superscript>3+</Superscript> doped CaF<Subscript>2</Subscript>

We present the spectroscopic properties and room temperature of a cw tunable laser operation with Yb³⁺ doped CaF₂ single crystals grown in our laboratory. A laser slope efficiency of 50% with respect to the absorbed 920 nm pump power was obtained, and the laser wavelength could be tuned between 1000 and 1060 nm. PACS 42.55R; 42.70An erratum to this article can be found at     

Spectroscopic properties of Er<Superscript>3+</Superscript> in Sc:LiNbO<Subscript>3</Subscript> crystal

The results of Er³⁺ ion spectroscopic analysis in Sc:LiNbO₃ crystals were reported. The line strengths from the ground state to the excited state were evaluated from the measured unpolarized absorption spectrum and analyzed by using standard Judd–Ofelt theory. For Sc(3 mol. %):Er (1 mol. %):LiNbO₃ crystal, the obtained intensity parameters are: Ω₂=3.72×10^-20 cm², Ω₄=1.07×10^-20 cm², and Ω₆=0.98×10^-20 cm². The fluorescence spectra and microsecond time-resolved spectra were investigated in the visible region. The excited state absorption transition strengths at 800 nm excitation were evaluated based on Judd–Ofelt theory. The results obtained here were compared to results from other research on Er:LiNbO₃ crystals. PACS 71.20.Eh; 77.84.Dy; 42.70.Hj; 42.62.Fi; 42.65.Ky     

Wavelength tunable femtosecond laser based on short Er<Superscript>3+</Superscript>/Yb<Superscript>3+</Superscript> codoped phosphate fiber

A wavelength tunable laser system mode-locked by nonlinear polarization evolution based on a 3 cm-long homemade Er³⁺/Yb³⁺ codoped phosphate fiber has been reported. By simply adjusting the polarization controllers, the central wavelength of the mode-locked spectrum can be tuned over 1537.1–1563.3 nm continuously. Moreover, 264-fs pulse with 3-dB spectral width of 39.6 nm and peak power of 7.8 kW at a 7.55 MHz repetition rate is generated directly from the all-fiber ring cavity.     

Intense White Luminescence from Combustion Synthesized Ca<Subscript>12</Subscript>Al<Subscript>14</Subscript>O<Subscript>33</Subscript>: Yb<Superscript>3+</Superscript>/Yb<Superscript>2+</Superscript> Single Phase Phosphor

The Ca₁₂Al₁₄O₃₃: Yb³⁺/Yb²⁺ single phase nano-phosphor has been synthesized through combustion route and its luminescence and lifetime studies have been carried out up to 20 K using 976 and 266 nm excitations. The samples heated in open atmosphere have shown the presence of Yb in Yb³⁺ and Yb²⁺ states. The 976 nm excitation results a cooperative upconversion emission at 486 nm due to the Yb³⁺ state and a broad band in the blue region and has been assigned to arise from the defect centers. The 266 nm excitation on the other hand results a broad emission band even from as-synthesized phosphor without doping of Yb, the width of which increases in presence of Yb due to the emission from Yb²⁺ ions formed in heated samples. The white emission covers almost whole visible region with bandwidth 190 nm. The ions in Yb²⁺ state has been found to increase with the increase in heating temperature up to 1,273 K. A back conversion of Yb²⁺ to Yb³⁺ has been observed for higher temperatures. Effect of boric and phosphoric acids as flux on the emission properties of Yb³⁺ and Yb²⁺ states have been examined and discussed. Quantum yield of emission has also been determined for different samples.     

Spectroscopic Properties and Energy Transfer Analysis of Tm<Superscript>3+</Superscript>-Doped BaF<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript> Glass

This paper reports on the spectroscopic properties and energy transfer analysis of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂-La₂O₃ glasses with different Tm₂O₃ doping concentrations (0.2, 0.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0 wt%). Mid-IR fluorescence intensities in the range of 1,300 nm−2,200 nm have been measured when excited under an 808 nm LD for all the samples with the same pump power. Energy level structure and Judd-Ofelt parameters have been calculated based on the absorption spectra of Tm³⁺, cross-relaxation rates and multi-phonon relaxation rates have been estimated with different Tm₂O₃ doping concentrations. The maximum fluorescence intensity at around 1.8 μm has been obtained in Tm₂O₃-3 wt% sample and the maximum value of calculated stimulated emission cross-section of Tm³⁺ in this sample is about 0.48 × 10⁻²⁰ cm² at 1,793 nm, and there is not any crystallization peak in the DSC curve of this sample, which indicate the potential utility of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂- La₂O₃ glass for 2.0-μm optical fiber laser.     

Sensitizer-dependent up-conversion of Ho<Superscript>3+</Superscript> in nanocrystalline Y<Subscript>2</Subscript>O<Subscript>3</Subscript>

Ho³⁺–Yb³⁺ co-doped Y₂O₃ nanocrystals were synthesized by firing hydroxy carbonate precursors. Yb³⁺-concentration-dependent up-conversion properties of Ho³⁺ in Y₂O₃ nanocrystals have been investigated. The relative intensity of up-converted red emission increases more quickly than that of the green and the near-infrared ones with the enhancement of the concentration of Yb³⁺. It is believed that the energy process ⁵ S ₂ (⁵F₄) (Ho) + ⁵ I ₇ (Ho) →⁵ I ₆ (Ho)+⁵ F ₅ (Ho) plays an important role in the population of the ⁵ F ₅ level of Ho³⁺. The result indicates that the intensity ratio of the green emission to the red one can be tuned by changing the sensitizer concentration. PACS 78.55.-m     

Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

Quasi-three level laser based on diode-pumped Nd<Superscript>3+</Superscript>:YAlO<Subscript>3</Subscript> crystal

We describe the output performances of the 930 nm ⁴ F _3/2 → ⁴ I _9/2 transition in Nd³⁺:YAlO₃ (Nd:YAP) under in-band pumping with diode laser at the 803 nm wavelength. An end-pumped Nd:YAP crystal yielded 1.13 W of continuous-wave (CW) output power for 17.8 W of incident pump power. Moreover, intracavity second-harmonic generation has also been achieved with a power of 172 mW at 465 nm by using a LiB₃O₅ (LBO) nonlinear crystal. The blue beam quality factor M ² was less than 1.3. The blue power stability was less 3% in 60 min.     

Optical spectra and excited state relaxation dynamics of Sm<Superscript>3+</Superscript> in Gd<Subscript>2</Subscript>SiO<Subscript>5</Subscript> single crystal

Single crystals of gadolinium orthosilicate Gd₂SiO₅ containing 0.5 at% and 5 at% of Sm³⁺ were grown by the Czochralski method. Optical absorption spectra, luminescence spectra and luminescence decay curves were recorded for these systems at 10 K and at room temperature. Comparison of optical spectra recorded in polarized light revealed that the anisotropy of this optically biaxial host affects the intensity distribution within absorption and emission bands related to transitions between multiplets rather than the overall band intensity. It has been found that among four bands of luminescence related to the ⁴G_5/2→⁶H_J (J=5/2–11/2) transitions of Sm³⁺ in the visible and near infrared region the ⁴G_5/2 →⁶H_7/2 one has the highest intensity with a peak emission cross section of 3.54×10⁻²¹ cm² at 601 nm for light polarized parallel to the crystallographic axis c of the crystal. The luminescence decay curve recorded for Gd₂SiO₅:0.5 at% Sm³⁺ follows a single exponential time dependence with a lifetime 1.74 ms, in good agreement with the ⁴G_5/2 radiative lifetime τ _rad=1.78 ms calculated in the framework of Judd-Ofelt theory. Considerably faster and non-exponential luminescence decay recorded for Gd₂SiO₅:5 at% Sm³⁺ sample was fitted to that predicted by the Inokuti-Hirayama theory yielding the microparameter of Sm³⁺–Sm³⁺ energy transfer C _da=1.264×10⁻⁵² cm⁶×s⁻¹.     

Luminescence of transparent glass ceramics containing Er<Superscript>3+</Superscript> and Yb<Superscript>3+</Superscript> zirconate-titanate nanocrystals

Luminescence regularities have been studied in new erbium/ytterbium materials based on glasses and glass ceramics of a magnesium-aluminosilicate system containing nanoscale erbium/ytterbium zirconate titanate crystals with the pyrochlore structure. Lifetimes of Yb³⁺ and Er³⁺ ions in the ² F_5/2 state and in the ⁴I_11/2 and ⁴I_13/2 states, respectively, and the efficiency of Yb³⁺ → Er³⁺ energy transfer have been evaluated. The identified spectral-luminescent characteristics of the studied glasses and glass ceramics co-doped with erbium and ytterbium ions show that these materials are promising media for producing laser generation in the spectral range around 1.5 μm.