Optical and EPR spectroscopy of Zn:Cr:ZnSe and Zn:Fe:ZnSe crystals

Optical and EPR characterization of Cr and Fe doped ZnSe crystals annealed in Zn vapor revealed a strong bleaching of the divalent state of transition metal ions. Photo induced EPR kinetics were studied in 20–80 K temperature range. Analysis of time-dependent data reveals Cr¹⁺ signal rise time decreases with increasing temperature. The non-exponential decay of Cr¹⁺ concentration were analyzed using Auger-type recombination process. The photoluminescence quantum yield of Cr²⁺ ions at ⁵E(D) → ⁵T₂(D) mid-IR transition excited via chromium ionization process was measured to be close to 100%.     

Vapor growth of CdSe:Cr and CdS:Cr single crystals for mid-infrared lasers

Single crystals of CdSe:Cr and CdS:Cr with the doping level up to 10¹⁹ cm⁻³ were grown by a vapor phase contact-free technique. An efficient room-temperature pulsed and continuous wave (CW) lasing with the CdSe:Cr crystal was achieved. First a pulsed lasing with the CdS:Cr crystal was also demonstrated. The slope efficiency on the absorbed energy was as high as 46.5% for Cr²⁺:CdSe and 39% for Cr²⁺:CdS lasers. Using an intra-cavity prism, the Cr²⁺:CdSe laser wavelength was continuously tuned from 2.26 to 3.61 μm while the Cr²⁺:CdS laser from 2.2 to 3.3 μm. For the laser wavelength, the crystal passive loss coefficient was estimated to be smaller than 0.045 cm⁻¹ for CdSe:Cr crystals and 0.039 cm⁻¹ for CdS:Cr crystals. For the Cr²⁺:CdSe laser, the CW output power up to 1.07 W was achieved.     

Color centers and charge state recharge in γ-irradiated Yb:YAP

Gamma-ray irradiation induced color centers and charge state recharge of impurity and doped ion in 10 at.% Yb:YAP have been studied. The change in the additional absorption (AA) spectra is mainly related to the charge exchange of the impurity Fe²⁺, Fe³⁺ and Yb³⁺ ions. Two impurity color center bands at 255 and 313 nm were attributed to Fe³⁺ and Fe²⁺ ions, respectively. The broad AA band centered at 385 nm may be associated with the cation vacancies and F-type center. The transition Yb³⁺ → Yb²⁺ takes place in the process of γ-irradiation. Oxygen annealing and γ-ray irradiation lead to an opposite effect on the absorption properties of the Yb:YAP crystal. In the air annealing process, the transition Fe²⁺ → Fe³⁺ and Yb²⁺ → Yb³⁺ take place and the color centers responsible for the 385 nm band was destroyed.     

Toxicity assessment of three-component Fe–Cr–Ni biomedical materials using an augmented simplex design

This study firstly establishes the toxicity assessment of three-component Fe–Cr–Ni biomedical materials using Probit dose–response model and augmented simplex design. The individually determined toxicity rankings of these three cations is in the order Ni²⁺ ≥ Cr³⁺ > Fe³⁺. The ternary Fe–Cr–Ni system's EC₅₀ contour plot shows a hump with EC₅₀ = 897.5 mg/L, and a saddle with EC₅₀ = 637.5 mg/L. Ternary Fe–Cr–Ni biomedical implants may possess good biocompatibility when the chemical compositions of selectively leached metal ions approach the hump region, but present at increased toxic risk when close to the saddle region. Toxicity of Fe–Cr–Ni three-component biomedical materials with various chemical compositions can be predicted and verified economically and efficiently using an augmented simplex design.     

Synthesis,EPR and optical spectroscopy of the Cr-doped tetraborate glasses

Electron paramagnetic resonance (EPR), optical absorption and photoluminescence spectra as well as luminescence kinetics of the Li₂B₄O₇:Cr and KLiB₄O₇:Cr tetraborate glasses were investigated at T = 300 K. The Li₂B₄O₇:Cr and KLiB₄O₇:Cr glasses containing 0.4 and 1.6 mol.% Cr₂O₃ of high optical quality were obtained from polycrystalline compounds by fast cooling of the melts. The X-band EPR spectroscopy shows that the Cr impurity is incorporated in the tetraborate glass network as isolated Cr³⁺ centers and Cr³⁺–Cr³⁺ pairs coupled by magnetic dipolar and exchange interactions. The EPR spectral parameters (g_eff and ΔB_pp) of both Cr³⁺ and Cr³⁺–Cr³⁺ centers in the Li₂B₄O₇:Cr and KLiB₄O₇:Cr glasses were measured and analyzed. All transitions in optical absorption, luminescence excitation and emission spectra of these glasses are identified. Broad complex bands that peak near 615, 405, and 350 nm in optical absorption and luminescence excitation spectra correspond to the ⁴A_2g(F) → ⁴T_2g(F), ⁴A_2g(F) → ⁴T_1g(F), and ⁴A_2g(F) → ⁴T_1g(P) spin-allowed transitions of the Cr³⁺ centers in distorted octahedral sites of the tetraborate glass network. The octahedral (cubic) crystal field strength (10Dq) and Racach parameters (B and C) for Cr³⁺ centers in Li₂B₄O₇:Cr and KLiB₄O₇:Cr glasses are estimated. Narrow and broad emission bands in red – NIR regions are assigned to the ²E_g(F) → ⁴A_2g(F) (R₁ line) and ⁴T_2g(F) → ⁴A_2g(F) (electron-vibration) transitions, which correspond to the Cr³⁺ centers in high-field and low-field sites, respectively. All observed emission bands are characterized by non-exponential decay. Measured average lifetimes and local structure of the Cr³⁺ centers in high-field and low-field sites of the Li₂B₄O₇:Cr and KLiB₄O₇:Cr glass network have been discussed.     

Enhanced emission of 2.7 μm from Er3+/Nd3+-codoped LiYF4 single crystals

An Er³⁺/Nd³⁺ co-doped LiYF₄ single crystal of ～Φ 12 mm × 95 mm size with high quality was grown by a Bridgman method. The luminescent properties of the crystals with different Er³⁺ and Nd³⁺ concentrations were studied. Compared with the Er³⁺ single-doped LiYF₄ crystal extremely enhanced emission at 2.7 μm from the Er³⁺/Nd³⁺ co-doped LiYF₄ was observed upon excitation of an 800 nm laser diode. Meanwhile, the green up-conversion emission and near infrared emission at 1.5 μm from Er³⁺ in the co-doped crystals were effectively restricted. The luminescent mechanisms for the Er³⁺/Nd³⁺ co-doped crystals were analyzed and the possible energy transfer processes were proposed. The energy transfer efficiencies for (Er³⁺:⁴I_13/2, Nd³⁺:⁴I_9/2) → (Er³⁺:⁴I_15/2, Nd³⁺:⁴I_15/2) and (Nd³⁺:⁴F_3/2, Er³⁺:⁴I_15/2) → (Nd³⁺:⁴I_9/2, Er³⁺:⁴I_9/2) were calculated. It was found that Er³⁺/Nd³⁺ co-doped single crystal may be a potential host for 2.7 μm lasers.     

Al2O3:Cr3+ microfibers by hydrothermal route: Luminescence properties

Uniform Al₂O₃:Cr³⁺ microfibers were synthesized by using a hydrothermal route and thermal decomposition of a precursor of Cr³⁺ doped ammonium aluminum hydroxide carbonate (denoted as AAHC), and characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL) spectra and decay curves. XRD indicated that Cr³⁺ doped samples calcined at 1473 K were the most of α-Al₂O₃ phase. SEM showed that the length and diameter of these Cr³⁺ doped alumina microfibers were about 3–9 μm and 300 nm, respectively. PL spectra showed that the Al₂O₃:Cr³⁺ microfibers presented a broad R band at 696 nm. It is shown that the 0.07 mol% of doping concentration of Cr³⁺ ions in α-Al₂O₃:Cr³⁺ was optimum. According to Dexter's theory, the critical distance between Cr³⁺ ions for energy transfer was determined to be 38 Å. It is found that the curve followed the single-exponential decay.     

EPR,optical absorption and luminescence studies of Cr3+-doped antimony phosphate glasses

Antimony phosphate glasses (SbPO) doped with 3 and 6 mol% of Cr³⁺ were studied by Electron Paramagnetic Resonance (EPR), UV–VIS optical absorption and luminescence spectroscopy. The EPR spectra of Cr³⁺-doped glasses showed two principal resonance signals with effective g values at g = 5.11 and g = 1.97. UV–VIS optical absorption spectra of SbPO:Cr³⁺ presented four characteristics bands at 457, 641, 675, and 705 nm related to the transitions from ⁴A₂(F) to ⁴T₁(F), ⁴T₂(F), ²T₁(G), and ²E(G), respectively, of Cr³⁺ ions in octahedral symmetry. Optical absorption spectra of SbPO:Cr³⁺ allowed evaluating the crystalline field Dq, Racah parameters (B and C) and Dq/B. The calculated value of Dq/B = 2.48 indicates that Cr³⁺ ions in SbPO glasses are in strong ligand field sites. The optical band gap for SbPO and SbPO:Cr³⁺ were evaluated from the UV optical absorption edges. Luminescence measurements of pure and Cr³⁺-doped glasses excited with 350 nm revealed weak emission bands from 400 to 600 nm due to the ³P₁ → ¹S₀ electronic transition from Sb³⁺ ions. Cr³⁺-doped glasses excited with 415 nm presented Cr³⁺ characteristic luminescence spectra composed by two broad bands, one band centered at 645 nm (²E → ⁴A₂) and another intense band from 700 to 850 nm (⁴T₂ → ⁴A₂).     

Judd–Ofelt analysis and energy transfer processes of Er3+ and Nd3+ doped fluoroaluminate glasses with low phosphate content

Spectroscopic property and energy transfer processes of singly doped and codoped Er³⁺ and Nd³⁺ fluoroaluminate glasses with low phosphate content are systematically analyzed. The absorption spectra of these glasses are tested, and the Judd–Ofelt (J–O) and radiative parameters are discussed based on J–O theory and the parameters changes substantially because of the other codoping ions. As for Nd³⁺: the main emission bands at 0.9 and 1.05 μm decrease in the codoped sample under the excitation of an 800 nm laser diode from the emission spectra because the Er³⁺: ⁴I_11/2 level reduces the Nd³⁺: ⁴F_3/2 level effectively through the energy transfer process Nd³⁺: ⁴F_3/2 → Er³⁺: ⁴I_11/2. For Er³⁺, the emission at 1.5 μm is restrained by codoping with Nd³⁺ ions from the energy transfer process Er³⁺: ⁴I_13/2 → Nd³⁺: ⁴I_15/2. The emission at 2.7 μm is enhanced because the Nd³⁺ ions deplete the lower level and exert a positive effect on the upper laser level. The microparameters of the energy transfer between the Er³⁺ and Nd³⁺ ions are calculated and discussed using Forster–Dexter theory. The energy transfer efficiencies of the Nd³⁺: ⁴F_3/2 to the Er³⁺: ⁴I_11/2 and the Er³⁺: ⁴I_13/2 to the Nd³⁺: ⁴I_15/2 are 28.8% and 74.5%, respectively. These results indicate that Nd³⁺ can be an efficient sensitizer for Er³⁺ to obtain Mid-infrared (Mid-IR) emission and the codoped Er³⁺/Nd³⁺ fluoroaluminate glass with low phosphate content is suitable to be used as the fiber optical gain media for 2.7 μm laser generation.     

Study on 2.0 μm fluorescence of Ho-doped water-free fluorotellurite glasses

Ho³⁺-doped water-free fluorotellurite glasses with composition of 60TeO₂–30ZnF₂–10NaF (mol%, TZNF60) were made by using specially-designed physical and chemical dehydration technique. 2.04 μm fluorescence (Ho³⁺: ⁵I₇ → ⁵I₈) was observed experimentally and presented in this paper: A broad bandwidth of ∼149 nm, large simulated emission cross-section of 7.2 × 10⁻²¹ cm², and the longest reported fluorescence lifetime of ∼10 ms among all the reported Ho³⁺-doped oxide glasses. Thanks to the absence of OH groups and low phonon energy with the addition fluorides into tellurite oxide glasses, 1.00Ho-TZNF60 glass demonstrates the maximum figure of merit (σ_em × τ_f) of 7.13 × 10⁻²⁷ m² s, thus regarded as a promising optical material for the development of 2.0 μm fiber lasers.     

Low energy implantation to inhibit wear in N+ ions implanted WC–Co composite

This work discusses the influence of nitrogen ion (N⁺) implantation on wear resistance of WC–Co composite. The WC–Co samples were bombarded at low N⁺ ions energies of 20 and 30 keV and doses of 10¹⁷ and 2 × 10¹⁷ ions cm⁻². Tribological tests were conducted against cylindrical 100Cr6 pin at 200 N load and 180 mm s⁻¹ speed. The tests use water lubrication and four sample types with Co binder content ranging in 6.5–25%. The X-ray spectra reveal that implantation is able to transform the original [CFC] Co structure of virgin surface to harder amorphous phase. However, it was found that excessive low binder content alters the wear behavior on non-implanted samples since it causes wear rate transition from 0.59 × 10⁻⁷ to 2.1 × 10⁻⁷ mm³/(mm² s) imposing hence instable wear regime. The SEM micrographs confirm the formation of transferred film within the implanted worn surface owing to (i) an enhancement in Co flow and (ii) a generation of oxides (Fe₂O₃, Fe₃O₄, Co₂O₃, WO₂). While the formed film acts to inhibit severe abrasion, the material removal process combining cobalt flow and carbide grains pull-out seems to be associated with oxidation mechanisms to be accentuated with energy increase. The most improvements in wear resistance were observed on samples with the highest Co content and the results were found more sensitive to N⁺ ions implantation energy than dose.     

Visible to infrared energy conversion in Pr3+–Yb3+ co-doped fluoroindate glasses

Processes involving visible to infrared energy conversion are presented for Pr³⁺–Yb³⁺ co-doped fluoroindate glasses. The emission in the visible and infrared regions, the luminescence decay time of the Pr³⁺:³P₀ → ³H₄ (482 nm), Pr³⁺:¹D₂ → ³H₆ (800 nm), Yb³⁺:²F_5/2 → ²F_7/2 (1044 nm) transitions and the photoluminescence excitation spectra were measured in Pr³⁺ samples and in Pr³⁺–Yb³⁺ samples as a function of the Yb³⁺ concentration. In addition, energy transfer efficiencies were estimated from Pr³⁺:³P₀ and Pr³⁺:¹D₂ levels to Yb³⁺:²F_7/2 level. Down-Conversion (DC) emission is observed due to a combination of two different processes: 1-a one-step cross relaxation (Pr³⁺:³P₀ → ¹G₄; Yb³⁺:²F_7/2 → ²F_5/2) resulting in one photon emitted by Pr³⁺ (¹G₄ → ³H₅) and one photon emitted by Yb³⁺ (²F_7/2 → ²F_5/2); 2-a resonant two-step first order energy transfer, where the first part of energy is transferred to Yb³⁺ neighbor through cross relaxation (Pr³⁺:³P₀ → ¹G₄; Yb³⁺:²F_7/2 → ²F_5/2) followed by a second energy transfer step (Pr³⁺:¹G₄ → ³H₄; Yb³⁺:²F_7/2 → ²F_5/2). A third process leading to one IR photon emission to each visible photon absorbed involves cross relaxation energy transfer (Pr³⁺:¹D₂ → ³F₄; Yb³⁺:²F_7/2 → ²F_5/2).     

Tm3+ doped Ga–As–S chalcogenide glasses and fibers

Tm³⁺ doped Ga–As–S chalcogenide glass samples were produced using As₂S₃ pure glass as starting materials. Their photoluminescence properties were characterized and strong emission bands were observed at 1.2 μm (¹H₅ → ³H₆), 1.4 μm (³H₄ → ³F₄) and 1.8 μm (³F₄ → ³H₆) under excitation wavelengths of 698 nm and 800 nm. The thulium and gallium concentrations were optimized to achieve the highest photoluminescence efficiency. From the optimal composition, a Tm³⁺ doped Ga–As–S fiber was drawn and its optical properties were studied.     

Influence of manganese on magnetic and electronic properties of ZnCr2Se4

The Zn_1?xMn_xCr₂Se₄ crystals were prepared by chemical vapor transport in closed silica tubes using ZnSe and MnSe with CrCl₃ as the transport agent. Four crystals with different Mn content (x = 0.12, 0.13, 0.18 and 0.24) were studied by X-ray photoelectron spectroscopy (XPS) and magnetic measurements in order to determine influence of manganese on their magnetic and electronic properties. The XPS revealed no change of chemical shifts of Cr core lines indicating a Cr³⁺ (3d³) electronic configuration. Magnetization measurements revealed a systematic increase in saturation magnetic moments from 6.32 μ_B/mol for x = 0.12 to 7.63 μ_B/mol for x = 0.24, as well as effective paramagnetic Bohr magneton numbers from 4.87 μ_B/mol for x = 0.12 to 6.91 μ_B/mol for x = 0.24.     

Spectroscopic properties and energy transfer in Er–Tm co-doped bismuth silicate glass

In this paper, we investigate the spectroscopic properties of and energy transfer processes in Er–Tm co-doped bismuth silicate glass. The Judd–Ofelt parameters of Er³⁺ and Tm³⁺ are calculated, and the similar values indicate that the local environments of these two kinds of rare earth ions are almost the same. When the samples are pumped at 980 nm, the emission intensity ratio of Tm:³F₄ → ³H₆ to Er:⁴I_13/2 → ⁴I_15/2 increases with increased Er³⁺ and Tm³⁺ contents, indicating energy transfer from Er:⁴I_13/2 to Tm:³F₄. When the samples are pumped at 800 nm, the emission intensity ratio of Er:⁴I_13/2 → ⁴I_15/2 to Tm:³H₄ → ³F₄ increases with increased Tm₂O₃ concentration, indicating energy transfer from Tm:³H₄ to Er:⁴I_13/2. The rate equations are given to explain the variations. The microscopic and macroscopic energy transfer parameters are calculated, and the values of energy transfer from Er:⁴I_13/2 to Tm:³F₄ are found to be higher than those of the other processes. For the Tm singly-doped glass pumped at 800 nm and Er–Tm co-doped glass pumped at 980 nm, the pumping rate needed to realize population reversion is calculated. The result shows that when the Er₂O₃ doping level is high, pumping the co-doped glass by a 980 nm laser is an effective way of obtaining a low-threshold ∼2 μm gain.     

The 1.53 μm spectroscopic properties of Er3+/Ce3+/Yb3+ tri-doped tellurite glasses containing silver nanoparticles

The metallic silver nanoparticles (NPs) was introduced into the Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses with composition TeO₂–ZnO–La₂O₃ to improve the 1.53 μm band fluorescence. The UV/Vis/NIR absorption spectra, 1.53 μm band fluorescence spectra, fluorescence lifetimes, X-ray diffraction (XRD) curves, differential scanning calorimeter (DSC) curves and transmission electron microscopy (TEM) image of tri-doped tellurite glasses were measured, together with the Judd–Ofelt intensity parameters, emission cross-sections, absorption cross-sections and radiative quantum efficiencies were calculated to investigate the effects of silver NPs on the 1.53 μm band spectroscopic properties of Er³⁺ ions, structural nature and thermal stability of glass hosts. It is shown that Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glasses can emit intense 1.53 μm band fluorescence through the combined energy transfer (ET) processes from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions under the 980 nm excitation. At the same time, the introduction of an appropriate amount of silver NPs can further improve the 1.53 μm band fluorescence owing to the enhanced local electric field effect induced by localized surface Plasmon resonance (LSPR) of silver NPs and the possible energy transfer from silver NPs to Er³⁺ ions, and an improvement by about 120% of fluorescence intensity is found in the studied Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass containing 0.5 mol% amount of silver NPs with average diameter of ∼15 nm. The energy transfer mechanisms from Yb³⁺ to Er³⁺ ions and Er³⁺ to Ce³⁺ ions were also quantitatively investigated by calculating energy transfer microparameters and phonon contribution ratios. Furthermore, the thermal stability of glass host increases slightly with the introduction of silver NPs while the glass structure maintains the amorphous nature. The results indicate that the prepared Er³⁺/Ce³⁺/Yb³⁺ tri-doped tellurite glass with an appropriate amount of silver NPs is an excellent gain medium applied for 1.53 μm band EDFA pumped with a 980 nm laser diode (LD).     

Spectroscopic and laser characterization of Yb,Tm:KLu(WO4)2 crystal

We report on a comprehensive spectroscopic and laser characterization of monoclinic Yb,Tm:KLu(WO₄)₂ crystals. Stimulated-emission cross-section spectra corresponding to the ³F₄ → ³H₆ transition of Tm³⁺ ions are determined. The radiative lifetime of the ³F₄ state of Tm³⁺ ions is 0.82 ms. The maximum Yb³⁺ → Tm³⁺ energy transfer efficiency is 83.9% for 5 at.% Yb – 8 at.% Tm doping. The fractional heat loading for Yb,Tm:KLu(WO₄)₂ is 0.45 ± 0.05. Using a hemispherical cavity and 5 at.% Yb – 6 at.% Tm doped crystal, a maximum CW power of 227 mW is achieved at 1.983–2.011 μm with a maximum slope efficiency η = 14%. In the microchip laser set-up, the highest slope efficiency is 20% for a 5 at.% Yb– 8 at.% Tm doped crystal with a maximum output power of 201 mW at 1.99–2.007 μm. Operation of Yb,Tm:KLu(WO₄)₂ as a vibronic laser emitting at 2.081–2.093 μm is also demonstrated.     

Microstructure and tensile strength of PM304 composite

PM304 composite comprising NiCr (80/20) matrix (60 wt.%) combined with Cr₂O₃ (20 wt.%), Ag (10 wt.%) and eutectic BaF₂/CaF₂ (10 wt.%) as solid self-lubricants additives has been successfully prepared by mechanical alloying and powder metallurgy. The sintered PM304 samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS). The density of PM304 composite sintered at 1100 °C was 7.3 g/cm³, and the mean tensile strength 47 MPa. The size of Cr₂O₃, BaF₂/CaF₂ particles was less than 1 μm, and that of Ag particles below 5 μm. Fracture morphology indicates that the fracture of PM304 is mainly along Ni80Cr20 grains.     

Density functional investigation of Fen clusters (n ≤ 6) with Cr substitutions: UB3LYP/LanL2DZ calculation

Geometric parameters, binding energies, natural populations, natural electron configurations and magnetic moments are obtained for the clusters of Fe_n, Cr_n and Fe_n?xCr_x (n = 2–6, x = 1–6) optimized under the constraint of well-defined point group symmetries at the UB3LYP/LanL2DZ level. The substitutional effects of Cr in Fe_n are found in optimized structure, binding energies and electronic structures. The binding energies of Fe_n are generally decreased by successive substitutions of Cr atoms for Fe atoms in the clusters. In the mixed Fe–Cr clusters most of Cr–Cr bond lengths are larger than the Fe–Fe and Fe–Cr bond lengths because of the strong repulsion due to the magnetic frustration between atoms. Among the Fe–Fe, Fe–Cr and Cr–Cr bond lengths in the mixed clusters, the trend is found to become larger in that order. The larger distances between atoms in the mixed clusters are mostly caused by the strong magnetic repulsion. The changes are associated with those of electronic structure caused by the Cr substitutions, especially with the extent of contribution of 4s and 3d electrons to bond.     

Tritium digital autoradiography with a Medipix2 hybrid silicon pixel detector

We report on the first measurements of ³H beta autoradiography obtained using a room temperature hybrid pixel detector, consisting of the Medipix2 single particle counting read-out chip bump-bonded to a 300 μm thick silicon pixel detector. This system has 256×256 square pixels of 55 μm pitch for a total sensitive area of 14×14 mm². Each pixel contains a double threshold discriminator and a 13-bit counter. Using a detection threshold equivalent to less than 6 keV and a background count rate of 5×10⁻³ counts mm⁻² s⁻¹, with exposures up to several hours, real-time images have been obtained of tritium-labeled solution drops spotted on a thin mylar foil placed in contact with the continuous top electrode of the silicon detector, in open air condition.