Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystal

Focused infrared femtosecond laser pulses (wavelength ∼800 nm, emission pulse duration 100 fs) were employed to fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F₂ color centers of about 2 × 10¹⁸ cm⁻³ and refractive index change of about 1% at 633 nm were induced by the fs-laser irradiation. This technique was applied to fabricate a distributed-feedback (DFB) F₂ color center laser structure inside LiF single crystal. The LiF DFB laser exhibited laser oscillation at 707 nm at room temperature. The slope efficiency of ∼10% and beam divergence of ∼20 mrad were achieved.     

Synthesis and characterization of bulk amorphous Zr65Cu18Ni9Al8 and [Zr0.65Cu0.18Ni0.09Al0.08]98Er2 alloys

Bulk metallic glasses (BMGs), especially Zr-based BMGs, have attracted lot of attention of materials scientists because of their very attractive physical, thermal and mechanical properties and a few unique applications. In the present study, Zr₆₅Cu₁₈Ni₉Al₈ alloy was designed according to the criterion of conduction electron/atom (e/a ratio) ∼1.395 and average atomic size of alloy (R_a) ∼0.1498 nm. Addition of 2 at.% Er was carried out in the base alloy to investigate its effect on thermal and mechanical properties. Characterization of alloys was performed using the techniques of XRD, DSC, and SEM/EDS. Mechanical properties like Vicker’s microhardness, nanohardness, elastic modulus, density and fracture strength were measured. Average shear angle was found to be ∼35 ± 1° for base alloy and about 31 ± 1° for alloy containing 2 at.% Er. Wide supercooled liquid regions of 129 K and 119 K were found for the base alloy and the alloy containing 2 at.% Er.     

Electromechanical and electro-optic properties of xBiScO3–yBiGaO3–(1−x−y)PbTiO3 single crystals

Single crystals in the xBiScO₃–yBiGaO₃–(1−x−y)PbTiO₃ (BS–BG–PT) system were grown by the high temperature solution method using Pb₃O₄ and Bi₂O₃ as the flux. The dielectric permittivity (ε_r) at room temperature for unpoled tetragonal crystals was determined to be 500–600 with dielectric loss tangents less than 0.3%. The Curie temperature was found to be around ∼420–450°C, with a dielectric maximum, exhibiting relaxor behavior. The longitudinal piezoelectric coefficient (d₃₃) was found to be ∼300 pC/N for 〈0 0 1〉 oriented tetragonal crystals with electromechanical coupling factor (k₃₃) of 75%, with a shear mode, d₁₅∼290 pC/N and k₁₅∼45%, lateral mode, d₃₁∼−55 pC/N and k₃₁∼−37%. The remnant polarization (P_r) was 46 μC/cm² with a coercive field (E_c) of 43 kV/cm at 1 Hz and DC field of 60 kV/cm. The linear electro-optic (E-O) coefficients of poled crystals determined using an automated scanning Mach–Zehnder interferometer method at room temperature and wavelength of 632.8 nm were r₃₃=36 and r₁₃=4 pm/V, respectively.     

Optical and kinetic properties of cathodically deposited amorphous tungsten oxide films

Optical properties and the coloration–decoloration kinetics of electrochromic films of amorphous tungsten oxide (a-WO₃), produced by cathodic deposition from a sodium tungstate based aqueous peroxide electrolyte, have been investigated. As films color in 1 N H₂SO₄, sequential appearance of bands with maxima at ∼1 eV, 1.6 eV, 2 eV, and 2.4 eV is observed in their optical absorption and electrosorption spectra, is the same as in the case of reduction of nanosized hydrated-WO₃ colloids with a gradual decrease in their size to that of 12-tungsten polyanions with Keggin structure, indicating the presence of such polytungstates in cathodically deposited a-WO₃, too. When polytungstate is reduced by one electron, an absorption band with a maximum at ∼1.6 eV appears in the optical spectrum of the film. This band corresponds to the optical excitation of charge transfer of the W⁵⁺ → W⁶⁺ type between two adjacent tungsten atoms. The reduction of polytungstate by a second electron with potential shift towards more negative values is accompanied by the appearance of an analogous band with a maximum at ∼2 eV. The reduction of such polytungstates involves participation of the bulk of injected electrons, indicating their dominate role in the nanostructure of the films investigated. The effective co-diffusion coefficient of electrons and protons in cathodically deposited a-WO₃ exhibits a potential dependence with a maximum at 0.1 V against a silver-chloride electrode, where its value is ∼10⁻⁸ cm²/s. It has been shown that the decrease in this coefficient at potential values of over 0.1 V is caused by a decrease in electron mobility.     

Magnetic and magnetostrictive behavior of Dy3+ doped CoFe2O4 single crystals grown by flux method

We studied the effect of Dy³⁺ content on the magnetic properties of cobalt ferrite single crystal. The single crystals of CoFe_1.9Dy_0.1O₄ were grown by the flux method using Na₂B₄O₇.10 H₂O (Borax) as a solvent (flux). The black and shiny single crystals were obtained as a product. The X-ray diffraction analysis at room temperature confirmed the spinel cubic structure with lattice constant a=8.42 Å of the single crystals. The compositional analysis endorses the presence of constituents Co, Fe and Dy elements after sintering at 1300 °C within the final structure. The magnetic hysteresis measurements at various temperatures viz. 10 K, 100 K, 200 K and 300 K reveal the soft ferrimagnetic nature of the single crystal than that of for pure CoFe₂O₄. The observed saturation magnetization (M_s) and coercivity (H_c) are found to be lower than that of pure CoFe₂O₄ single crystal. The magnetostriction (λ) measurement was carried out along the [001] direction. The magnetic measurements lead to conclude that the present single crystals can be used for magneto-optic recording media.     

Growth of PBI2 single crystals from stoichiometric and Pb excess melts

We have successfully grown high-purity and -quality PbI₂ single crystals by the vertical Bridgman method. The rocking curves of four-crystal X-ray diffraction (XRD) show 120 arcsec in full-width at half-maximum (FWHM). The photoluminescence (PL) spectra at 7.8 K show the resolved intensive exciton emission line and the weak DAP emission band. The deep-level emissions are not observed. The measurement of the electrical and radiographic properties show that Leadiodide (PbI₂) single crystal has a resistivity of 5×10¹⁰ Ω cm and imager lag is 8 s, respectively. In order to improve the controllability of crystal growth, PbI₂ single crystals were also grown from a lead (Pb) excess PbI₂ source. The experimental results show very good reproducibility. In addition, the growth models of crystal are proposed, and the growth mechanism is discussed.     

Vertical Bridgman growth and characterization of large ZnGeP2 single crystals

The growth of ZnGeP₂ crystals by seeded Vertical Bridgman method was studied. High-quality near-stoichiometric ZnGeP₂ single crystals obtained were of 20–30 mm in diameter and 90–120 mm in length. By selection of the seed crystallographic orientation the single crystal ingots without cracks and twins were grown, as shown by X-ray diffraction. The infrared transmission property of the ZnGeP₂ crystals was studied by the calculated optical absorption coefficient spectra. The results showed that after thermal annealing of the crystals the optical absorption coefficient was ～0.10 cm^?1 at 2.05 μm, and ～0.01 cm^?1 at 3–8 μm. The rocking curves patterns of the (4 0 0) reflection demonstrated that the as-grown single crystals possessed a good structural quality. The composition of the crystals was close to the ideal stoichiometry ratio of 1:1:2. The low-loss typical ZnGeP₂ samples of 6 mm×6 mm×15 mm in sizes were cut from the annealed ingots for optical parametric oscillation experiments. The output power of 3.2 W was obtained at 3–5 μm when the incident pumping power of 2.05 μm laser was 9.4 W, and the corresponding slope efficiency and the conversion efficiency were 44% and 34%, respectively.     

Fabrication of Cr4+,Nd3+:YAG transparent ceramics for self-Q-switched laser

Transparent 0.1 at.%Cr,1.0 at.%Nd:YAG (Y₃Al₅O₁₂) ceramics were fabricated by a solid-state reaction and vacuum sintering with CaO as a charge compensator and tetraethyl orthosilicate (TEOS) as a sintering aid using high-purity powders of Al₂O₃, Y₂O₃, Nd₂O₃ and Cr₂O₃. The mixed powder compacts were sintered at 1800 °C for 5 h and 30 h under vacuum. The optical transmittance of the Cr,Nd:YAG ceramics sintered at 1800 °C for 5 h and 30 h is ∼63% and ∼78% in the infrared wavelengths, respectively. The two samples exhibit pore-free structures and the average grain size is about 10 and 20 μm. For the sample sintered at 1800 °C for 5 h, the dominant fracture mechanism is the transgranular fracture. With increase of holding time up to 30 h, the ratio of intergranular fracture surfaces increase and more Cr³⁺ ions in the Cr,Nd:YAG ceramic transform to Cr⁴⁺. High-quality Cr⁴⁺,Nd³⁺:YAG transparent ceramics may be a potential self-Q-switched laser material.     

Structure of rare-earth phosphate glasses by X-ray and neutron diffraction

Previous diffraction studies of the structures of rare-earth phosphate glasses (R₂O₃)_x(P₂O₅)_1−x are extended to glasses with smaller R³⁺ ions with R = Sm, Gd, Dy, Er, Yb, Y for x = ∼0.25 and with R = Nd, Sm, Gd for x = ∼0.15. Parameters for the P–O, R–O and O–O first-neighbor peaks were obtained by Gaussian fitting. P–P and R–P distances were estimated from the positions of peak maxima. Effects of residual silica or alumina contents present as a result of glass processing were taken into account for selected samples. The P–O coordination number, N_PO, and the P–O, O–O, P–P distances are consistent with the presence of phosphate tetrahedra and are insensitive to the R species and the R₂O₃ content. Rare-earth coordination numbers, N_RO, decrease from ∼8 to ∼6.5 when x is increased from ∼0.15 to ∼0.25. N_OO and N_PP decrease with increasing R₂O₃ content indicating the network disintegration. The numbers N_RO of the metaphosphate glasses (x = ∼0.25) decreases from ∼7 to ∼6 when R is changed from La to Yb. This change is also indicated by the behavior of the R–O distances and by constant number densities of atoms. The decrease in N_RO with increasing R₂O₃ content is due to the reduction in the number of terminal O (O_T) available for coordination of the R³⁺ ions (six at metaphosphate composition). Especially for smaller R³⁺ ions sharing O_T between two R sites is not favored. The decrease by ∼0.04 nm of the prominent R–R first-neighbor distance with a change of R from La to Yb at the metaphosphate composition is indicated by a shift to higher magnitude of scattering vector of the shoulder occurring in front of the first main diffraction peak.     

Luminescence and absorption spectroscopy of Sn-related impurity centers in silica

We report an experimental study on the absorption and luminescence spectra of oxygen deficient point defects in Sn-doped silica. The absorption band at 4.9 eV (B_2β band) and the two related photoluminescence bands at ∼4.2 eV (singlet–singlet emission, S₁ → S₀) and at ∼3.2 eV (triplet–singlet emission, T₁ → S₀), linked by a thermally activated T₁ → S₁ inter-system crossing process (ISC), are studied as a function of temperature from 300 to 20 K. This approach allows us to investigate the dynamics properties of the matrix in the surroundings of the point defects and the effects of local disorder on the two relaxation processes from S₁: the radiative channel to S₀ and the ISC process to T₁. We observe that the S₁ → S₀ decay kinetics at higher temperatures do not follow a single-exponential law and the ISC rate shows a temperature dependence that cannot be rationalized by a single activation process, suggesting the presence of a complex landscape of configurational energies. The comparison with analogous data for Ge-doped silica reveals that the local dynamics of the matrix, the defect–matrix electron–phonon coupling, and the ISC rate dispersion are not substantially modified by the isoelectronic and isostructural substitution Sn–Ge. On the contrary, the Sn-related ISC process is ∼5 times more efficient than the Ge-related one. Since we observed that the coupling with local phonons increases the ISC efficiency by four order of magnitudes in the investigated temperature range, the reported data strongly suggest that, even if the presence of the spin–orbit coupling is needed for ISC processes, it has not play a primary role in the ISC processes in silica, where it acts as a homogenous and temperature-independent scale factor.     

Microstructural analysis of Ga2S3–2MCl (M = K, Rb, Cs) glasses using Raman scattering

Raman scattering spectra of Ga₂S₃–2MCl (M = K, Rb, Cs) glasses have been conducted at room temperature. Based on the analysis of the local co-ordination surroundings of Cs⁺ ions, the similarities and differences of Raman spectra for the glass Ga₂S₃–2CsCl and the bridged molecular GaCl₃ were explained successfully. Through considering the effect of M⁺ ions on mixed anion units [GaS_4?xCl_x] and bridged units [Ga₂S_6?xCl_x] and the corresponding microstructural model, the Raman spectral evolution of the Ga₂S₃–2MCl (M = K, Rb, Cs) glasses was reasonably elucidated.     

Single crystal growth and physical properties of superconducting ferro-pnictides Ba(Fe,Co)2As2 grown using self-flux and Bridgman techniques

We have employed the high temperature solutions growth technique and the Bridgman technique to grow cm-size high-quality single crystals of pristine BaFe₂As₂ (Ba122) and its Co-doped superconducting variants. In the first approach, self-flux (Fe, Co)As was used to achieve a homogeneous melt of composition Ba(Fe, Co)_3.1As_3.1 at T=1463 K. The melt was then cooled slowly under a temperature gradient in a double-wall crucible assembly to obtain large and flux-free single crystals of Ba(Fe, Co)₂As₂. In the second approach, single crystals were grown directly from a stoichiometric Ba(Fe, Co)₂As₂ melt at T=1723 K employing the Bridgman technique associated with a vertical tube furnace. Using both techniques single crystals with lateral dimensions up to 25×10 mm² and thickness up to 1 mm were obtained. Details of the two methods are given and a comparative study of the magnetic and transport properties of the single crystals obtained using the two methods is presented.     

Spectroscopic properties of Co2+ ions in La2O3–MgO–Al2O3–SiO2 glass-ceramics

Co²⁺-doped La₂O₃–MgO–Al₂O₃–SiO₂ (LMAS) glass-ceramics was synthesized by conventional method. The microstructure of LMAS GCs heat-treated at 760 °C/12 h + 930 °C/4 h was characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The spectroscopic properties of Co²⁺-doped LMAS GCs were studied by absorption spectrum, excitation spectrum, and temperature dependent luminescence spectra. XRD results revealed the sizes of MgAl₂O₄ crystalline phases are about 9.1 ± 1.5 nm. The three peaks in the visible absorption band of LMAS GCs at 549 nm, 585 nm and 626 nm are connected with the transitions from ⁴A₂ level to ²A₁/²T₂(²G), ⁴T₁(⁴P) and ²E/²T₁(²G) levels, respectively, and excitations into them emit the radiation at around 666 nm. The luminescence intensity increased with temperature increasing from 10 K to 150 K, while it weakened with temperature increasing from 150 K to 350 K. These features were explained by the effects of two competing mechanisms.     

ac conductivity in a-Ge–Se–Bi glasses

In the present paper the effect of Bi impurity (low ∼4 at.% and high ∼10 at.%) on the ac conductivity (σ_ac) of a-Ge₂₀Se₈₀ glassy alloy is studied and the experimentally deduced values are fitted with theoretically deduced values by using correlated barrier hopping model (CBH). Frequency dependent ac conductance of the samples over a frequency range of 100–50 kHz has been taken in the temperature range (268–360 K). At frequency 2 kHz and temperature 298 K, the value of ac conductivity (σ_ac) decreases at low concentration of Bi (4 at.%). However, the value of σ_ac increases at higher concentration of Bi (10 at.%). The ac conductivity is proportional to ω^s for undoped and doped samples. The value of frequency exponent (s) decreases as the temperature increases. These results have been explained on the basis of some structural changes at low and high concentration of Bi impurity.     

Optical properties and infrared-to-visible upconversion in Er3+-doped GeO2–Bi2O3 and GeO2–PbO–Bi2O3 glasses

Luminescence properties and upconversion studies of germanate glasses in ternary GeO₂–PbO–Bi₂O₃ and binary GeO₂–Bi₂O₃ systems containing Er₂O₃ (0.1–1.0 wt%) are presented for the first time. The Judd-Ofelt parameters found for these glasses are: Ω₂ = 4.50 × 10⁻²⁰ cm², Ω₄ = 1.55 × 10⁻²⁰ cm² and Ω₆ = 0.69 × 10⁻²⁰ cm² for binary glasses and Ω₂ = 4.44 × 10⁻²⁰ cm², Ω₄ = 1.82 × 10⁻²⁰ cm² and Ω₆ = 0.39 × 10⁻²⁰ cm² for ternary glasses. The refractive index of these glasses is found to be ∼2. The transition ⁴I_13/2 → ⁴I_15/2 is peaked at ∼1.53 μm and shows a radiative lifetime around 5 ms. Both systems exhibit similar emission cross-section at 1.53 μm around 0.8 × 10⁻²⁰ cm². Upconverted green emission at ∼530 nm (²H_11/2 → ⁴I_15/2) and ∼550 nm (⁴S_3/2 → ⁴I_15/2) and red emission at ∼668 nm (⁴F_9/2 → ⁴I_15/2) are observed under 980 nm cw excitation. Our results suggest that these glasses are promising candidates for applications in photonics.     

Vapor growth and characterization of pyrite (FeS2) doped with Co,Ni, and As: Variations in semiconducting properties

Pyrite (FeS₂) crystals doped with As, Ni and Co were synthesized with chemical vapor transport over an 18 cm horizontal gradient of 700–600 °C in evacuated quartz tubes, from a mixture of FeS and S, with FeBr₃ as a transport agent. Sulfur fugacity and thus S:Fe stoichiometry was constrained by the Fe_1−xS/FeS_2−y buffer. As, Ni and Co concentrations were ∼3–800 ppm, ∼200–1500 ppm and ∼ 450–3700 ppm, respectively.Semiconducting properties were measured at room temperature using a van der Pauw and Hall measurement system. Ni and Co-doped pyrite are n-type while As-doped pyrite tends to be p-type. Resistivity for Co-doped pyrite ranged from 0.009 to 0.02 Ω cm while for Ni- and As-doped pyrite, resistivity ranged from 2 to 17 Ω cm. Undoped pyrite resistivity ranged from 15 to 85 Ω cm. Carrier concentration was similar for undoped and Ni-doped pyrite, ranging from 10¹⁵ to 10^16.6 cm⁻³, while for Co-doped pyrite it ranged from 10^18.7 to 10^19.3 cm⁻³ and for As-doped pyrite it ranged from 10¹⁴ to 10¹⁸ cm⁻³. Hall mobility was similar for Co and Ni-doped pyrite ranging from 60 to 270 cm² v⁻¹ s⁻¹ while for undoped pyrite it ranged from 8 to 70 cm² v⁻¹ s⁻¹. Hall mobility for As-doped pyrite ranged from 55.0 to 0.2 cm² v⁻¹ s⁻¹ for electrons and from 0.1 to 11.3 cm² v⁻¹ s⁻¹ for holes with the exception of one sample (of 22). These values should be viewed more as trends than as definitive. The results obtained for Ni, Co, and undoped pyrite are similar to those reported in the literature while results for As-doped synthetic pyrite have not previously been reported.     

Fabrication and characterization of transparent conductive ZnO:Al thin films prepared by direct current magnetron sputtering with highly conductive ZnO(ZnAl2O4) ceramic target

Using a highly conductive ZnO(ZnAl₂O₄) ceramic target, c-axis-oriented transparent conductive ZnO:Al₂O₃ (ZAO) thin films were prepared on glass sheet substrates by direct current planar magnetron sputtering. The structural, electrical and optical properties of the films (deposited at different temperatures and annealed at 400°C in vacuum) were characterized with several techniques. The experimental results show that the electrical resistivity of films deposited at 320°C is 2.67×10⁻⁴ Ω cm and can be further reduced to as low as 1.5×10⁻⁴ Ω cm by annealing at 400°C for 2 h in a vacuum pressure of 10⁻⁵ Torr. ZAO thin films deposited at room temperature have flaky crystallites with an average grain size of ∼100 nm; however those deposited at 320°C have tetrahedron grains with an average grain size of ∼150 nm. By increasing the deposition temperature or the post-deposition vacuum annealing, the carrier concentration of ZAO thin films increases, and the absorption edge in the transmission spectra shifts toward the shorter wavelength side (blue shift).     

Growth and scintillation properties of Pr doped Gd3(Ga,Al)5O12 single crystals

Pr:Gd₃(Ga,Al)₅O₁₂ single crystals were grown by the micro-pulling down (μ-PD) method. All grown crystals were greenish and transparent with 3.0 mm in diameter, 15–30 mm in length. Neither visible inclusions nor cracks were observed. Luminescence and scintillation properties were measured. The substitution at the Al³⁺ sites by Ga³⁺ in garnet structure has been studied. In these crystals, Pr³⁺ 5d–4f emission is observed with 340 nm wavelength. Pr1%:Gd₃Ga₃Al₂O₁₂ shows highest emission intensity. The light yield of Pr:Gd₃Ga₃Al₂O₁₂ sample with 3 mmφ×1 mm size was around 4500 photon/MeV. Scintillation decay time was 7.9 ns (0.5%), 46 ns (0.7%) and 214 ns (98.8%).     

Formation of bulk Pt–Pd–Ni–P glassy alloys

The thermal behavior of (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x (x = 16–25 at.%) glassy alloys has been investigated. It is found that the crystallization behavior of the (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x glassy alloys changes from a single-stage exothermic reaction to a two-stage exothermic reaction depending on phosphorous content. When the phosphorous content is 23 at.%, the glassy alloy exhibits the largest supercooled liquid region (ΔT_x) and a sharp single exothermic peak. Fixing the phosphorous content at 23 at.%, the Pt_77−x−yPd_xNi_yP₂₃ (x = 7.7–61.6 at.%, y = 7.7–61.6 at.%) glassy alloys have a wide composition range in which the glassy alloys exhibit a large supercooled liquid region (ΔT_x beyond 60 K). In this range, the Pt_30.8Pd_23.1Ni_23.1P₂₃ glass has the largest ΔT_x (77 K) and a high reduced glass transition temperature (T_rg) of 0.60. This alloy can be cast into fully glassy rods with a diameter of 3 mm. Under uni-axial compression, bulk Pt_30.8Pd_23.1Ni_23.1P₂₃ glassy alloy has an elastic strain of ∼2%, an ultimate strain (to fracture) of ∼6.4%, a Young’s modulus of ∼106 GPa and a failure strength of ∼1390 MPa.     

Microscopic collective dynamics in liquid tellurium

The spectra of collective excitations of liquid tellurium have been studied by means of inelastic neutron scattering. Here we report on the dynamics of liquid Te as measured at two different temperatures, just above melting (T_m = 723 K) and at ∼1000 K. Estimates for the velocity of propagating excitations for both temperatures have been obtained from the experimental data and a contrasting behavior is found with respect to anomalies shown by the adiabatic sound velocity measured by ultrasound methods.