Crystal growth and optical characterization of Cr,Ca: Y3Al5O12

Single crystals of Cr,Ca: Y₃Al₅O₁₂(YAG) with different Cr and Ca concentrations were grown by the Czochralski (CZ) and floating zone (FZ) method. The analysis of absorption and emission spectra of both as-grown and oxygen-annealed crystals in relation to concentration ratio of Ca and Cr (Ca/Cr) showed that Cr⁴⁺ concentration ratio to total Cr concentration increased up to about Ca/Cr = 6, and Cr⁶⁺ can also exist. Luminescence lifetime at 1400 nm was measured as a function of Cr⁴⁺ concentration.     

Energy transfer and annealing behavior of luminescence in Dy-doped PbWO4 single crystal

The Dy³⁺: PWO single crystal was subsequently annealed in air atmosphere at a temperature of 500°C, 550°C, 600°C, 700°C, 800°C, 900°C, and 1000°C, respectively. X-ray excited luminescence spectra were measured before and after each step of annealing. Annealing experiments confirmed the energy transfer-taking place from PbWO₄ (PWO) host to Dy³⁺ ions, followed by the enhancing characteristic emission of Dy³⁺ ions. In the process of annealing, the luminescence of PWO host was significantly reduced while that of Dy³⁺ was increased simultaneously. Annealing at a temperature below 700°C suppressed the blue luminescence of the PWO host and enhanced its green components, while the emission of Dy³⁺ is increased to some extent. Further annealing at higher temperature strongly reduced the luminescence of the PWO host, while the emission of Dy³⁺ was greatly increased. Interstitial oxygen O_i could play an important role in the luminescence. Annealing could modify the luminescence of Dy³⁺ ions in PWO, which may be useful in terms of some application purposes.     

Compositional dependence of the valency state of Cr ions in oxide glasses

Absorption spectra of Cr ions in multi-component oxide glasses melted in Ar were measured. The integrated extinction coefficient is used as a parameter to analyze the redox reaction of Cr³⁺ and Cr⁶⁺ in glass samples. The relative content of Cr⁶⁺ increased and that of Cr³⁺ decreased with increasing basicity in silicate and borate glasses; this result is consistent with previous studies. On the contrary, only Cr³⁺ exist in phosphate glasses. Cr⁴⁺ ions were found only in aluminate, gallate and alumino-silicate glasses with modifier contents ≥ 60 mol%. The relationship between point defects and Cr⁴⁺ formation is examined on the basis of electron spin resonance measurements. We concluded that the superoxide ion radical (O₂⁻) and the peroxy bonding (-O-O-) oxidize Cr³⁺ to Cr⁴⁺ in aluminate, gallate, and alumino-silicate glasses.     

Evidence of structural anisotropies in silicate glass fibres by ESR

The ESR spectra of Cr³⁺ and Cr⁵⁺ in soda-lime-silicate glass fibres were investigated with respect to defined parameters of the fibre drawing process from a nozzle. The following results concerning the structure of the fibres were obtained: Although the short-range order of the Cr³⁺ and Cr⁵⁺ ions is identical in the structure of the bulk glass as well as in the fibre glass, changes of the covalent portion of the bonds between the Cr⁵⁺ ion and its oxygen ligands were observed. The fibre glass structure is a frozen-in deformed or orientated (this depends on definition) silicate glass network, which influences the bonds in the square bipyramidal [Cr⁵⁺O₆] ⁷⁻ complexes in the following manner: the σ-bonds between Cr⁵⁺ and the four oxygens in the square plane of the bipyramid are more covalent in the fibre than in the bulk glass. They are also orientation-dependent: more covalent for those of the randomly distributed complexes with their c-axes perpendicular to the fibre axis than parallel to it. The π-bond of the CrO³⁺ oxygen is less covalent in the fibre than in the bulk glass and is also orientation-dependent: more covalent for those complexes with their c-axes parallel to the fibre axis than perpendicular to it.

The ability of the glass network to form [Cr⁵⁺O₆] ⁷⁻ or CrO³⁺ complexes increases with the following increasing parameters of the fibre drawing process: melting temperature, oxygen partial pressure, mean cooling rate, and tensile stress. This fact is primarily a consequence of the shift of the redox-equilibrium and of the difficulty for the structural change from an octahedral symmetry of the Cr³⁺ and Cr⁵⁺ to a tetrahedral symmetry of the Cr⁶⁺ and vice versa during the fibre drawing process. The [Cr⁵⁺O₆] ⁷⁻ complex is highly unstable and is an intermediate state.     

Growth and characterization of a new nonlinear optical material: Potassium p-nitrophenolate dihydrate (NO2–C6H4–OK·2H2O) with new bonding properties

Single crystals of potassium p-nitrophenolate dihydrate (NPK·2H₂O) have been grown successfully using the isothermal solvent evaporation technique. It is a new semiorganic nonlinear optical crystal, possessing a d_eff of about 1.5 times that of lithium niobate and in which the K⁺ ions are bonded to the nitro group instead of bonding with the phenolic O⁻. Large single crystals of dimension upto 20×6×4 mm³ are harvested within a period of 60 days. The grown crystals are subjected to single crystal X-ray, FTIR and DRS-UV visible spectral, thermal and microhardness analyses. Single crystal X-ray analysis confirms the molecular formula and the structure of the crystal. FTIR spectral studies verify the functional groups present in the crystal. The DRS-UV visible spectrum proved the optical transparency of the crystal in the entire visible and near infrared region. Thermal studies reveal that the crystals are stable upto 180 °C. Microhardness measurements on the cleaved plane (1 1 0) explain the strength and slip direction in the crystal. The SHG efficiency of the crystal is examined by performing the Kurtz powder test using Nd:YAG laser.     

射频磁控溅射氧气流量对制备的Ga2O3∶Cr薄膜光致发光性能的影响

在不同氧气流量下,采用双靶射频磁控共溅射的方法在蓝宝石(α-Al₂O₃)基底上制备得到系列掺Cr的Ga₂O₃(Ga₂O₃∶Cr)薄膜,详细研究了薄膜在900 ℃退火前后的结构和光学性能。结果表明,未退火的Ga₂O₃∶Cr薄膜为非晶结构,其发光主要位于蓝绿波段。经900 ℃退火后,薄膜的结构由非晶变为多晶,且在近红外波段观测到了来源于Cr³⁺掺杂的发光。退火后的薄膜结晶质量和近红外发光均与氧气流量密切相关,而其光学带隙不受氧气流量的影响。在所研究的氧气流量范围,4 mL/min氧气流量下薄膜的近红外发光强度最强,这与此条件下薄膜结晶质量较好以及Cr³⁺替代Ga³⁺的数量较多有关。以上研究成果可为制备高质量Ga₂O₃∶Cr薄膜提供参考。     

Ambient-condition growth of superconducting YBa2Cu4O8 single crystals using KOH flux

YBa₂Cu₄O₈ is a stoichiometric oxide superconductor of T_c80 K. Unlike YBa₂Cu₃O_7−δ, this compound is free from oxygen vacancy or twin formation and does not have any microscopic disorder in the crystal. Doping with Ca raises its T_c to 90 K. The compound is a promising superconductor for technological application. Up to now, single crystals have not been grown without using specialized apparatus with extremely high oxygen pressure up to 3000 bar and at over 1100 °C due to the limited range of reaction kinetics of the compound. This fact has delayed the progress in the study of its physical properties and potential applications. We present here a simple growth method using KOH as flux that acts effectively for obtaining high-quality single crystals in air/oxygen at the temperature as low as 550 °C. As-grown crystals can readily be separated from the flux and exhibit a perfect orthorhombic morphology with sizes up to 0.7×0.4×0.2 mm³. Our results are reproducible and suggest that the crystals can be grown using a conventional flux method under ambient condition.     

Growth and properties of (Cr, Nd) doped GdCa4O(BO3)3 crystals

Optical quality Nd:GdCa₄O(BO₃)₃ (Nd:GdCOB) crystal and chromium sensitized crystal of Cr:Nd:GdCa₄O(BO₃)₃ (Cr:Nd:GdCOB) were grown by the Czochralski (CZ) method. The transmittance spectra was measured at room temperature on 7at%Nd:GdCOB and lat%Cr:7at%Nd:GdCOB samples. The absorption in the blue-UV region of Cr:Nd:GdCOB was larger than that of Nd:GdCOB, which matched the xenon flash lamp very well. A 3×3×10mm³ Nd:GdCOB sample without any antireflective coating was sidepumped using a pulsed dye-laser tuned to 595.4nm. The green-light output energy was 1.35mJ at a pump energy of 17.5mJ. The conversion efficiency was 7.7%, and the threshold energy was less than 1.2mJ which are the best results up to now to our knowledge. 3×3×7mm³ Nd:GdCOB and Cr:Nd:GdCOB samples without antireflective coatings were pumped using a flash pump source. The threshold for Cr:Nd:GdCOB was 0.9J while for Nd:GdCOB it was I.O.J. The output green light energy was 2.46mJ for Cr:Nd:GdCOB at a 10J flash pump energy and was 1.96mJ for Nd:GdCOB at the same pump energy, which clearly shows that Cr³⁺ ions have the sensitizing effect on the Nd³⁺ ions in Nd:GdCOB crystal.     

The growth and spectroscopic characteristics of Ca3Y2(BO3)4:Er laser crystal

The Ca₃Y₂(BO₃)₄:Er³⁺ crystal with a size up to 20 mm×30 mm was grown by the Czochralski method. The absorption spectrum was measured and its absorption peaks were assigned to the corresponding transitions between the Er³⁺ energy levels. A broad emission spectrum from 1429.4 to 1662.8 nm was exhibited from 530 nm wavelength pumping. This crystal is promising as a tunable infrared laser crystal.     

Growth and spectral properties of Nd-doped Sr3Y(BO3)3 crystal

A new crystal of Nd³⁺:Sr₃Y(BO₃)₃ with dimension up to 25×35 mm² was grown by Czochralski method. Absorption and emission spectra of Nd³⁺: Sr₃Y(BO₃)₃ were investigated . The absorption band at 807 nm has a FWHM of 18 nm. The absorption and emission cross sections are 2.17×10⁻²⁰ cm² at 807 nm and 1.88×10⁻¹⁹ cm² at 1060 nm, respectively. The luminescence lifetime τ_f is 73 μs at room temperature     

Growth and spectroscopic properties of Er single doped and Er–Yb co-doped YAl3(BO3)4 crystals

Er³⁺-doped and Er³⁺–Yb³⁺ co-doped yttrium aluminum borate (YAB) single crystals have been grown by the top-seeded solution growth method using a new flux system, namely NaF–MoO₃–B₂O₃. The Er³⁺ concentrations were 1.3 mol% for both single doped and co-doped crystals and the Yb³⁺ concentration in the Er³⁺–Yb³⁺ co-doped crystal was 20.0 mol% in the raw materials. The distribution coefficients of Er³⁺ single doped and Er³⁺–Yb³⁺ co-doped crystals were measured. The polarized absorption and fluorescence spectra of Er³⁺–Yb³⁺ co-doped crystal were recorded and compared with those of Er³⁺ single doped crystal. The results demonstrate that Er³⁺–Yb³⁺ co-doped YAB crystal is a potential candidate for 1.55 μm laser materials.     

The effect of oxygen pressure on the microstructures of Co-doped rutile TiO2 thin films grown by pulsed laser deposition

Comprehensive microstructures of 7% cobalt-doped rutile TiO₂ thin films grown on c-plane sapphire by pulsed laser deposition were characterized using transmission electron microscopy (TEM). The effects of oxygen pressure during growth on the Co distribution inside the films were investigated, and the detailed growth mechanism of both TiO₂ and TiO₂+Co was discussed. The similar oxygen sublattices and low mismatch between (1 0 0) rutile and c-plane sapphire favors the rutile phase. However, the three-fold symmetry of the substrate surface resulted in three rutile domain orientation variants, and they grow adjacent to each other. Cobalt was found to precipitate out as nanocrystals inside the TiO₂ matrix as the growth pressure of oxygen was decreased. At 0.05 mTorr oxygen pressure, almost all of the Co segregates into crystallographically aligned nanocrystals with a particle size of 4.4±0.15 nm. All the samples have magnetic coercivity at room temperature. The magnetic moment per Co atom increased with decreased oxygen pressure, suggesting that the Co that replaced the Ti²⁺ in the TiO₂ lattice does not have a large magnetic moment.     

Studies of the facetting of the polished (100) face of CaF2

The present paper deals with studies of the facetting of the polished (1 0 0) surface of CaF₂ during annealing and growth in UHV using low energy electron diffraction (LEED), atomic force microscopy (AFM), and transmission electron microscopy (TEM). First morphological modifications of the polished surfaces become visible at temperatures of T=874 K. Surfaces annealed at T=974 K exhibit a micro-roughening with pyramidal protrusions and corresponding depressions. LEED studies indicate the evolution of {1 1 1} facets. Reflexes from the (1 0 0) surface are not seen. After growth of about 660 monolayers of CaF₂ at T=1093 K and a saturation ratio S=33 from the vapor phase, larger pyramid-like or hip roof-like crystallites are developed. The results of AFM height profiles as well as of the LEED investigations indicate again the formation of {1 1 1} facets as proved by their angles of 54.7 ^o with the base (1 0 0) surface. This shows that the crystallites are homoepitaxially grown on the underlying CaF₂ substrate.     

Growth and properties of Tm: YAG crystals

The high optical quality Tm: YAG single crystals with dopant concentrations of 2, 4, 8 at % Tm³⁺ ions have been grown by the Czochralski (CZ) method .The segregation coefficients of Tm³⁺ ions in CZ-grown 2, 4, 8 at % doped Tm: YAG crystals determined by ICP-AES method are equal to 0.93, 1.04, 1.10, respectively. The absorption spectra in the range of 190–900 nm and the IR absorption spectra in the range of 1000–4500 cm⁻¹ of the Tm: YAG crystals were also measured at room temperature. The results indicate that Tm: YAG is favorable for laser diode-pumped system, and air annealing can improve the quality of the Tm: YAG crystal.     

Mössbauer and optical spectroscopic study of temperature and redox effects on iron local environments in a Fe-doped (0.5 mol% Fe2O3) 18Na2O-72SiO2 glass

Local environments of ferric and ferrous irons were systematically studied with Mössbauer (at liquid helium temperature) and ultraviolet-visible-near infrared spectroscopic methods for various 18Na₂O-72SiO₂ glasses doped with 0.5 mol% Fe₂O₃. These were prepared at temperatures of 1300-1600 °C in ambient air or at 1500 °C under reducing conditions with oxygen partial pressures from 12.3 to 0.27×10⁻⁷ atmospheres. The Mössbauer spectroscopic method identified three types of local environments, which were represented by the Fe³⁺ sextet, the Fe³⁺ doublet, and the Fe²⁺ doublet. The Fe³⁺ sextet ions were assigned to ‘isolated’ octahedral ions. Under reducing conditions, the octahedral Fe³⁺ ions were readily converted into octahedral ferrous ions. The Fe³⁺ doublet exists both in octahedral and tetrahedral environment, mainly as tetrahedral sites in the reduced samples. The tetrahedral ions were found stable against reduction to ferrous ions. The Fe²⁺ doublet sites existed in octahedral coordination. Combining results from both spectroscopic studies, the 1120- and 2020-nm optical bands were assigned to octahedral ferrous ions with a different degree of distortion rather than different coordinations. Further, we assigned the 375-nm band to the transition of octahedral ferric ions that are sensitive to the change of oxygen partial pressure in glass melting and 415-, 435-, and 485-nm bands to the transitions of the tetrahedral ferric ions that are insensitive to oxidation states of the melt. The effect of ferric and ferrous ions with different coordination environments on the glass immiscibility was elucidated.     

Thermal and laser properties of Yb:YAl3(BO3)4 crystal

Large optical-quality Yb:YAl₃(BO₃)₄(Yb:YAB) crystals have been grown by the flux method. The thermal properties of Yb:YAB crystal were measured for the first time. The thermal properties of Yb:YAB crystal with different Yb³⁺ ion concentrations are also reported. The results show that the ytterbium concentration influences the properties of Yb:YAB crystal. The specific heat decreases with the increase of Yb³⁺ ion concentrations in the experiment range. Apparently, the thermal expansion coefficient increases along the c-direction with the increase of Yb³⁺ ion concentrations, while it changes slightly along the a-direction. The output laser in 1120–1140 nm ranges has been demonstrated pumped by InGaAs laser. The slope efficiency is 3.8%. The self-frequency-doubling output power of 1 mW is achieved.     

Growth and spectral properties of Nd:LaVO4 crystal

This paper reports the growth and spectral properties of 3.5 at% Nd³⁺:LaVO₄ crystal with diameter of 20×15 mm² which has been grown by the Czochralski method. The spectral parameters were calculated based on Judd–Ofelt theory. The intensity parameters Ω_λ are: Ω₂=2.102×10⁻²⁰ cm², Ω₄=3.871×10⁻²⁰ cm², Ω₆=3.235×10⁻²⁰ cm². The radiative lifetime τ_r is 209 μs and calculated fluorescence branch ratios are: β₁(0.88μm)=45.2, β₂(1.06μm)=46.7, β₃(1.34μm)=8.1. The measured fluorescence lifetime τ_f is 137 μm and the quantum efficiency η is 65.6%. The absorption band at 808 nm wavelength has an FWHM of 20 nm. The absorption and emission cross sections are 3×10⁻²⁰ and 6.13×10⁻²⁰ cm², respectively.     

Preparation of NaCl color center laser crystals and perturbation effect of ions

NaCl:(F₂⁺)_AH-type color center laser crystals were prepared by the crystal growth, additional coloration, annealing, and by light aggregation at 290 and 77 K. The formation process of color center in NaCl crystals were investigated, the influence of doped cation and anion ions on the stability and optical properties of color centers is discussed in this paper.     

Changes in the refractive index of fused silica due to implantation of transition-metal ions

The changes in the refractive index of high purity silica glass produced by implantation of Ti⁺, Cr⁺, Mn⁺, Fe⁺ and Cu⁺ ions at 160 keV were measured using ellipsometry at a wavelength of 633 nm. Implantation doses ranged from 10¹⁵ to 6 × 10¹⁶ ions cm⁻². At the highest doses, a relative increase in refractive index of approximately 15% for implanted Cu ions and an increase of about half that for implanted Cr, Mn and Fe ions are observed. These variations in index probably result from colloid formation. For implanted Ti, virtually no change in the measured index of refraction was observed even at the highest doses.     

Formation of oxygen-deficient type structural defects and state of ions in SiO2 glasses implanted with transition metal ions

The properties and concentrations of oxygen-deficient type structural defects in type III SiO₂ glasses implanted with Ti⁺, Cr⁺, Mn⁺, Fe⁺, or Cu⁺ to doses from 0.5×10¹⁶ to 6×100¹⁶ ions/cm² at an energy of 160 keV have been measured by using vacuum UV and EPR spectroscopies. An intense absorption band centered around 7.5 eV is observed in all the samples except for Cu-implanted ones and is attributed primarily to Si---Si homo-bonds with the bond distance close to that of the Si₂H₆ molecule. The homo-bond and implanted ion concentrations are of the same order of magnitude in the implanted layers. An E′ type center associated with the homo-bond is observed in all the samples except for Cu-implanted ones. Anomalous behaviors of the Cu-implanted samples are attributed to the formation of Cu-colloids. An enhanced formation of metallic particles or colloids is suggested for the samples implanted with Cr, Mn or Fe to doses higher than 3×10¹⁶ ions/cm².