Hyperfine structure of EPR spectra of Cr5+ ions in irradiated sodium dichromate crystals

EPR spectra are studied of X-irradiated sodium dichromate crystals grown from an aqueous solution by evaporation at 31°C. Doublet lines of EPR-absorption are attributed to the Cr⁵⁺ ions in the CrO₄³⁻ and CrO₃⁻ radicals resulting from radiation decomposition of Cr₂O₇²⁻ and being in the lattice in two unequivalent positions. Hyperfine structure caused by interaction of an unpaired electron with Cr⁵³ nucleus were observed both at liquid nitrogen and room temperatures. For the line, caused by CrO₄³⁻ radical, g_y and A_y directions coincide and angles both between A_x, g_x and A_z, g_z make up ∼ 25°. The spectrum is described by usual spin Hamiltonian for S = 1/2 with following parameters (T = 77 K): for CrO: g_z = 1.984, g_y = 1.970, g_x = 1.961, |A_z| = 8.2 · 10⁻⁴ cm⁻¹, |A_y| = 13.7 × 10⁻⁴ cm⁻¹, |A_x| = 36.1 · 10⁻⁴ cm⁻¹; for CrO₃⁻: g_z = 1.915, g_y = 1.975, g_x = 1.985, |A_z| = 32.2 · 10⁻⁴ cm⁻¹.     

Hyperfine structure of Cr5+ EPR spectra in X-irradiated potassium halide crystals doped with CrO and Ca2+

The hyperfine structure has been investigated in the EPR spectra of X-irradiated KCl: CrO, Ca²⁺ and KBr: CrO, Ca²⁺ crystals. Preliminarily spectra were simplified by means of heating the crystals up to 400 K for KCl and 440 K for KBr, that destroys the less stable CrO · V centers. It is ascertained, that principal directions of g- and A-tensors do not coincide, and principal A-values are determined. Calculations, making use of the experimental meanings of g- and A-tensor components, showed, that the degree of distortion of oxygen tetrahedra in CrO · Ca²⁺ · V centers is almost the same in both KCl and KBr crystals; besides, these calculations suggest the existence of a strong covalent bonding between the central Cr⁵⁺ ion and four oxygen ligands in CrO ions.     

Growth and some properties of Ce3+-doped LiYbF4 single crystals

LiYbF₄ single crystals, nominally pure and doped with Ce³⁺ ions, of optical quality and up to 60 mm in diameter, have been grown by vertical directed crystallization. The optical and mechanical properties of the crystals have been studied. The refractive index dispersion for LiYbF₄ in the range of 0.4–0.6 μm can be described by the dependence n ²(λ) − 1 = Aλ²/(λ² − λ ₀²), where A = 1.14 and 1.21 and λ₀ = 0.074 and 0.080 μm for n _o and n _e, respectively. The sample microhardness exceeds 2.6 GPa. LiYbF₄ crystals are transparent in the range of 0.17–9 μm and have an absorption band in the range of 0.9–1.2 μm. It is shown that LiYbF₄ crystals doped with Ce³⁺ ions can be used as optical cut-off UV filters in the operating range λ = 0.25−0.28 μm.     

Spectroscopy and crystal-field analysis of Cr4+-doped transparent silicate glass-ceramics

We report transparent Cr⁴⁺-doped SiO₂–Al₂O₃–ZnO–Li₂O–K₂O glass-ceramics with broadband infrared luminescence. After heart-treatment, Li₂ZnSiO₄ crystallite was precipitated in the glasses, and its average size increased with increasing heat-treatment temperature. Racah parameters of Cr⁴⁺–Li₂ZnSiO₄ glass-ceramics have been calculated, and it was confirmed from absorption spectra that the energy levels of Cr⁴⁺-doped glass-ceramics are close to the cross point ¹E and ³T states. No infrared emission was detected in the as-made glass samples, but the broadband infrared emission centered at 1210 nm with the full width at half maximum (FWHM) of more than 250 nm was observed by exciting the glass-ceramics with excitation of an 808 nm laser diode. In order to analyze the located crystal field of Cr⁴⁺ ions, the emission spectra are fitted by multi-peak Gauss fitting. It is seen that the fluorescence spectra are fitted into two Gaussian bands at around 1195 and 1263 nm with band widths of 208 and 278 nm, respectively. The two Gaussian bands at around 1195 and 1263 nm have about the same decay rate, and hence they would probably originate from the same luminescent centers. The observed infrared emission could be attributed to Cr⁴⁺ ions at low-field sites in Li₂ZnSiO₄ glass-ceramics.     

Colour centre in YAG: Cr3+ crystals

YAG crystals doped with 0.01 wt% Cr were grown by Czochralski method using 98% Ar + 2% H₂ protective atmosphere. Four colour varieties of the crystals were prepared in the dependence of the Al₂O₃ :Y₂O₃ ratio in the melt and the sign of electrical potential above the melt level. New absorption bands were attributed to the O⁻ centre (400 to 480 nm), F centre (357, 500, and 833 nm) or Cr³⁺ at another site than octahedral (455, and 612 nm).     

Cr3+ luminescence quenching in stoichiometric lithium niobate crystals

Cr³⁺-doped stoichiometric LiNbO₃ crystals were grown and detailed spectroscopic investigations were performed. The samples are characterized by extremely low frequency factor of the luminescence thermal quenching. Its numerical value 1.7 × 10⁸ s⁻¹ is about 10⁴ times lower than in crystals of lithium-aluminum fluorides. Under such conditions, radiative transitions of Cr³⁺ ions compete successfully with non-radiative ones, resulting in a relatively high quantum yield of the broadband luminescence at room temperature. The quenching of luminescence is counteracted by the effect of the dynamic removal of the radiative transition exclusion, and as a result, Cr³⁺ radiative lifetime in LiNbO₃ decreases from 8 to 4 μs when temperature grows up from 77 to 300 K. Therefore quantum yield of the broadband luminescence at room temperature is not 5–10%, as reported previously, but about 30%. The results obtained in the present study show the stoichiometric lithium niobate doped with Cr³⁺ ions to be a potential active media for tunable lasers.     

High-order picosecond SRS and self-SRS generation in Nd3+-doped CaMoO4, SrMoO4, and SrWO4 laser crystals

The results of a stimulated Raman scattering (SRS) spectroscopy study of Nd³⁺-doped CaMoO₄, SrMoO₄, and SrWO₄ crystals are presented. All the wavelengths of the Stokes and anti-Stokes generation observed in these crystals are identified. For the first time, the self-SRS conversion of the emission frequency of activator Nd³⁺-ions is attained in these lasing molybdates and tungstate with scheelite-type structures.     

EPR spectra of irradiated Na2Cr2O7 · 2 H2O crystals

The EPR spectra of X-irradiated Na₂Cr₂O₇ · 2 H₂O crystals grown from the aqueous solution at room temperature are studied. Three groups of lines are detected, which marked conventionally as C₁, C₂ and C₃. Each of the C₁ and C₂ line groups is a superposition of two doublets. The lines are interpreted as due to the Cr⁵⁺ ions in the distorted (stretched) oxygenous tetrahedron. The doublet structure is due to hyperfine interaction with nuclei of 1/2 spin which are protons of water. It is shown that the C₁ and C₂ line groups are due to the centers of the same type with different orientations in the lattice. The principal g-tensor values for these centres were as follows: g_z = 1.916, g_y = 1.978, g_x = 1.986.     

Photoluminescence of annealed LLGG:Cr3+ crystals under high pressure

High pressure photoluminescence has been used for analysis of lattice disorder in La_2.7Lu_2.29Cr_0.01Ga₃O₁₂ and La_2.32Lu_2.59Cr_0.02Ga_3.07O₁₂ crystals. Photoluminescence of samples before annealing and those annealed for 5 h in oxygen and hydrogen atmosphere at 923 K has been measured. The pressure dependence of Cr³⁺ luminescence has been used to obtain the crystal field distribution (lattice disorder) and its dependence on the thermal treatment. The distribution energy of the ⁴T₂ state has been considered as a measure of disorder. Annealing in oxygen has been found to reduce the energies of ⁴T₂ states for all sites, and annealing in hydrogen – to increase these energies.     

EPR and Optical Absorption Spectra of Cr3+ Ions in Lithium Sulphate Single Crystals

Optical absorption and EPR spectra of Li₂SO₄ · H₂O crystals doped with Cr³⁺ are studied at liquid nitrogen temperature. The bands are found in absorption spectra with maxima about 17000, 23 800 and 37 200 cm⁻¹, assigned to the ⁴A₂ → ⁴T₂, ⁴A₂ → ⁴T₁ and ⁴A₂ → ⁴T₁ (⁴P) transitions, respectively. The crystal field theory parameters were determined and appeared to be as follows: Dq = 1700 cm⁻¹, B = 667 cm⁻¹, C = 3002 cm⁻¹. The lines resulting from Cr³⁺ ions are found in EPR spectra. All lines are doublets, which is indicative of presence of two magnetically unequivalent centre positions, and have the hyperfine structure resulting from interaction of the unpaired electron spin with Cr⁵³ isotope nucleus. Centres are oriented in such a way, that z-axes, corresponding to two centre positions, are situated at both sides of a-axis at an angle of about 3°. Spin Hamiltonian parameters were found as follows: g_x = 1.985, g_y = 1.984, g_z = 1.988, D = 0.130 cm⁻¹, E = 0.016 cm⁻¹, |A| = 17.8 · 10⁻⁴ cm⁻¹.     

GdAl3（BO3）4及其Yb^3＋激活激光晶体的研究

采用顶部籽晶法从K2Mo3O10-B2O3助熔剂体系中生长出尺寸达到20 mm的GdAl3(BO3)4(简称GAB)和Yb3 激活的晶体.测试了GAB晶体的<100>和(001)轴向热膨胀系数,确定了透光波长范围,计算了有效倍频系数和相位匹配角随波长的变化,结果表明GAB晶体在整个透光范围内均可实现相位匹配.测定了Yb3 :GAB晶体在室温下的偏振吸收和荧光光谱,进行了光谱计算,测试了晶体的激光性能,在1046 nm处实现了2.1 W激光输出,斜率效率达到58%.     

Peculiarities of the growth of PbWO4:Nd3+ and PbMoO4:Nd3+ single crystals

Crystallography Reports - The dependence of the Nd segregation coefficient on Nd concentration in PbWO4 and PbMoO4 melts and on the method of its introduction is analyzed. In a study of the...     

Crystallization behavior of Dy3+-doped selenide glasses

Rare earth (RE)-doped chalcogenide glasses are an important promising material for active photonic devices, including mid-infrared (mid-IR) fiber lasers and amplifiers. Here we report on dysprosium ion (Dy³⁺)-doped GeAsGaSe chalcogenide glasses based on 10 atomic (at.) % Ga. A series of Dy³⁺-doped GeAsGaSe glasses, with increasing levels of Dy³⁺ dopant from 0 ppm to 2000 ppm added to the Ge_16.5As₉Ga₁₀Se_64.5 (at. %) base glass, is synthesized and characterized using: Fourier transform infrared spectrometry; X-ray diffraction (XRD); imaging and analysis using a high resolution transmission electron microscope, with selected area electron diffraction (HRTEM-SAED), and energy dispersive X-ray spectroscopy (HRTEM-EDX) and an environmental scanning electron microscope with energy dispersive X-ray spectroscopy (ESEM-EDX) and with secondary electron mapping. At the higher levels of Dy³⁺ doping, the glasses exhibit bulk crystallization; XRD, HRTEM-EDX and ESEM-EDX indicate the crystals are predominantly a modified, face centered cubic α-Ga₂Se₃, with some substitution of Ge. In addition, features on the bulk glass surface are shown to comprise Dy³⁺, sometimes accompanied by Si and [O] which, it is suggested, are due to contamination from the silica glass melting ampoule.     

Excited-state absorption spectroscopy of crystals doped with Cr3+, Ti3+, and Nd3+ ions. Review

Basic results of the investigations of the absorption and luminescence spectra of strongly pumped crystals doped with Cr³⁺, Ti³⁺, and Nd³⁺ ions are considered. These investigations have been systematically carried out since 1968 in the Laboratory of Physics of Optical Crystals and the Laboratory of Spectroscopy of Crystals of the Shubnikov Institute of Crystallography of the Russian Academy of Sciences. The results obtained are compared with the data in the literature.     

Synthesis and characterization of Cr4+-doped CaO–GeO2–Li2O–B2O3(Al2O3) transparent glass-ceramics

Synthesis and devitrification behavior of Cr-doped CaO–GeO₂–Li₂O–B₂O₃(Al₂O₃) glasses have been studied. A range of glass compositions was found to yield transparent glass-ceramics after devitrification. The size of crystallites is below 1 μm. Glass-ceramic samples exhibit 1050–1600 nm broad-band emission with a maximum around 1260 nm, very similar to the emission of Cr⁴⁺:Ca₂GeO₄ bulk crystals. X-ray diffraction measurements indicate that the structure of crystallites exhibiting near infrared emission in glass-ceramics may be assigned to Cr⁴⁺:Ca₂GeO₄ with increased lattice parameters.     

Colour Centre in Nd3+-doped Yttrium Aluminates

Spectroscopic and laser properties of YAG:Nd and YAP:Nd grown from the nonstoichiometric melts or those containing Mo and Fe impurity were compared. YAG:Nd grown from the melt with surplus of Y₂O₃ and annealed in H₂ at ∼ 1400 °C have an increased laser performance. Mo admixture in hydrogen annealed YAP:Nd laser crystals suppresses effectively the detrimental action of iron impurity.     

The study of Sm3+-doped low-phonon-energy chalcohalide glasses

The Sm³⁺-doped low-phonon-energy (LPE) Ge–Ga–Se–CsI glasses were studied. Upon excitation at 980 nm diode laser, intense 1.25 and 1.49 μm near-infrared fluorescence bands with broad full width at half maximum (FWHM) of 49 and 53 nm were observed, respectively. About 180–300 μs fluorescence lifetimes were obtained for the 1.49 μm emission. The thermal properties and structure of glasses were investigated by differential thermal analysis (DTA) and Raman spectra, respectively. Spectroscopic characteristics of the optical transitions have been calculated by using the Judd–Ofelt theory and evaluated for excited levels.