Charge compensation and the luminescence of Cr3+ in KMgF3

Studies of the photoluminescence spectra of Cr³⁺ ions in KMgF₃ crystals co-doped with Cr³⁺ and Ni²⁺ ions are reported. Several crystal field sites are identified by the different R-line spectra due to the ² E⁴ A ₂ transition and broadband luminescences associated with the ⁴ T ₂⁴ A ₂ transitions. Cr³⁺ ions substituting without local charge compensation on the octahedral Mg²⁺ site give rise to a low temperature R line in photoluminescence at =702.3 nm with a radiative decaytime of 3 ms at T=14 K. At T=300 K this isotropic centre gives rise to an unpolarized broadband ⁴ T ₂⁴ A ₂ emission, which results from the thermal occupancy of an excited ⁴ T ₂ state just above the ² E level which, at lower temperature, gives rise to emission in the R-line. Other crystal field sites are due to some Cr³⁺ ions having Mg²⁺ or K⁺ vacancies in nearest-neighbour positions, these vacancies being required to maintain charge neutrality in doped fluoride perovskites. The Cr³⁺–K⁺ vacancy complex results in the centre having trigonal symmetry, and low temperature, photoluminescence via R ₁ and R ₂ lines at 716.8 nm and 716.0 nm, respectively. Finally, Cr³⁺ ions having a nearest neighbour Mg²⁺ vacancy have tetragonal symmetry, experiencing weak crystal fields. In consequence, the ⁴ T ₂ level lies below ² E and the photoluminescence spectrum at low temperature takes the form of a polarized broad ⁴ T ₂⁴ A ₂ band with peak at 760 nm and radiative decaytime of 54 s.     

Temperature dependence of the lifetime of Cr3+ luminescence in garnet crystals

The coexistence of sharp R-lines from the ² E state and the broad band from the ⁴ T ₂ state in the photoluminescence spectra of Cr³⁺:Gd₃Sc₂Ga₃O₁₂ (GSGG) and Cr³⁺:Y₃Ga₅O₁₂ (YGG) is observed at low temperature (10K). The decay lifetimes of the broad emission bands of Cr³⁺ in GSGG and YGG are very close to those of the R-lines being, respectively, 0.23 ms and 2.5 ms. These results are explained in terms of the extent of the mixing of the ⁴ T ₂ vibronic wavefunction with that of the ² E lowest excited state by tunnelling.     

Cooperative emission of photons by weakly coupled chromium ions in YAG and LaAlO3

In YAG:Cr³⁺ and LaAlO₃:Cr³⁺ we have found cooperative emissions of one photon by two excited Cr³⁺ ions analogous to those recently observed by us in Al₂O₃:Cr³⁺. The vibronic sideband of this fluorescence confirms our earlier identification of the photons involved with the polar longitudinal optical modes of the host lattice. Fluorescence decay measurements again indicate radiationless decay processes of the doubly excited pair states leading to non-equilibrium R₂ emission. A fluorescence ²A₁→⁴A₂ of chromium ions in YAG has been detected for the first time.     

Disorder and the shape of the R-lines in Cr3+-doped garnets

This paper describes measurements of the R-line emission from Cr³⁺ ions in Y₃Ga₅O₁₂, Gd₃Sc₂Ga₃O₁₂, and Gd₃Sc₂Al₃O₁₂. Discrete splittings of the lines are interpreted as due to disorder on the cation sublattices. Assuming statistical distributions of the cations on the different cation sublattices enables estimates to be made of the degree of non-stoichiometry in the crystals.     

调谐激光晶体Cr3+:ZnWO4光致发光特性的研究

研究了调谐激光晶体Ｃｒ³⁺：ＺｎＷＯ₄的光致发光特性。报道了它的吸收光谱、激发光谱、发射光谱及其随温度的变化、零声子跃迁和发射寿命等实验结果，并讨论了激发特性、电子－声子耦合作用、ＺｎＷＯ₄中Ｃｒ³⁺的发射寿命曲线等相关问题。 关键词：     

Dominant Cr3+ centers in LiNbO3 under hydrostatic pressure

Low-temperature luminescence spectra of stoichiometric Cr: LiNbO₃ and of congruent Cr, Mg: LiNbO₃ were studied. Cr³⁺ impurity ions preferentially occupy Li⁺ sites (Cr_Li) in the LiNbO₃ crystal lattice, while Cr³⁺ ions substituting for Nb⁵⁺ ions (Cr_Nb) occur in addition to Cr_Li centers in codoped Cr, Mg: LiNbO₃ crystals. Application of a high hydrostatic pressure leads to a transformation of (dominant in concentration) Cr³⁺ centers from low-to high-crystal-field centers. Due to a strong pressure-induced blue shift of the ⁴ T ₂ state resulting in crossing with the ² E state, the replacement of the broad band ⁴ T ₂ → ⁴ A ₂ emission by a narrow R-line emission ² E → ⁴ A ₂ occurs in the luminescence spectra of the samples. This effect of level crossing was observed for the dominant Cr _Li ³⁺ and Cr _Nb ³⁺ centers at pressures which correlated well with estimations based on the ⁴ T ₂-² E energy gap (230 and 1160 cm^?1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^?1/kbar, respectively).     

Optical spectroscopy of Cr3+ ions in LiF single crystals

This paper reports on the luminescence spectrum, its excitation-wavelength dependence and decaytime of Cr³⁺:LiF crystals. In contrast to Al₂O₃ and MgO, where the Cr³⁺ luminescence at low temperature is due to the R-line and its vibronic sideband, luminescence from Cr³⁺:LiF is assigned to the broad-band ⁴ T ₂⁴ A ₂ transition. A periodic structure in the vibronic sidebands of this transition occurs with energy separations of ca. 60 meV. The linear polarisation of the vibronic sidebands shows that their radiative transition takes place from the lowest excited level of the ⁴ T ₂ state, i.e. ⁴ B ₁, into the ⁴ A ₂ ground state of Cr³⁺ ions in orthorhombic symmetry sites. The crystal-field parameters calculated from the experimental data, and luminescence decay-time of the two polarized configurations of the ⁴ B ₁(⁴ T ₂)⁴ A ₂ transition are also reported.     

Investigation of local lattice structure around Cr3+ ions at the trigonal and tetragonal sites in RbCdF3: Cr3+ crystal

The local lattice structure and EPR, optical spectra for Cr³⁺ doped in RbCdF₃ crystal have been studied by diagonalizing the complete energy matrices. The results show that the local structure of the Cr³⁺ ions in RbCdF₃ exhibits a compressed distortion at the trigonal and tetragonal sites. The compressed distortion can be ascribed to the fact that the radius of Cr³⁺ ion is smaller than that of Cd²⁺ ion, and therefore Cr³⁺ ion will draw the fluorin ligands inwards. The variational ranges of the local structural parameters for Cr³⁺ doped in RbCdF₃ crystal R =?1.9491 Å ～?1.9814 Å, θ?= 55.234° ～?55.286° at the trigonal site and R ₁ =?1.8617 Å ～?1.8928 Å, R ₂ =?1.9527 Å ～?1.9851 Å at tetragonal site are obtained respectively, and the EPR and optical spectra agree well with the experimental results.     

Cr3+掺杂的Cd3Al2Ge3O12光谱特性及晶场参数计算

采用高温固相法合成了Cd₃Al₂Ge₃O₁₂：Cr³⁺多晶材料,利用X射线衍射对其结构进行了分析,通过Cr³⁺的室温吸收光谱、室温和77K发射光谱分别对其光谱特性和晶场参数进行了分析和计算.结果表明：在450 nm的蓝光激发下,Cd₃Al₂Ge₃O₁₂：Cr³⁺室温发 关键词： 3Al₂Ge₃O₁₂：Cr³⁺')" href="#">Cd₃Al₂Ge₃O₁₂：Cr³⁺ 荧光光谱 晶场参数 可调谐激光     

Spectral separation of Cr<Superscript>3+</Superscript> optical centers in stoichiometric magnesium-doped lithium niobate crystals

The broadband luminescence of chromium optical centers with strongly overlapping spectral lines and similar emission probabilities from excited ⁴ T ₂ states of red and green Cr³⁺ centers in stoichiometric magnesium-doped lithium niobate crystals has been separated for the first time. The spectral-luminescence characteristics and parameters of intracenter interaction between red and green optical Cr³⁺ centers in stoichiometric lithium niobate have been calculated. The luminescence quantum efficiencies of red and green chromium centers are determined.     

Influence of high hydrostatic pressure on Eu-luminescence in KMgF3:Eu crystal

Spectroscopic investigations are presented of KMgF₃:Eu²⁺ crystal under high hydrostatic pressure from ambient to 310 kbar. The sample was excited by 30 ps pulses generated by optical parametric generator (OPG) system with wavelength controlled between 210 and 325 nm. The Grüneisen parameters of individual phonons are obtained from the pressure shift of the Eu²⁺ emission related to the ⁶P_7/2→⁸S_7/2 transition accompanied by phonon sideband. The luminescence decays exponentially for the pressure below 135 kbar with lifetime of 3.30 ms and slightly nonexponential above 135 kbar, while the average decay time is nearly independent of the pressure. The results obtained for KMgF₃:Eu²⁺ are compared with those for LiBaF₃:Eu²⁺ in which the ⁶P_7/2→⁸S_7/2 emission is replaced by the broadband emission of the 4f⁶5d¹→4f⁷ transition at high hydrostatic pressure.     

Photoluminescence and photoinduced charge transfer in KTaO3:Cr,Li

Kinetics and spectral composition of the luminescence of a KTaO₃:0.1%Cr,10%Li sample (molar charge percentages) have been studied within a broad temperature range (2–300 K). The luminescence spectra of KTaO₃:0.1%Cr,10%Li have been found to have a strong narrow R-line characteristic of Cr³⁺ ions, with a clearly pronounced vibronic wing. Whereas the R-line width in Li-codoped samples is only higher by 120% than that of crystals doped only with chromium, the R-luminescence intensity in the presence of lithium increases by several orders of magnitude. It is shown that the luminescence excitation, besides the intracenter mechanism, is dominated by a process involving photoinduced charge transfer, which results in specific kinetics of the photoluminescence. The luminescence excitation model considered in the work is based, in particular, on the substantial role played in this process by electronic polaron capture by the Li⁺ dipole center caused by the existence of a characteristic shallow electron trap. Fiz. Tverd. Tela (St. Petersburg) 39, 2172–2178 (December 1997)     

Spectroscopic and laser properties of Cr3+:Yb3+:Er3+: fluoride phosphate glass

Sensitization of the fluorescence of Er³⁺ in fluoride phosphate glass containing up to 20 mol% phosphates by codoping with Cr³⁺ and Yb³⁺ is shown. The low order of ligand field strength for Cr³⁺ (Dq/B=2.04) results in broad Cr³⁺ fluorescence overlapping the Yb³⁺ absorption. The electronic energy transfer efficiency approaches 100%. Deviations of donor decay from the Förster law are interpreted in terms of the inhomogeneously acceptor distribution. The electronic energy transfer efficiency of Yb³⁺ → Er³⁺ reaches a maximum value of 75% for glasses containing 20 mol% phosphates. The transfer is shown to be migrationally accelerated by means of GAF-LAF-FB theory. From Judd-Ofelt parameters a stimulated emission cross-section for the transition⁴I_13/2 →⁴I_15/2 of Er³⁺ of 6.2×10⁻²⁰ cm² is derived. The c.w. laser action of Er³⁺ by Cr³⁺ excitation and double-step energy transfer is shown. The output is tuned continuously from 1536 to 1596 nm. Flashlamp pumping is also shown.

     

Cr3+ doped Al2O3 nanoparticles: Effect of Cr3+ content in intensifying red emission

Cr³⁺-doped α-Al₂O₃ nanoparticles (Al_2−xCr_xO₃, 0.005 ≤ x ≤ 0.05) were synthesized by co-precipitation method. X-ray diffraction (XRD) patterns of Cr³⁺:Al₂O₃ nanoparticles revealed the crystallite size of ∼53 nm and electron microscopy (SEM & TEM) confirmed the spherical nanoparticle formation. Diffuse reflectance spectra (DRS) displayed peaks at 406 and 558 nm corresponding to the Cr³⁺ transitions which became prominent with the increase in Cr³⁺ concentration which was also evidenced by the gradually increasing pink coloration of the samples. Photoluminescence (PL) studies showed the sharp red emission at 694 nm (ruby line) which was observed for all samples. The Dq/B value for all samples was found to be greater than 2.3 confirming the presence of Cr³⁺ ions in the octahedral sites. Chromaticity diagrams displayed the maximum red appearance for the sample with x = 0.01 and a lifetime of 4 ms. The synthesized Cr³⁺:Al₂O₃ nanoparticles with smaller crystallite sizes and narrow near monochromatic emission can be used in various applications including sensing, lasing, and bioimaging applications.     

Optical characterization of Cr3+ centers in LiNbO3

Low-temperature luminescence spectra of stoichiometric Cr:LiNbO₃, congruent Cr:LiNbO₃ and congruent Cr,Mg:LiNbO₃ were studied. Dominant low-field and minor high-crystal-field optical centers are the Cr³⁺ impurity ions that preferentially occupy Li⁺ sites (Cr_Li) in the Cr:LiNbO₃ crystals. Low-field centers related to Cr³⁺ substitution of Nb⁵⁺ (Cr_Nb) occur in addition to Cr_Li in co-doped Cr,Mg:LiNbO₃ samples. Application of high hydrostatic pressure leads to the transformation of dominant Cr³⁺ centers from low- to high-field type due to strong pressure-induced blue shift of the ⁴ T ₂ state, resulting in its crossing with the ² E state of Cr³⁺. This level-crossing effect was observed for the dominant Cr³⁺ _Li and Cr³⁺ _Nb centers at pressures that correlate well with estimations based on the ⁴ T ₂-² Eenergy gap (230 cm^-1 and 1160 cm^-1) and on the rate of their pressure-induced change (14.35 and 11.4 cm^-1/kbar, respectively). We also studied inhomogeneous broadeningof the ² E?⁴ A ₂transitions at ambient pressure for the minor high-field “defect” Cr³⁺ _Li centers in congruent LiNbO₃. A fine structure in the spectral response of these centers was observed. The obtained results are discussed on the basis of a microscopic hierarchic model for perturbed Cr³⁺ ions in the LiNbO₃ lattice. Received: 25 June 2001 / Published online: 2 November 2001     

混合形成体对掺铒碲酸盐玻璃的热力学稳定性和发光性能的影响

研究了混合形成体效应对掺铒碲酸盐玻璃热力学稳定性、1.53 μm发光特性和上转换发光强度的影响.通过拉曼光谱测试,分析了WO₃,Nb₂O₅,GeO₂等氧化物对Er³⁺离子配位场结构,以及对发光谱的非均匀展宽机理的作用.结果表明,通过掺杂适当声子能量的网络形成体氧化物,不但可获得热力学稳定性较好的玻璃,还可有效降低Er³⁺离子⁴I_11/2能级的寿命,在抑制Er³⁺离子在可见光波段的上转换发光的同时不致劣化其在1.53 μm的发光特性.本文制得的碲酸盐玻璃具有较大的受激发射截面((9.64—10.96)×10^-21cm²)和荧光半高宽(FWHM)(50—67 nm),热力学稳定性良好,是一种理想的掺Er³⁺宽带有源光纤用基质玻璃. 关键词： 掺铒碲酸盐玻璃 混合形成体 1.53 μm发光 声子能量     

The impurity-induced luminescence of RbMnF3

The impurity-induced luminescence of RbMnF₃ has been studied in the temperature range, 4–120 K. Multimagnon sidebands of impurity-induced Mn²⁺ emission lines have been detected spanning the spectral range, 5515–5960 Å. The broad emission bands, which peak at 5750 Å and 6440 Å, have been attributed to phonon-assisted transitions from two impurity-induced Mn²⁺ traps of different depths. Pulsed luminescence measurements indicate that the quenching of the shallow trap emission is by a thermal activation to the E1 exciton level whereas the quenching of the deeper trap emission is by a multiple phonon decay. The coupling between Mn²⁺ ions is strong enough to support a detectable transfer of excitation between these two traps.The results of this research also include the observation in absorption of the E1 exciton line and its 1-magnon and possible 2-magnon sidebands.     

Spectroscopic properties of fluorophosphate glass with high Er<Superscript>3+</Superscript> concentration

Fluorophosphate glass with 4 mol. % ErF₃ content was prepared. The different scanning calorimetry was conducted. Raman spectrum, infrared transmission spectrum, absorption spectrum were measured. Fluorescence spectrum and lifetime of emission around 1.53 μm were measured under 970 nm laser diode excitation. The metaphosphate content in the composition is limited, but the maximum phonon energy of glass amounts to 1290 cm^-1, and is comparatively high. The full width at half maximum is about 56 nm, and is wider than for most of the materials investigated. The measured lifetime of ⁴ I _13/2→⁴ I _15/2 transition, contributed by the high phonon energy, inefficient interaction of Er³⁺ ions, and low water content, amounts to no less than 7.36 ms though the Er³⁺ concentration is high. This work might provide useful information for the development of compact optical devices. PACS 78.20.-e; 42.70.Ce; 42.70.Hj; 32.70.Cs     

Spectroscopic properties of Cr4+-doped LiAlO2

The spectroscopic properties of the new potential laser material Cr⁴⁺:LiAlO₂ are presented. LiAlO₂ exhibits tetrahedrally coordinated lattice sites only. Doping the crystal with chromium only as well as additional codoping with magnesium yields the incorporation of Cr⁴⁺ on the Al site. The Cr⁴⁺ emission extends from 1.1 to 1.7µm. In the case of doping just with chromium, the lifetime is single exponential in the whole temperature range between 12 and 550 K with a room-temperature lifetime of 29µs and a low-temperature lifetime of 95µs, which are the longest lifetimes observed until now for Cr⁴⁺ systems. Codoping with magnesium yields an additional Cr⁴⁺ center, which is clearly observed both in the decay dynamics and in the low-temperature emission spectrum.     

Studies of the local compressibility of Cr 3+-centered octahedron in LiSc (WO 4)2 crystal from the pressure dependence of the optical spectra

The pressure-induced shifts of optical spectral bands ⁴T₂ and ²E for the preferential Cr³⁺-centered octahedron for Cr³⁺ at the Sc³⁺ site of the LiSc (WO ₄)₂ crystal have been calculated from crystal-field theory. From the calculation, the local linear compressibility d ln R/d p≈?3.3×10^?4 kbar^?1 for the Cr³⁺ center in a LiSc (WO ₄)₂ crystal is obtained. The result is discussed.