可见光波段稀土激光晶体的研究进展

可见光激光在数据存储、光通讯、激光显示、激光医疗、激光打印以及科学研究等领域具有非常重要的应用价值。随着蓝光LD泵浦源的商用化, 直接泵浦稀土离子掺杂激光晶体实现可见光激光输出吸引了人们极大的研究兴趣。目前, 可见光稀土离子主要集中在Pr ³⁺、Dy ³⁺、Tb ³⁺和Sm ³⁺等。其中, Pr ³⁺的研究较多, 发光波长涵盖面较广, 发射波段覆盖蓝光、绿光、红光、橙光; Dy ³⁺和Tb ³⁺因为能够发射黄光以填补Pr ³⁺的不足也吸引了广泛的研究; 此外, Sm ³⁺和Eu ³⁺也是典型的可见波段稀土发光离子。本文综述了近几年可见波段稀土离子掺杂激光晶体的研究现状, 主要以Pr ³⁺、Dy ³⁺、Tb ³⁺和Sm ³⁺掺杂YAlO₃ (YAP)、Mg : SrAl₁₂O₁₉ (SRA)等晶体为研究对象, 总结了一套适合Pr ³⁺掺杂材料的判据, 对晶体生长、结构、热学性能、偏振光谱性能和激光性能进行了系统的研究。     

Feasibility study of in-beam polarization of fluorine

The feasibility of in-beam polarization of nuclear spin in F⁺ beams was studied. It was demonstrated that metastable atomic fluorine could be optically pumped on the closed transition (3s ⁴P_5/2→3p ⁴D_7/2) at 686 nm. The metastable production efficiency in sodium vapor was measured using optical depopulation pumping of the metastable on the (3s ⁴P_5/2→3p ⁴D_5/2) transition at 678 nm. At low helium density, 0.24 (+0.16/−0.03) of the reionized beam was derived from metastables, implying an ion beam polarization of 24% would be feasible if the metastables were fully polarized. The hyperfine structures of the 3s ⁴P_5/2, 3p ⁴D_5/2 and 3p ⁴D_7/2 states in ¹⁹F were measured via laser-induced fluorescence and modulated optical depopulation pumping. Based on these results, a scheme is suggested for polarizing ²⁰F.     

紫外光激发Ce3+掺YVO4蓝光发射性能研究

利用提拉法生长了YVO₄和掺2.0at% CeO₂(或Ce₂(CO₃)₃)的YVO₄: Ce³⁺晶体。样品的XRD测试表明Ce³⁺代替Y³⁺进入晶格, Ce³⁺的加入并没有影响YVO₄的晶格结构。XPS测试显示YVO₄: Ce³⁺晶体中Ce离子3d分裂为882.0、885.8、902.9、908.0和915.9 eV等5个峰, 峰位表明样品中铈离子是以Ce³⁺和Ce⁴⁺两种价态形式存在。YVO₄和YVO₄: Ce³⁺激发谱都呈现出260~360 nm宽带激发, 此激发带源于基质中VO₄^3-离子团的配体O到V的电荷迁移吸收。使用325 nm的紫外线激发时, 两种样品均可发出以440 nm 为中心的宽带蓝光,其中YVO₄发射峰应归属于VO₄^3-离子团中³T₂→¹A₁和³T₁→¹A₁跃迁; 而YVO₄: Ce³⁺的蓝光发射则来源于Ce³⁺的5d→4f 的跃迁。分析可知YVO₄: Ce³⁺中VO₄^3-的π轨道和Ce³⁺的电子波函数能有效地重叠, 使得VO₄^3-和Ce³⁺可通过交换作用有效地向Ce³⁺传递能量, 可大幅提高Ce³⁺的蓝光发射强度。实验结果显示YVO₄: Ce³⁺可作为UV-LED管芯激发的白光发光二极管用高亮度蓝色发光材料。     

Optical and structural characterizations of Cu-doped KCl films

The production of highly Cu⁺-doped KCl films and the properties of their 266 nm absorption band, which has an off-center property in doped single crystals, open the possibility of application of these films as ultra-violet optical filters. The investigated films, of approximately 1 μm thickness, were prepared by resistive co-evaporation of KCl and CuCl powders on different substrates of CaF₂, Al₂O₃, SiO₂, KCl and Si. The Cu⁺ concentration, as determined by energy-dispersive X-ray, ranges from 10²⁰ to 10²¹ cm⁻³, for 1–15% CuCl nominal mole percent concentration. Structural and optical properties were investigated through scanning electron microscopy, X-ray diffraction, ellipsometry, optical absorption and transmission. The films are polycrystalline, and the gain size decreases with increasing Cu⁺ impurity concentration, yielding an increase of visible transmission to a limited CuCl concentration. These films show a 6.295 Å lattice parameter with a f.c.c. structure and an index of refraction of 1.53 at 266 nm. When the Cu⁺ concentration is increased, the UV band position remains the same and no clusters are evidenced even to the high 15% CuCl concentration investigated, which differs very much from single crystals samples grown by the Kyropoulos-Czochralski method. For a Cu⁺ concentration of 8×10²⁰ cm⁻³ the film shows a transmission better than 88% at 350 nm wavelength.     

A photoluminescence study of vanadium-related defects in n-type, semi-insulating and p-type Cd(Zn)Te

A photoluminescence (PL) study of vanadium-related defects in semi-insulating and co-doped p-type and n-type CdTe:V crystals gives evidence of the presence of the V²⁺–Zn complex. In addition to the ³T₂(F)→³A₂(F) emission of V³⁺ near 0.5 eV and the ⁴T₂(F)→⁴T₁(F) transition of V²⁺ near 0.45 eV, two further luminescence bands are detected at higher energies. The first emission band (I), peaking around 0.8 eV, is correlated to the V²⁺–Zn complex and the second one (II), peaking around 0.6 eV, is attributed to the acceptor level introduced by the cadmium vacancies. Varying the zinc concentration in CdTe, we analyse the behaviour of the vanadium impurity charge state. We show that the V²⁺ internal transition decreases with zinc alloying due to the formation of the V²⁺–Zn complex. The emission bands related to isolated V_Cd are present with high intensity only in the p-type crystals, in which all the vanadium content is in the V³⁺ oxidation state, whereas, in the semi-insulating and n-type crystals, the PL spectrum is dominated by Emission I related to the V²⁺–Zn complex. The presence of this complex in the semi-insulating crystals used in photorefractive (PR) applications and the dominance of this complex over the optical properties of Cd(Zn)Te:V imply the contribution of this complex to the PR processes.     

Synthesis and the luminescent properties of europium-activated Ca2SnO4 phosphors

The synthesis and photoluminescent (PL) properties of calcium stannate crystals doped with europium grown by mechanically activated in a high energy vibro-mill have been investigated. The characteristics of Ca₂SnO₄:Eu³⁺ phosphors were found to depend on the amounts of europium ions. The XRD profiles revealed that the system, (Ca_1−xEu_x)₂SnO₄, could form stable solid solutions in the composition range of x = 0–7% after being calcined at 1200 °C. The calcined powders emit bright red luminescence centered at 618 nm due to ⁵D₀ → ⁷F₂ electric dipole transition. Both XRD data and the emission ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) reveal that the site symmetry of Eu³⁺ ions decreases with increasing doping concentration. The maximum PL intensity has been obtained for 7 mol% concentration of Eu³⁺ in Ca₂SnO₄.     

Spectroscopic evidence of inhomogeneous distribution of Nd in YVO4, YPO4 and YAsO4 crystals

Nd³⁺ : YVO₄ is one of the most interesting laser hosts for micro and diode-pumped solid state lasers. We have studied magnetic and optical properties of Nd³⁺ in three zircon type crystals YMO₄ (M=V, As, P). In particular, Nd³⁺ ions exhibit in the three hosts a multisite character observed in the absorption and emission spectra. However, the emission and its dynamics are strongly dependant on the reabsorption mechanisms. In Nd : YVO₄, single crystals containing 7 ± 1 × 10¹⁸ Nd³⁺ ions/cm³, the lifetime is 95 ± 2 μs in good agreement with the calculated radiative lifetime. Electron Paramagnetic Resonance (EPR) measurements are performed to identify the nature of the different substitution sites for Nd³⁺ ions. Nd³⁺ ions are found to be inhomogeneously distributed in tetragonal D_2d symmetry sites, in isolated ions, “shallow clusters” and pairs. Proportions of the different local environments depend on the total neodymium concentration. For instance, 15% of the Nd³⁺ ions are gathered in Nd³⁺–Nd³⁺ pairs for 7.2 ± 0.2 × 10¹⁹ Nd³⁺ ions/cm³.     

Spot F positron source electro-deposited on a graphite rod

2.2 GBq of ¹⁸F (half-life 110 min) was effectively deposited on a graphite electrode of diameter 3 or 5 mm. The fraction of the electro-deposited ¹⁸F was as high as 97% of the total ¹⁸F produced in the solution. The fraction of the positrons emitted out of the electrode was 44% of the total β⁺. Thus, as much as 36% of the β⁺ from the produced ¹⁸F is available for the source of slow positron beams.     

HPLC Elution Behavior of La-Fullerenes Investigated by Radiochemical Method

HPLC elution behaviors of La-fullerenes were investigated by use of the radiochemical methods. Elution curves of La-fullerenes could be observed in detail for the first time, and the persistent presence of La in the whole elution curve was demonstrated. Production of La-fullerenes containing a radioactive ¹⁴⁰La could be achieved for the first time.     

Temperature dependencies of excited states lifetimes and relaxation rates of 3–5 phonon (4–6 μm) transitions in the YAG, LuAG and YLF crystals doped with trivalent holmium, thulium, and erbium

The temperature dependencies of the nanosecond multiphonon relaxation (MR) rates of the ³F₃ state of Tm³⁺ in the YLF crystal and of the ⁵F₅ state of Ho³⁺ ion in the YAG and LuAG crystals and of the microsecond MR rates of the ⁴F_9/2 (²H_9/2) state of Er³⁺ ions in YLF were measured in the wide temperature range using direct laser excitation and selective fluorescence kinetics decay registration. For YLF the observed relations are explained by 4-phonon process in the frame of a single-frequency model with hω_eff=450±30 cm⁻¹ for the ³F₃ state of Tm³⁺ and by 5-phonon process with hω_eff=445 cm⁻¹ for the ⁴F_9/2 (²H_9/2) state of Er³⁺. For YAG and LuAG crystals these dependencies are explained by the 3-phonon process with hω_eff=630 cm⁻¹. The decrease of the relaxation rate with the temperature in the range from 13 to 80 K was observed for the ⁴F_9/2 (²H_9/2) state of Er³⁺ in the YLF crystal. It is explained by the redistribution of excited electronic states population of erbium ions over the higher lying Stark levels with different MR probabilities. A good fit of experimental temperature dependence (including the dropping part of the experimental curve) was obtained for single-frequency model (hω_eff=450 cm⁻¹) with W₀₁=8.0×10⁴ s⁻¹ and W₀₂=4.7×10⁴ s⁻¹ accounting Boltzmann distribution of population over two excited Stark levels of the excited state of erbium ions. Employment of this model improves the fit between the experiment and the theory for the ⁵F₅ state of Ho³⁺ ion in YAG as well. Strong influence of the parameters of the non-linear theory of MR, i.e. the reduced matrix elements U^(k) of electronic transitions and the phonon factor of crystal matrix η on the spontaneous MR rates was observed experimentally. The smaller these parameters the slower the spontaneous MR W₀. This fact can be used for searching new active crystal laser media for the mid-IR generation.     

NaGd(WO4)2:Yb3+/Tm3+反蛋白石光子晶体的制备与上转换发光调制研究

三维光子晶体具有长程有序的结构特点, 在可见和近红外光谱范围内有着广泛的应用。光子晶体的一个重要性质是其对嵌入其中的发光中心自发辐射具有调制作用。本研究利用自组装和模板辅助法制备高质量的三维NaGd(WO₄)₂:Yb³⁺/Tm³⁺反蛋白石光子晶体, 探究了光子带隙对Tm³⁺离子上转换荧光发射与发光动力学的调制作用。通过对比分析发现, 由于反蛋白石光子晶体独特的周期性大孔结构和光子带隙效应, 处于光子带隙内的Tm³⁺离子¹G₄-³H₆的发光强度被抑制约45%, 自发辐射速率(SDR)被抑制约30%, 同时上转换局域热效应得到有效的调制。本实验结果对探索新型高效稀土掺杂上转换发光材料和提高上转换发光效率有指导意义。     

Growth and spectroscopy of Dy doped in ZnWO4 crystal

Single-crystal ZnWO₄:Dy³⁺ was grown by Czochralski technique. The XRD, absorption spectra as well as fluorescence spectrum are investigated and the Judd–Ofelt intensity parameters Ω₂, Ω₄, Ω₆ are obtained to be 7.76 × 10⁻²⁰ cm², 0.57 × 10⁻²⁰ cm², 0.31 × 10⁻²⁰ cm², respectively. Calculated radiative transition rate, branching ratios and radiative lifetime for different transition levels of ZnWO₄:Dy³⁺ crystals are presented. Fluorescence lifetime of ⁴F_9/2 level is 158 μs and quantum efficiency is 66%.The most intense fluorescence line at 575 nm correlative with transition ⁴F_9/2 → ⁶H_13/2 is potentially for application of yellow lasers.     

CeO2修饰Mn-Fe-O复合材料及其NH3-SCR脱硝催化性能

氨选择性催化还原(NH3-SCR)技术需要进一步研发在相对较低温度(<300℃)下具有良好催化活性、高稳定性及环境友好的脱硝催化材料。本工作采用草酸共沉淀法制备Mn-Fe-O催化材料,并对其进行不同含量CeO2修饰,用于低温NH3-SCR脱硝催化反应。通过X射线衍射(XRD)、扫描电子显微镜(SEM)、氮气吸附-脱附、X射线光电子能谱(XPS)、程序升温还原或脱附(H2-TPR、NH3-TPD)等手段对催化剂进行了表征。催化结果表明,在相同反应条件下适量CeO2修饰后的Mn-Fe-O样品比纯Mn-Fe-O表现出更优异的NH3-SCR脱硝催化性能,在80℃时NO转化率在95%以上,且具有较高的N2选择性。CeO2修饰提高了Mn-Fe-O氧化物表面的Fe^3+、Mn^3+和Mn4+含量及表面酸性位点数量,从而有助于NH3的吸附及催化反应的进行,并且Fe^2+/Fe^3+、Mn^2+/Mn^3+/Mn^4+以及Ce^3+/Ce^4+电子对之间的相互氧化还原反应提高了催化剂的氧化还原能力及稳定性。     

Feasibility study of a He-magnetometer for neutron electric dipole moment experiments

We report on a ³He-magnetometer capable of detecting tiny magnetic field fluctuations of less than 10⁻¹⁴ T in experiments for measuring the electric dipole moment (EDM) of the neutron. It is based on the Ramsey technique of separated oscillating fields and uses nuclear spin-polarized ³He gas which is stored in two vessels of V10 l in a sandwich-type arrangement around the storage bottle for ultra-cold neutrons (UCN). The gas is polarized by means of optical pumping in a separate, small discharge cell at pressures around 0.5 mbar and is then expanded into the actual magnetometer volume. To detect the polarization of ³He gas at the end of the storage cycle the gas is pumped out by means of an oil-diffusion pump and compressed again into the discharge cell where optical detection of nuclear polarization is used.     

Energy transfer and radiationless relaxation processes in Nd and Cr doped mixed garnet crystals

Four-wave mixing and flourescence dynamics measurements have been used to investigate the energy transfer processes in (Gd,Ca)₃(Ga,Mg,Zr)₂Ga₃O₁₂ crystals doped with Nd³⁺ or codoped with Cr³⁺ and Nd³⁺ ions. The results show that strong, short range energy transfer takes place between Cr³⁺ and Nd³⁺ ions and this is enhanced by long range energy migration among the Cr³⁺ ions. Energy migration was also found to occur among the Nd³⁺ ions. The energy migration properties are affected by the active ion concentration, the distribution of active ions, ion-ion iteraction rates, and temperature. In addition, the pump band to metastable state radiationless relaxation process in the Cr³⁺ ions was characterized by determining the values of the dephasing time from the properties of the four-wave mixing signal. The effects of anharmonic coupling are needed to explain the variation of the T₂-dephasing time as a function of the crystal field splitting between the ²E and ⁴T₂ levels for a series of Cr³⁺-doped materials.     

Effect of Tb ions on X-ray-induced defect formation in phosphate containing glasses

The study of radiation-induced defect formation in glasses is of growing interest for applications in optics and photonics. The influence of Tb³⁺ ions on X-ray-induced defects has been examined in glasses with different phosphate contents. The defects have been characterized by optical absorption and ESR spectroscopy. (Tb³⁺)⁺ ions are formed by X-ray irradiation. They exhibit a broad charge transfer band at 370 nm. The formation of (Tb³⁺)⁺ hole centres suppresses the formation of intrinsic phosphate-related hole centres, absorbing in the visible region. PO₃ defects absorbing in the ultraviolet region contribute mainly to the electron centres corresponding to the (Tb³⁺)⁺ hole centres. Immediately after X-ray irradiation, 5–10% of the Tb³⁺ ions have been oxidized. The amount of (Tb³⁺)⁺ ions increases with increasing phosphate content. The stability of the (Tb³⁺)⁺ ions at room temperature depends on glass composition and melting conditions.     

Crystal growth and spectral properties of pure and Co-doped Mg3B2O6 crystal

Novel pure and cobalt-doped magnesium borate crystals (Mg₃B₂O₆) have been grown successfully by the Czochralski technique for the first time. Crystal growth, X-ray powder diffraction (XRD) analysis, absorption spectrum, fluorescence spectrum as well as fluorescence decay curve of Co²⁺:Mg₃B₂O₆ (MBO) were described. From the absorption peaks for the octahedral Co²⁺ ions, the crystal-field parameter Dq and the Racah parameter B were estimated to be 943.3 cm⁻¹ and 821.6 cm⁻¹, respectively. The fluorescence lifetime of the transition ⁴T₁(⁴P) → ⁴T₂ centered at 717 nm was measured to be 9.68 ms.     

Spectroscopic analysis and the effects of color centers on the laser performance of Nd: CaZn2Y2Ge3O12

Spectroscopic and EPR investigations of Nd³⁺-doped CaZn₂Y₂Ge₃O₁₂ (CAZGAR) have been performed. The absorption, fluorescence, excitation spectra and fluorescence lifetime have been measured at room temperature. The Judd-Ofelt theory has been applied to the measured optical absorption intensities to predict the radiative decay rates, branching ratios, and peak stimulated emission cross section from the metastable ⁴F_3/2 state to the ⁴I_9/2 manifold. The fluorescence lifetime of the ⁴F_3/2 level of Nd³⁺ at low concentration in this host was measured to be 285 ± 10 μs, which is longer than that for Nd³⁺: YAG. Color centers located at zinc octahedral sites have been produced in these crystals by ultraviolet irradiation and have been detected by EPR techniques. The effects of the color centers on the potential laser characteristics of this materials are discussed.     

Enhancement of Er I13/2 population in Y2O3 by energy transfer to Ce

We report measurements of the energy transfer between Er³⁺ and Ce³⁺ in Y₂O₃. The transition between the Er^{3+ 4}I_11/2 and ⁴I_13/2 excited states can be stimulated by energy transfer to Ce³⁺, augmenting the population in the ⁴I_13/2 state at the expense of that in the ⁴I_11/2 state. Experiments were performed on Y₂O₃ planar waveguides doped with 0.2 at.% erbium and 0–0.42 at.% cerium by ion implantation. From measurements of Er³⁺ decay rates as a function of cerium concentration we derive an energy transfer rate constant of 1.3×10⁻¹⁸ cm³/s. The efficiency of the energy transfer amounts to 0.47 at 0.42 at.% cerium. The energy transfer rate constant measured in Y₂O₃ is two times smaller for Er³⁺→Ce³⁺ than that for Er³⁺→Eu³⁺ in the same material.     

Spectroscopic investigation of Nd ion in LiNbO3, MgO:LiNbO3 and LiTaO3 single crystals relevant for laser applications

The polarized absorption and luminescence properties of Nd³⁺ doped isostructural LiNbO₃, MgO:LiNbO₃ and LiTaO₃ nonlinear bulk single crystals are reported. Pump-probe experiments associated with the Judd-Ofelt approach are used to estimate two types of room temperature cross sections: polarized emission cross sections of the dominant ⁴F_3/2 → ⁴I_1//2 transition near 1085 and 1093 nm and polarized excited-state absorption cross sections in the same spectral domain and in the green spectral range corresponding to self frequency doubling. Self frequency-doubling results are also given in Nd:LiNbO₃ and Nd:MgO:LiNbO₃ versus sample temperature.