Ce~(3+)在稀土取代的卤磷酸盐中的发光与能量传递的研究

本工作用固相反方法、以炭保护或在N_2-H_2还原气氛中，两次灼烧，合成了一系列烯土取代的卤磷酸盐发光体，其反应是M_2~ICO_3 + M_3~(II)(PO_4)_2 + M_2~(III)O_3 + (NH_4)_2HPO_4 + M~(II)F_2 → M_x~IM_(10-2x)~(II)M_x~(III)(PO_4)_6F_2 + NH_3 + H_2O式中：M~I = Li~+, Na~+, K~+; M~(II) = Ca~(2+), Sr~(2+); M~(III) = Y~(3+), La~(3+), Gd~(3+); X = 0.5, 1, 2, 3。通过X－射线分析法和以Eu~(3+)作为荧光离子探针方法确定了样品的物相和晶体结构，表明它与M_(10)~(II)(PO_4)_6F_2同属六方晶系(Pb_3/m)。并用图解外推法求得样品的晶胞参数a和c分别为9.41和6.89 A。在研究了M~I, M~(II)和M~(III)为不同阳离子，X为不同值时，取代物的物相和结构变化以及对Ce~(3+)发光和能量传递影响规律的基础上，以Na_2Ca_6La_2(PO_4)_6F_2为基本体系比较详细地研究了Ce~(3+)的发光，Ce~(3+)-Mn~(2+), Ce~(3+)-Re~(3+) (Re~(3+) = Pr~(3+)、Nd~(3+)、Sm~(3+)、Tb~(3+)、Dy~(3+)、Tm~(3+)、Ho~(3+)、Er~(3+))以及Ce~(3+)-Mn~(2+)-Re~(3+) (Re~(3+) = Dy~(3+), Nd~(3+))的能量传递。实验发现，Na_2Ca_6La_2(PO_4)_6F_2:Ce~(3+)是较强的紫外光发射体，发射谱带是由338和358nm两个宽带峰组成的，相应于~2D-~2F_(5/2)和~2D-~2F_(1/2)跃迁。发射强度和~2D-~2F_(6/2)，~2D-~2F_(7/2)两种辐射跃迁几率相对大小与温度，Ce~(3+)的浓度有关；F~-含量对发射强度也有强烈的影响。我们认为，浓度和温度的影响是由于Ce~(3+)-Ce~(3+)的能量迁移作用。实验发现，Ce~(3+)能有效地敏化Mn~(2+)的发光，得到高效的黄色荧光发射体。Ce~(3+)对Re~(3+)的作用可以分为三类：A）. Ce~(3+)-Sm~(3+)、Tb~(3+)、Dy~(3+)、Tm~(3+)；B）．Ce~(3+)-Nd~(3+)，Pr~(3+)；C）. Ce~(3+)-Ho~(3+), Er~(3+)。在A，B类型中，Ce~(3+)能有效地敏化这些稀土离子，但能量传递效率(η_T)_A > (η_T)_B。在Ce~(3+)-Ho~(3+)、Ce~(3+)-Er~(3+)类型中，Ho~(3+)、Er~(3+)不产生可见辐射，Ce~(3+)的发射强度也显著降低。在Ce~(3+)-Mn~(2+)-Re~(3+)三元中心体系中，Ce~(3+)的能量可直接传递给Mn~(2+)和Re~(3+)中心，还可通过Ce~(3+)-Mn~(2+)-Re~(3+)的途径传递。通过研究中心离子的浓度效应，温度效应、测定激发光谱、荧光光谱、荧光衰减曲线的荧光寿命，能够证实：在Ce~(3+)-Mn~(2+)、Ce~(3+)-Re~(3+)、Ce~(3+)-Mn~(2+)-Re~(3+)的能量传递属于无辐射共振传递相互作用类型。符合M. Yokata等人提出的偶极－偶极相互作用扩散限制传递历程。我们还利用Ce~(3+)、Ce~(3+)-Mn~(2+)、Ce~(3+)-Re、Ce~(3+)-Mn~(2+))-Re~(3+)激活体系中Ce~(3+)的发射强度和荧光寿命等数据计算了Ce~(3+)的能量传递效率。发现两种结果相差较大，为了准确地表达Ce~(3+)的敏化效果，在分析上了产生这种误差的原因后，提出了两种传递效率表示式(η_f, η_R)。

Comparative studies of semiconductor saturable absorber mirror mode-locking dynamics in pulsed diode-end-pumped picosecond Nd:GdVO4 and Nd:YAG lasers

Ultrashort pulses were generated in passively mode-locked Nd:YAG and Nd:GdVO4 lasers pumped by a pulsed laser diode with 10-Hz repetition rate. Stable mode-locked pulse trains were produced with the pulse width of 10 ps. The evolution of the mode-locked pulse was observed in the experiment and was discussed in detail. Comparing the pulse evolutions of Nd:YAG and Nd:GdVO4 lasers, we found that the buildup time of the steady-state mode-locking with semiconductor saturable absorber mirrors (SESAMs) was relevant to the upper-state lifetime and the emission cross-section of the gain medium.

在p-Si(100)上溅射法生长ZnO的结构和光学特性

室温下在p-Si(100)上采用直流反应磁控溅射法外延生长了ZnO薄膜。XRD测量表明了ZnO是高度c轴单一取向生长的，XRC测量则表明了ZnO的高质量。在室温下的PL测量中见到了带边发射，其强度与晶体质量有关。

GaN发光二极管的负电容现象

采用串联等效电路分析了较大正向电压下半导体二极管的交流电学特性，由此可以同时测量结电容和串联电阻，并能判断二极管是否有界面层。首次发现了在较低的测试频率和较大的正向电压下，GaN二极管的结电容具有负值，并且测试频率越低，正向偏压越大，负电容现象越显著。这种负电容效应可能与大正向电压下强注入造成的电子-空穴复合发光有关。

ZnSxTe1-x三元混晶的光学性质研究

国家自然科学基金

The diphasic nc-Si/a-Si : H thin film with improved medium-range order

A series of silicon film samples were prepared by plasma enhanced chemical vapor deposition (PECVD) near the threshold from amorphous to nanocrystalline state by adjusting the plasma parameters and properly increasing the reactions between the hydrogen plasma and the growing surface. The microstucture of the films was studied by micro-Raman and Fourier transform infrared (FTIR) spectroscopy. The influences of the hydrogen dilution ratio of silane (R-H = [H-2]/[SiH4]) and the substrate temperature (T-s) on the microstructural and photoelectronic properties of silicon films were investigated in detail. With the increase of RH from 10 to 100, a notable improvement in the medium-range order (MRO) of the films was observed, and then the phase transition from amorphous to nanocrystalline phase occurred, which lead to the formation of diatomic hydrogen complex, H-2* and their congeries. With the increase of T-s from 150 to 275 degreesC, both the short-range order and the medium range order of the silicon films are obviously improved. The photoconductivity spectra and the light induced changes of the films show that the diphasic nc-Si/a-Si:H films with fine medium-range order present a broader light spectral response range in the longer wavelength and a lower degradation upon illumination than conventional a-Si:H films. (C) 2004 Elsevier B.V. All rights reserved.