Ultrafast Saturable Absorption of Core/Shell Colloidal Quantum Dots

Ultrafast saturable absorption (SA) materials that are capable of blocking the optical absorption under strong excitation have extensive applications in photonic devices. This work presents core/shell colloidal quantum dots (CQDs) which have the quantized energy levels, excellent band gap tunability, and possess significant SA performance. When the band gap is close to the pump pulse energy, the CQDs show significant resonant SA response. At the same excitation conditions, the core/shell CQDs dispersions show better SA response than graphene dispersions, and comparable to the recently reported molybdenum disulfide. The carrier dynamics of the SA of the CQDs is analyzed systematically. The research has also found that the two‐photon absorption of the CQDs show nearly cubic power law of the band gap, while the SA performance keeps almost the same in the nonresonant regime. Further, superior passive Q‐switched laser behavior is observed using the CQDs as a saturable absorber. The results directly reveal the physical processes of this basic problem and broaden the applications of CQDs in photonic devices.     

Narrow Linewidth Tm:YLF Laser with Volume Bragg Grating Dual-End-Pumped by Equidirectional-Polarization Fiber-Coupled Laser Diode

We demonstrate a narrow linewidth Tm:YLF laser with a volume Bragg grating (VBG) that was pumped by an equidirectional-polarization fiber-coupled laser diode using a dual-end-pumping configuration. For an optimized output coupler with a radius of curvature of 150 mm and transmission of 15% at a wavelength of 1.91 μm, the maximum output power was 15.6 W for an absorbed pump power of 51.7 W at 1,907.93 nm, and the output had a narrow linewidth of 0.22 nm. This corresponds to an optical-to-optical conversion efficiency of 30.2% and the slope efficiency was 36.7%.     

Removal of oxygen from silicon by electron beam melting

Small amounts of multicrystalline silicon were melted in an electron beam furnace in different experimental conditions in order to investigate the oxygen evaporation behavior during the electron beam melting (EBM) process. The oxygen content level before and after EBM was determined by secondary ion mass spectroscopy. The oxygen content was reduced from 6.177 to 1.629 ppmw when silicon was melted completely at 15 kW with removal efficiency up to 73.6 %. After that, it decreased continually to <0.0517 ppmw when the refining time exceeded 600 s with a removal efficiency of more than 99.08 %. During the melting process, the evaporation rate of silicon is 1.10 × 10^?5 kg/s. The loss of silicon could be reduced up to 1.7 % during oxygen removal process to a desirable figure, indicating EBM is an effective method to remove oxygen from silicon and decrease the loss of silicon.     

Evolution analysis of the states of the EZ model

Based on suitable choice of states, this paper studies the stability of the equilibrium state of the EZ model by regarding the evolution of the EZ model as a Markov chain and by showing that the Markov chain is ergodic. The Markov analysis is applied to the EZ model with small number of agents, the exact equilibrium state for N = 5 and numerical results for N=18 are obtained.     

The structure and optical characters of the ZnO film grown on GaAs/Al2O3 substrate

In this paper ZnO films are grown on GaAs/Al₂O₃ substrates at different temperature by metal-organic chemical vapor deposition (MOCVD). The GaAs/Al₂O₃ substrates are formed by depositing GaAs layer (∼35 nm) on the Al₂O₃ substrate. The results of X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) demonstrate that most of the Ga and As atoms form Ga-As bond and the GaAs layer does not present any orientation. The characters of the ZnO films grown on GaAs/Al₂O₃ substrates are investigated by XRD, photoluminescence (PL), atomic force microscopy (AFM) and Raman scattering. Compared with ZnO film grown on Al₂O₃ substrate, ZnO film prepared by our fabrication scheme has good crystal and optical quality. Meanwhile its grain size becomes bigger according to the AFM image. Raman analysis indicates that the intrinsic defects and the in-plane tensile stress are obviously reduced in ZnO/GaAs/Al₂O₃ samples.     

Structural, optical, and electronic properties of colloidal CuO nanoparticles formed by using a colloid-thermal synthesis process

Colloidal cupric oxide (CuO) nanoparticles were formed by using a colloid-thermal synthesis process. X-ray diffraction patterns, transmission electron microscopy (TEM) images, high-resolution TEM images, and X-ray energy dispersive spectrometry profiles showed that the colloidal CuO nanoparticles were formed. The optical band-gap energy of CuO nanoparticles at 300 K, as determined from the absorbance spectrum, was 3.63 eV. A photoluminescence spectrum at 300 K showed that a dominant emission peak appeared at the blue region. X-ray photoelectron spectroscopy profiles showed that the O 1s and the Cu 2p peaks corresponding to the CuO nanoparticles were observed.     

Eu掺杂SiO2纳米基质的发光特性

采用溶胶-凝胶(sol-gel)法制备了Eu掺杂的SiO₂干凝胶,分别用光致发光(PL)光谱、透射电镜(TEM)、扫描电镜(SEM)、红外吸收(IR)谱等分析手段对样品进行了表征,研究了SiO₂的基质中Eu³⁺、Eu²⁺的发光特性以及退火温度对发射光谱的影响,并对其发光机理进行了分析。结果表明,样品掺杂均匀,颗粒尺寸大约在50~80 nm,硼(B)离子进入SiO₂网格,成为了基质的一部分,改变了基质的网络结构。当采用258 nm激发样品时,随着退火温度的升高,红光发射强度先增强后减弱。对于经800 ℃退火处理的样品红光发射最强,出现了576 nm(⁵D₀ →⁷F₀),620 nm(⁵D₀ →⁷F₂),658 nm(⁵D₀ →⁷F₃)3条谱线,其中主峰位于 620 nm红光发射,对应于Eu³⁺离子的⁵D₀ →⁷F₂超灵敏跃迁,进一步说明B离子参与到基质中,形成了Si—O—B键,导致Eu³⁺离子所处配位环境的对称性降低,从而有利于Eu³⁺离子的特征发射;当采用271 nm激发样品时,随着退火温度的升高,蓝光发射强度先增强后减弱,经850 ℃退火的样品400~500 nm蓝光发射最强,归属于Eu²⁺的5d→4f的跃迁发射,证明在铝离子(Al³⁺)存在的情形下,在高温退火过程中Al³⁺部分取代Si⁴⁺形成AlO^-₄基团,掺杂Eu³⁺填补AlO^-₄基团附近的空位,增加了Eu³⁺周围的AlO^-₄四面体中氧原子的电子给予能力,使得Eu³⁺还原成Eu²⁺,从而得到了较强的蓝光发射。但是,当退火温度达到900 ℃时,由于稀土离子发生位置的迁移形成团簇红光和蓝光都明显地降低。     

以最大原子密度定义合金相中的第一近邻团簇

本文提出利用不同壳层所包含的径向原子密度, 即单位体积内的原子个数随着径向的分布, 来方便而精确地定义团簇, 即具有最大径向原子密度的且表面呈现三角密堆结构的完整壳层为第一近邻团簇. 最后以Al-Ni-Zr合金相为例说明了该方法的合理性与适用性, 及此方法所定义的团簇与非晶形成的关系.     

Sequential over-barrier ionization of multi-electron atoms in the tens-to-hundreds keV/u energy range

Our previous work on the classical over-barrier ionization model for helium double ionization is extended to the complex multi-electron system of Ne.The total and q-fold ionization cross sections are calculated at energies ranging from a few tens to several hundred keV/u.The calculation results are in good agreement with the experimental data,and the energy dependence of the cross sections suggests that the multi-ionization of a strong perturbated complex atom is probably the sequential over-barrier ionization process.