量子点单光子源的光纤耦合

半导体量子点在低温下产生谱线细锐的激子发光可制备单光子源.光纤耦合可避免低温共聚焦装置扫描定位和振动影响,是实现单光子源即插即用和组件化的关键技术.在耦合工艺上,基于微区定位标记发展出拉锥光纤与光子晶体腔或波导侧向耦合、大数值孔径锥形端面光纤与量子点样片垂直耦合等技术;然而,上述工艺需要多维度精密调节以避免柔软光纤的畸形弯曲实现对准和高效耦合.陶瓷插针或石英V槽封装的光纤无弯曲且具有大平滑端面,只要与单量子点样片对准贴合就可保证垂直收光, V槽封装的排式光纤还可通过盲对粘合避免扫描对准,耦合简单.本文在前期排式光纤粘合少对数分布Bragg反射镜(distributed Bragg reflector, DBR)微柱样片实现单光子输出基础上,经理论模拟采用多对数DBR腔提升样片垂直出光和光纤收光效率,使光纤输出单光子计数率大大提升.     

基于自适应核学习相关向量机的乳腺X线图像微钙化点簇处理方法研究

采用自适应核学习相关向量机方法, 结合形态学滤波和Kallergi分簇标准, 研究了乳腺X线图像中微钙化点簇的处理. 首先将微钙化点检测看作一个监督学习问题, 然后应用自适应核学习相关向量机作为分类器判断图像中每一个位置是否为微钙化点并采用形态学处理滤除干扰噪声, 最后对获得的微钙化点采用Kallergi标准进行分簇. 为提高运算速度, 在微钙化点检测时将整个图像分解为多个子图像并行运算, 实现了一种基于自适应核学习相关向量机的微钙化点簇快速处理方法. 实验结果和分析表明, 自适应核学习相关向量机方法算法性能优于相关向量机方法, 特别是实现的快速方法能进一步降低微钙化点簇的处理时间. 关键词： 乳腺X线图像 微钙化点簇 相关向量机 自适应核学习     

可压缩涡流场中空泡运动规律及声辐射特性研究

基于可压缩流体力学基本理论, 通过边界积分方程, 采用不同表面压力模型, 求解空泡在计及可压缩性的涡流场中的运动规律; 通过表面离散及坐标变换, 采用Kirchhoff动边界积分方程, 将空泡表面视为运动变形边界, 作为直接噪声源, 获得涡流场中空泡运动产生的时域声压分布; 分析了涡流场参数对空泡运动规律及声辐射特性的影响. 研究结果表明: 计及流场可压缩性, 空泡的脉动幅度会随时间减弱, 辐射声压幅值随之减小; 空泡在涡流场中会发生延展、 颈缩、 撕裂, 并在撕裂后子空泡中形成射流; 当流场中的压力减小时, 空泡运动过程中的最大半径与撕裂前的最大长度逐渐增加, 且当流场中压力较小时, 空泡撕裂时形成的子空泡增多; 空泡辐射声压的指向性较弱, 撕裂会使辐射声压产生突变, 形成极大峰值; 随着涡通量的增大或空泡数的减小, 空泡脉动周期及其诱导的辐射声压波动周期随之延长, 辐射声压峰值逐渐滞后并减小. 本文结果旨在为涡流场中空泡运动规律及声辐射特性的相关研究提供参考. 关键词： 可压缩 涡流场 空泡 声辐射     

高生长速度条件下的“层片↔棒状”共晶转变机理研究

低速生长条件下, 共晶“层片↔棒状”转变只由两相的体积分数控制. 高速情况下, 这种转变有时亦发生, 其转变机理不清楚. 本文应用竞争生长准则, 结合高速生长条件下层片共晶和棒状共晶生长模型研究了生长速度引起的“层片↔棒状”转变机理. 结果显示: 体积分数在临界值附近很小的范围内, 生长速度和溶质配分系数的增大可引起“棒状→ 层片”共晶转变; 而当体积分数远离临界值时, 转变不发生. 生长速度名义上引起“层片↔棒状”共晶转变实际上是由生长速度变化引起的体积分数的变化导致的. 关键词： “层片↔棒状”共晶转变 竞争生长 生长速度 体积分数     

强流ECR离子源引出系统研究

为了提高强流ECR 离子源的引出束流品质,分别设计了1# 和2# 引出系统,利用束流引出模拟软件PBGUNS 对1# 和2# 引出系统进行了质子束流引出与传输的模拟计算,结合实际测得的发射度数据分析引出系统,发现2# 引出系统比1# 引出系统引出束流品质高。对ECR 离子源引出系统的电势等位线分布等参数引起的球差进行了简单数学推导及MATLAB 绘图,并结合1# 和2# 引出系统束流相图模拟结果证明了球差会使引出束流品质有效发射度增长,通过适当加大电极孔径可改善束流聚焦情况,得到了束流光学聚焦较好的束流引出系统设计。To improve the quality of extracted ion beam from a high current ECR ion source, 1# and 2# extraction systems were designed and tested. The PBGUNS code was used to simulate the 1# and 2# extraction systems of proton ion beam. The emittance measurement results with the two different extraction systems were compared and analyzed with the simulation, the conclusion that more high quality beam extracted from 2# system than 1# system was got. The formula derivation of ECR ion source extraction system spherical aberration and MATLAB drawing was done by the analyzing on the distribution of extraction field equipotentials, effective emittance increasing caused by spherical berration was proved by 1# and 2# extraction systems beam phase space simulation result, beam focusing would be improved if electrode hole size increasing appropriately and a general concept on good optics focusing of ion beam extraction system was proposed finally.     

Ionic conductivity in three crystalline phases of LiBH4 under pressure

The AC electrical conductivity of LiBH₄ was investigated below 2 GPa between 1 Hz and 1.6 MHz. The high-temperature phase has an ionic conductivity of up to 0.01 S cm^?1, while the low-temperature phases have conductivities two orders of magnitude lower. All phases show an Arrhenius behaviour with activation energies E _a between 0.5 and 0.7 eV, in good agreement with earlier data except for phase III, which is found to have the highest activation energy of the phases studied. The high-temperature phase has a minimum in E _a near 1 GPa, close to the triple point, correlated with a sudden change in activation volume. These features may indicate an isostructural phase transition. The conductivities of the ambient temperature phases increase temporarily by an order of magnitude after transitions between these phases, probably due to new diffusion channels via structural defects. The phase diagram agrees well with earlier results.     

Photonic bandgap via nonlinear modulation assisted by spontaneously generated coherence

A four-level double-ladder atomic system with two upper states coupled to the excited state by a standing-wave trigger field is explored to generate photonic bandgap (PBG) structure. With the assistance of spontaneously generated coherence (SGC) from the two decay pathways, we can obtain single or double fully developed PBG when the trigger field is far away from resonance or resonant. While in the absence of SGC, the atomic medium becomes strong absorptive to the probe field, and therefore the resulting PBGs are severely malformed or even cannot be opened up. Numerical results show that the PBG structure is originated from the third-order cross Kerr nonlinear modulation between the probe and trigger fields. This mechanism differs from the recent schemes based on linear modulation.     

PM10 formation during the combustion of N2-char and CO2-char of Chinese coals

The formation of PM₁₀ (particles less than or equal to 10 μm in aerodynamic diameter) during char combustion in both air-firing and oxy-firing was investigated. Three Chinese coals of different ranks (i.e., DT bituminous coal, CF lignite, and YQ anthracite) were devolatilized at 1300 °C in N₂ and CO₂ atmosphere, respectively, in a drop tube furnace (DTF). The resulting N₂-chars and CO₂-chars were burned at 1300 °C in both air-firing (O₂/N₂ = 21/79) and oxy-firing (O₂/CO₂ = 21/79). The effects of char properties and combustion conditions on PM₁₀ formation during char combustion were studied. It was found that the formation modes and particle size distribution of PM₁₀ from char combustion whether in air-firing or in oxy-firing were similar to those from pulverized coal combustion. The significant amounts of PM_0.5 (particles less than or equal to 0.5 μm in aerodynamic diameter) generated from combustion of various chars suggested that the mineral matter left in the chars after coal devolatilization still had great contributions to the formation of ultrafine particles even during the char combustion stage. The concentration of PM₁₀ from char combustion in oxy-firing was generally less than that in air-firing. The properties of the CO₂-chars were different from those of the N₂-chars, which was likely due to gasification reactions coal particles experienced during devolatilization in CO₂ atmosphere. Regardless of the combustion modes, PM₁₀ formation in combustion of N₂-char and CO₂-char from the same coal was found to be significantly dependent on char properties. The difference in the PM₁₀ formation behavior between the N₂-char and CO₂-char was coal-type dependent.     

Control and scheduling in a two-station queueing network: Optimal policies and heuristics

Consider a two-station queueing network with two types of jobs: type 1 jobs visit station 1 only, while type 2 jobs visit both stations in sequence. Each station has a single server. Arrival and service processes are modeled as counting processes with controllable stochastic intensities. The problem is to control the arrival and service processes, and in particular to schedule the server in station 1 among the two job types, in order to minimize a discounted cost function over an infinite time horizon. Using a stochastic intensity control approach, we establish the optimality of a specific stationary policy, and show that its value function satisfies certain properties, which lead to a switching-curve structure. We further classify the problem into six parametric cases. Based on the structural properties of the stationary policy, we establish the optimality of some simple priority rules for three of the six cases, and develop heuristic policies for the other three cases.     

Polarimetric-Spectral-Sensitive Compressed Ultrafast Photography Enables Real-Time Intensity Visualization of Time-Varying Polarized Ultrafast Laser Pulses

Ultrafast laser pulses with complex spatiotemporal and polarization couplings have shown great application prospects in coherent control, particle acceleration, molecular dynamics excitation, and chiral or anisotropic detection. It is very important to observe the polarization-coupled ultrafast laser pulses for characterizing the performance of laser source and understanding the laser-matter interaction, but there is currently no detection technique that can completely characterize them, including the spatio-, spectral-, and polarimetric-temporal information. In this work, a polarimetric-spectral-sensitive has been compressed ultrafast photography (PS-CUP) to realize the real-time intensity visualization of time-varying polarized (TP) ultrafast laser pulses. PS-CUP can not only distinguish the polarization state of a TP pulse, but also realize the spatio-, spectral- and polarimetric-temporal measurement. To demonstrate the ability of polarization resolving detection, PS-CUP is used to successfully distinguish between the sinistral and dextral TP pulses and measure the angle and degree of linear polarization of a dextral TP pulse, respectively, and the experimental results are highly consistent with theoretical simulations. Owing to the unprecedented multidimensional detection ability, PS-CUP is prospected to provide a powerful tool for the diagnosis of various complex ultrafast laser pulses and the detection of ultrafast chiral optics.