Observation of four-wave mixing in caesium atoms using a noncycling transition

In this paper the generation of four-wave mixing (FWM) signal using a noncycling transition of caesium atoms is investigated when the pumping laser is locked to the transition $6{\rm S}_{1/2}F=4\to6{\rm P}_{3/2}F'=4$, and meanwhile the probe frequency is scanned across the $6{\rm S}_{1/2}F=4 \to6{\rm P}_{3/2}$ transition. The efficiency of the four-wave mixing signal as a function of the intensity of the pumping beams and the detuning of the pumping beams is also studied. In order to increase the detection efficiency, a repumping laser which is resonant with $6{\rm S}_{1/2} F=3\to 6{\rm P}_{3/2}F'=4$ transition is used. A theoretical model is also introduced, and the theoretical results are in qualitative agreement with experimental ones.     

Characteristics of selective oxidation during the fabrication of vertical cavity surface emitting laser

Taking into account oxidation temperature, N₂ carrier gas flow, and the geometry of the mesa structures this paper investigates the characteristics of selective oxidation during the fabrication of the vertical cavity surface emitting laser (VCSEL) in detail. Results show that the selective oxidation follows a law which differs from any reported in the literature. Below 435℃ selective oxidation of Al_0.98Ga_0.02As follows a linear growth law for the two mesa structures employed in VCSEL. Above 435℃ approximately increasing parabolic growth is found, which is influenced by the geometry of the mesa structures. Theoretical analysis on the difference between the two structures for the initial oxidation has been performed, which demonstrates that the geometry of the mesa structures does influence on the growth rate of oxide at higher temperatures.     

Two-dimensional numerical research on effects of titanium target bombarded by TEMPⅡ accelerator

Two-dimensional numerical research has been carried out on the ablation effects of titanium target irradiated by intense pulsed ion beam (IPIB) generated by TEMP II accelerator. Temporal and spatial evolution of the ablation process of the target during a pulse time has been simulated. We have come to the conclusion that the melting and evaporating process begin from the surface and the target is ablated layer by layer when the target is irradiated by the IPIB. Meanwhile, we also obtained the result that the average ablation velocity in target central region is about 10\,m/s, which is far less than the ejection velocity of the plume plasma formed by irradiation. Different effects have been compared to the different ratio of the ions and different energy density of IPIB while the target is irradiated by pulsed beams.     

Synthesis of single-crystalline wurtzite aluminum nitride nanowires by direct arc discharge

High-density hexagonal aluminum nitride (h-AlN) nanowires were synthesized through the direct reaction of Al with nitrogen gas without catalyst and template using a direct arc discharge method. The as-grown AlN nanowires were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The h-AlN nanowires have a diameter in the range 20–70 nm and a length of several tens of micrometers. Vapor–solid growth mechanisms can be employed to explain the formation of the h-AlN nanowires. PACS 81.05.Ea; 81.10.Bk; 81.16.Dn; 68.65.-k; 81.16.-c     

Mössbauer studies of the re-entrant spin-glass behaviour of Fe–Al alloys

Fe–Al alloys around the concentration of 30 at. % Al present re-entrant spin-glass behaviour at low temperatures. This behaviour is not completely understood and Mössbauer spectroscopy, combined with other experimental techniques, is useful to describe and explain this behaviour. Results show that the Mössbauer spectra coincide with the magnetic behaviour showed in literature and they can be explained as a magnetic cluster system whose magnetic clusters are getting smaller when the temperature is decreasing. When the temperature is reaching to the spin-glass transition at 92 K the spins in the paramagnetic matrix are moving slower and below this transition the spins are completely frozen.     

Working group report: Flavor physics and model building

This is the report of flavor physics and model building working group at WHEPP-9. While activites in flavor physics have been mainly focused on B-physics, those in model building have been primarily devoted to neutrino physics. We present summary of working group discussions carried out during the workshop in the above fields, and also briefly review the progress made in some projects subsequently.     

Stealth scalar field overflying a 2+1 black hole

A nontrivial scalar field configuration of vanishing energy-momentum is reported. These matter configurations have no influence on the metric and therefore they are not be “detected" gravitationally. This phenomenon occurs for a time–dependent nonminimally coupled and self-interacting scalar field on the 2+1 (BTZ) black hole geometry. We conclude that such stealth configurations exist for the static 2+1 black hole for any value of the nonminimal coupling parameter ζ≠0 with a fixed self-interaction potential U _ζ(Φ). For the range 0 < ζ≤1/2 potentials are bounded from below and for the range 0 < ζ < 1/4 the stealth field falls into the black hole and is swallowed by it at an exponential rate, without any consequence for the black hole.     

双缺陷模一维光子晶体的双光子吸收增强研究

采用真空镀膜工艺制备了具有762 nm和800 nm双缺陷模的含两个CdS缺陷层的TiO2/SiO2一维光子晶体,运用抽运探测技术测量了其双光子吸收。对于两个缺陷模,双光子吸收均得到很大的增强,其中缺陷模为800nm时的双光子吸收系数307 cm/GW要大于缺陷模为762 nm时的116 cm/GW,分别为单层CdS薄膜的48倍和18倍。这种双光子吸收的增强是由于光局域化导致一维光子晶体缺陷层内的电场强度增大而形成的。通过传输矩阵法计算了一维光子晶体的内部场强,发现800 nm波长光入射时缺陷层内的电场强度要大于762 nm波长光入射时的电场强度值。     

深空光通信中的图像信标捕获技术

为实现深空光通信链路建立过程中精确的对准,提出了一种深空光通信系统扩展信标的捕获方案。该方案以可视地球图像作为信标,在航天器上存储一幅信标图像作为参考图像,采用天线扫描的方式在各点对所瞄准的区域成像,利用像素扫描的方式,使参考图像和实际探测图像进行相关,在天线扫描完成以后,找出相关性最大的位置,即可认为在该点捕获到地球图像。在计算两图像相关系数的过程中,由于傅里叶梅林变换幅度谱具有伸缩及旋转不变性,因此利用傅里叶梅林变换即可消除两图像相关系数因为旋转和伸缩的影响。利用蒙特卡罗方法随机产生1000个视场,仿真结果表明,3σ内正确捕获到信标图像的概率为99.6%,表明这是一种可行的方法。     

大气光通信中大气闪烁时间平滑效应研究

利用大气闪烁的时间平滑效应,增加光电探测器曝光时间是克服大气闪烁对大气光通信的瞄准、捕获、跟踪链路影响的有效途径之一.基于科尔莫戈罗夫谱,导出了弱湍流起伏区无限平面波和球面波的大气闪烁时间平滑因子的一般表达式,并且用几何光学近似得到了在大孔径和小孔径接收情况下的时间平滑因子近似表达式.在孔径远大于菲涅耳区尺度时,平面波和球面波的时间平滑因子的近似表达式与精确表达式的误差分别小于6%和3%.分析结果表明,孔径越小,时间平滑效应越显著,并且菲涅耳区尺度对时间平滑效应影响的程度也随着孔径的增加而减弱.