Ultrafast dynamics of femtosecond laser-induced high spatial frequency periodic structures on silicon surfaces

Femtosecond laser-induced periodic surface structures(LIPSS)have been extensively studied over the past few decades.In particular,the period and groove width of high-spatial-frequency LIPSS(HSFL)is much smaller than the diffraction limit,making it a useful method for efficient nanomanufacturing.However,compared with the low-spatial-frequency LIPSS(LSFL),the structure size of the HSFL is smaller,and it is more easily submerged.Therefore,the formation mechanism of HSFL is complex and has always been a research hotspot in this field.In this study,regular LSFL with a period of 760 nm was fabricated in advance on a silicon surface with two-beam interference using an 800 nm,50 fs femtosecond laser.The ultrafast dynamics of HSFL formation on the silicon surface of prefabricated LSFL under single femtosecond laser pulse irradiation were observed and analyzed for the first time using collinear pump-probe imaging method.In general,the evolution of the surface structure undergoes five sequential stages:the LSFL begins to split,becomes uniform HSFL,degenerates into an irregular LSFL,undergoes secondary splitting into a weakly uniform HSFL,and evolves into an irregular LSFL or is submerged.The results indicate that the local enhancement of the submerged nanocavity,or the nanoplasma,in the prefabricated LSFL ridge led to the splitting of the LSFL,and the thermodynamic effect drove the homogenization of the splitting LSFL,which evolved into HSFL.

Optimization of Injection Parameters for Profile Control and Flooding in an Oilfield during High Water Cut Period

In order to improve the effect of water control and oil stabilization during high water cut period, a mathematical model of five point method well group was established with the high water cut well group of an Oilfield as the target area, the variation law of water cut and recovery factor of different injection parameters was analyzed, and the optimization research of injection parameters of polymer enhanced foam flooding was carried out. The results show that the higher the injection rate, the lower the water content curve, and the higher the oil recovery rate. As the foam defoamed when encountering oil, when the injection time was earlier than 80% of water cut, the later the injection time was, the better the oil displacement effect would be. When the injection time was later than 80% of water cut, the later the injection time was, the worse the oil displacement effect would be. The larger the injection volume, the lower the water content curve and the higher the recovery rate. After the injection volume exceeded 0.2 PV, the amplitude of changes in water content and recovery rate slowed down. The optimal injection parameters of profile control agent for high water content well group in Oilfield A were: injection rate of 15 m3/d, injection timing of 80% water content, and injection volume of 0.2 PV.

Periodical polarization reversal modulation in multiferroic MnWO_(4) under high magnetic fields

We report polarization reversal periodically controlled by the electric field in multiferroic MnWO_(4) with a pulsed field up to 52 T.The electric polarization cannot be reversed by successive opposite electric fields in low magnetic fields(<14 T)at 4.2 K,whereas polarization reversal is directly achieved by two opposite electric fields under high magnetic fields(<45 T).Interestingly,the polarization curve of rising and falling fields for H∥u(magnetic easy axis)is irreversible when the magnetic field is close to 52 T.In this case,the rising and falling polarization curves can be individually reversed by the electric field,and thus require five cycles to recover to the initial condition by the order of the applied electric fields(+E,-E,-E,+E,+E).In addition,we find that ferroelectric phaseⅣcan be tuned from parallel to antiparallel in relation to ferroelectric phase AF2 by applying a magnetic field approximated to the c axis.

Effects of High Temperature on Antioxidant Enzymes and Lipid Peroxidation in Flag Leaves of Wheat During Grain Filling Period

On the basis of the phytotron, the effects of high temperature （daily average temperature 25, 30, 35 and 40℃, respectively） on antioxidant enzymes and lipid peroxidation in flag leaves of wheat at 50% relative air moisture during grain fastest filling stage [19-21 days after anthesis （DAA）] were studied. The wheat cultivars tested were Yangmai 9 with weak-gluten and Yangmai 12 with medium-gluten. Compared with 25℃, the higher the temperature was, the higher was the MDA content in flag leaves, while lower were the SOD, POD, and CAT activities. SOD and CAT activities in Yangmai 12 appeared to be more sensitive to high temperature than that in Yangmai 9. But POD activity in Yangmai 12 was less sensitive to high temperature. MDA content in Yangmai 12 was higher than that in Yangmai 9. The 1000-grain weight declined with increase in temperature.

Re-study on Recurrence Period of Stokes Wave Train with High Order Spectral Method

Owing to the Benjamin-Feir instability, the Stokes wave train experiences a modulation-demodulation process, and presents a recurrence characteristics. Stiassnie and Shemer researched the unstable evolution process and provided a theoretical formulation for the recurrence period in 1985 on the basis of the nonlinear cubic Schrodinger equation （NLS）. However, NLS has limitations on the narrow band and the weak nonlinearity. The recurrence period is re-investigated in this paper by using a highly efficient High Order Spectral （HOS） method, which can be applied for the direct phase- resolved simulation of the nonlinear wave train evolution. It is found that the Stiassnie and Shemer＇s formula should be modified in the cases with most unstable initial conditions, which is important for such topics as the generation mechanisms of freak waves. A new recurrence period formula is presented and some new evolution characteristics of the Stokes wave train are also discussed in details.

Hierarchical microstructures with high spatial frequency laser induced periodic surface structures possessing different orientations created by femtosecond laser ablation of silicon in liquids

High spatial frequency laser induced periodic surface structures(HSFLs)on silicon substrates are often developed on flat surfaces at low fluences near ablation threshold of 0.1 J/cm2,seldom on microstructures or microgrooves at relatively higher fluences above 1 J/cm^2.This work aims to enrich the variety of HSFLs-containing hierarchical microstructures,by femtosecond laser(pulse duration:457 fs,wavelength:1045 nm,and repetition rate:100 kHz)in liquids(water and acetone)at laser fluence of 1.7 J/cm^2.The period of Si-HSFLs in the range of 110–200 nm is independent of the scanning speeds(0.1,0.5,1 and 2 mm/s),line intervals(5,15 and 20μm)of scanning lines and scanning directions(perpendicular or parallel to light polarization direction).It is interestingly found that besides normal HSFLs whose orientations are perpendicular to the direction of light polarization,both clockwise or anticlockwise randomly tilted HSFLs with a maximal deviation angle of 50°as compared to those of normal HSFLSs are found on the microstructures with height gradients.Raman spectra and SEM characterization jointly clarify that surface melting and nanocapillary waves play important roles in the formation of Si-HSFLs.The fact that no HSFLs are produced by laser ablation in air indicates that moderate melting facilitated with ultrafast liquid cooling is beneficial for the formation of HSFLs by LALs.On the basis of our findings and previous reports,a synergistic formation mechanism for HSFLs at high fluence was proposed and discussed,including thermal melting with the concomitance of ultrafast cooling in liquids,transformation of the molten layers into ripples and nanotips by surface plasmon polaritons(SPP)and second-harmonic generation(SHG),and modulation of Si-HSFLs direction by both nanocapillary waves and the localized electric field coming from the excited large Si particles.

High winding number of topological phase in non-unitary periodic quantum walk

Topological phases and their associated multiple edge states are studied by constructing a one-dimensional non-unitary multi-period quantum walk with parity-time symmetry.It is shown that large topological numbers can be obtained when choosing an appropriate time frame.The maximum value of the winding number can reach the number of periods in the one-step evolution operator.The validity of the bulk-edge correspondence is confirmed,while for an odd-period quantum walk and an even-period quantum walk,they have different configurations of the 0-energy edge state andπ-energy edge state.On the boundary,two kinds of edge states always coexist in equal amount for the odd-period quantum walk,however three cases including equal amount,unequal amount or even only one type may occur for the even-period quantum walk.

Middle Holocene warm period and sea level high in coastal areas,North China

Relevant geological, geographical, archaeological data were collected to study the characteristics of middle Holocene warm period and sea level high on North China coast. Middle Holocene climate and sea level change on North China coast were correlated to warm marine environment events in about 8–3 ka B.P. The sea level in about 8 ka B.P. was higher than present mean sea level, then fluctuated for 5 000 years and after that it became even in 3 ka B.P. The highest sea level occurred in about 6–5 ka B.P.; the maximum was about 2–3 m and minimum was about 1–2 m.

High Power VCSEL Device with Periodic Gain Active Region

High power vertical cavity surface emitting lasers(VCSEKLs) with large aperture have been fabricated through improving passivation, lateral oxidation and heat dissipation techniques. Different from conventional three quantum well structures, a periodic gain active region with nine quantum wells was incorporated into the VCSEL structure, with which high efficiency and high power operation were expected. The nine quantum wells were divided into three groups with each of them located at the antinodes of the cavity to enhance the coupling between the optical field and the gain region. Large aperture and bottom-emitting configuration were used to improve the beam quality and the heat dissipation. A maximum output power of 1.4 W was demonstrated at CW operation for a 400 μm-diameter device. The lasing wavelength shifted to 995.5 nm with a FWHM of 2 nm at a current of 4.8 A due to the internal heating and the absence of active water cooling. A ring-shape far field pattern was induced by the non-homogeneous lateral current distribution in large diameter device. The light intensity at the center of the ring increased with increasing current. A symmetric round light spot at the center and single transverse mode operation with a divergence angle of 16° were observed with current beyond 4.8 A.

High Speed Observation of Periodic Cavity Behavior in a Convergent-Divergent Nozzle for Cavitating Water Jet

Cloud cavitation shows an unsteady periodic tendency under a certain flow condition. In a cavitating water jet flow with cavitation clouds, the cavities or the clouds produce high impact at their collapse. In order to make clear a mechanism of the periodic cavity behavior, we experimentally examine the behavior in a transparent cylindrical convergent-divergent nozzle using a high-speed video camera. An effect of upstream pressure fluctuation due to a plunger pump is investigated from a viewpoint of unsteady behavior in a cavitating water jet. As a result, it is found that the cavitating flow has two kinds of oscillation patterns in the cavity length (cavitation cloud region). One is due to the upstream pressure fluctuation caused by the plunger pump. The other is much shorter periodic motion related to the characteristic oscillation of cavitation clouds accompanied with the shrinking (reentrant), growing and shedding motion of the clouds.

Finding Periodic Solutions of High Order Duffing Equations via Homotopy Method

This paper presents a detailed analysis of finding the periodic solutions for the high order Duffing equation x^（2n） ＋ g（x） = e（t） （n ≥ 1）. Firstly, we give a constructive proof for the existence of periodic solutions via the homotopy method. Then we establish an efficient and global convergence method to find periodic solutions numerically.

Examine the Relationship of Emotional Intelligence and Creativity with Academic Achievement of Second Period High School Students

The present study aimed to examine the relationship of emotional intelligence and creativity with academic achievement of second period high school students in Nikshahr. This was a descriptive-correlational study. The population of the current study included all second period high school students in the academic year of 2013-2014. The statistical sample was selected using simple random sampling method. To collect data, Mayer Salovey Caruso Emotional Intelligence Test (1995) as well as Creativity Questionnaire (Sultani) and to compare students’ academic achievements, their GPAs in the academic year of 2013-2014 were used. Obtained data were analyzed using both descriptive (frequency table, frequency, and charts) and inferential statistics (independent t-test and Pearson correlation coefficient). The results indicated that emotional intelligence and academic achievement were significantly correlated. Moreover, there was a significant and positive relationship between creativity and academic achievement. Additionally, no significant difference was found between males and females considering their academic achievement.

Ion acoustic shock and periodic waves through Burgers equation in weakly and highly relativistic plasmas with nonextensivity

A comparative study is carried out for the nonlinear propagation of ion acoustic shock waves both for the weakly and highly relativistic plasmas consisting of relativistic ions and qdistributed electrons and positions.The Burgers equation is derived to reveal the physical phenomena using the well known reductive perturbation technique.The integration of the Burgers equation is performed by the（G￠/G）-expansion method.The effects of positron concentration,ion–electron temperature ratio,electron–positron temperature ratio,ion viscosity coefficient,relativistic streaming factor and the strength of the electron and positron nonextensivity on the nonlinear propagation of ion acoustic shock and periodic waves are presented graphically and the relevant physical explanations are provided.

基于投影寻踪模型的特高含水油藏剩余油可采潜力评价方法

影响特高含水油藏剩余油可采潜力的因素极其复杂,且各因素的影响程度差异明显,常规方法多以剩余油饱和度或剩余油储量丰度等单一指标评价剩余油潜力,难以有效指导特高含水油藏剩余油挖潜。在充分考虑特高含水油藏剩余油可采潜力影响因素的基础上,综合表征储层非均质性、剩余油可采储量规模、水淹状况以及油水分流能力的差异,构建了特高含水油藏剩余油可采潜力量化评价指标体系,并考虑不同指标对剩余油可采潜力控制程度的差异,将加速遗传算法与投影寻踪模型相结合来确定各评价指标的客观权重,从而构建了剩余油可采潜力指数,形成特高含水油藏剩余油可采潜力量化评价新方法。以渤海Q油田南区主力产层NmIL砂体为例,开展特高含水油藏剩余油可采潜力量化评价,结果表明,新方法可综合表征不同区域位置的储层物性、可采储量丰度和油水分流能力对剩余油可采潜力的影响,实现了主力产层NmIL砂体剩余油可采潜力分布的差异化定量评价,优势可采潜力区域刻画明显,将其作为NmIL砂体下一步井网加密调整潜力区域,以精准指导加密水平井的部署,为特高含水油藏剩余油挖潜提供了一种全新的分析方法与思路。

Forecasting High-Frequency Long Memory Series with Long Periods Using the SARFIMA Model

This paper evaluates the efficiency of the SARFIMA model at forecasting high-frequency long memory series with especially long periods. Three other models, the ARFIMA, ARMA and PAR models, are also included to compare their forecasting performances with that of the SARFIMA model. For the artificial SARFIMA series, if the correct parameters are used for estimating and forecasting, the model performs as well as the other three models. However, if the parameters obtained by the WHI estimation are used, the performance of the SARFIMA model falls far behind that of the other models. For the empirical intraday volume series, the SARFIMA model produces the worst performance of all of the models, and the ARFIMA model performs best. The ARMA and PAR models perform very well both for the artificial series and for the intraday volume series. This result indicates that short memory models are competent in forecasting periodic long memory series.

溯源弹簧形变过程的断路器振动信号递归量化分析辨识方法

从锁止机构脱扣引起储能弹簧释能,经部件带动动触头运动再到静止的每个动作具有严格阶段特征,伴随断路器动作的机械振动展现了能量传递及设备健康状态。该文提出一种溯源弹簧形变过程的断路器振动信号递归量化分析方法,首先由高速相机捕捉断路器操动时储能弹簧的动作图像,通过计算机视觉跟踪动态提取反映弹簧形变特征帧,再依据特征帧时序划分操动过程;然后将不同阶段振动信号映射至高维相空间,经递归分析得到体现动力系统变化特征的递归图,并递归量化分析其纹理结构特征;最后利用支持向量机模型对正常及故障状态下的断路器振动特征样本进行分析辨识,对比结果证明,由弹簧释能时序细化振动信号特征分析有效提高了分类识别准确率。该文方法在断路器操动机构状态辨识中具有广阔的应用前景。

四川盆地深层页岩气钻井关键技术新进展及发展展望

四川盆地页岩气资源丰富,目前深层页岩气已成为该盆地天然气增储上产的重点领域,但随着埋深增加和构造背景变化,地质工程条件将更加复杂,钻井过程中将面临井漏风险高、井下工具高温易失效、水平井轨迹控制难度大等技术难题。为此,在系统分析已完钻井实钻数据的基础上,依据深层页岩气区块地质工程特征,系统梳理了影响安全优快钻井的关键技术难点,形成了以地质工程一体化导向技术、钻井提速技术、防漏治漏与复杂防治技术为主体的深层页岩气安全优快钻井关键技术系列。研究结果表明:(1)以精细地质建模优选地质工程“双甜点”、实时靶体追踪为主的地质工程一体化导向技术,实现了地质目标的精准优选和精确追踪;(2)以“高效PDC钻头选型+个性化优化+大扭矩螺杆”高效破岩技术、“MSE+CCS”参数实时优化技术、油基钻井液地面降温+高温旋转导向技术为主的钻井提速技术,实现了页岩气钻井提速提效;(3)以井壁稳定性评价、裂缝性漏层识别、井漏与复杂防治为主的复杂防治技术,从源头降低了井下漏失和卡钻风险。结论认为:(1)形成的深层页岩气安全优快钻井技术,显著提高了机械钻速和铂金靶体钻遇率,在现场规模化推广应用200余口井,单井平均钻井周期降低42.7%,钻井提速效果显著,有力支撑了深层页岩气效益规模开发;(2)深层页岩气钻井将聚焦“地质工程一体化、水平段一趟钻、防漏治漏及智能钻井决策”等方面的技术攻关。

近136年(1885~2020年)长江中下游伏旱期划分及其演变分析

少雨高温的伏旱期对长江中下游地区农业、水利和人体健康等具有重要影响,但目前缺乏对该地区伏旱期的历史划分,对其长期变化规律也缺乏充分认识。本文利用1885年以来已划定的长江中下游沿江五站(上海、南京、芜湖、九江和汉口)梅雨期和夏季日气温、降水资料,对出梅后的少雨高温时段分别确定了少雨和高温的定量标准,划定出1885~2020年长江中下游伏旱期,并用统一气温标准结合伏旱期长度,综合确定了近136 a(1885~2020年)伏旱期强度指数。这136 a(包含有16个无伏旱年及10个入秋伏旱年)里伏旱期的年平均长度为21.6 d,呈现出3~6 a、36 a和84 a周期。1951年以来,1959~1978年是伏旱期长度的主要峰期,1980~1987年是伏旱期长度的下降谷期,之后缓慢增加;1980年代之后虽然大于30 d的长伏旱期有减少趋势,但是15~30 d且高强度的伏旱期频现;1995年之后秋伏旱明显增多,夏季延迟。136 a里最长且强的伏旱期依次出现于1934、1967、1978和2013年;最早的伏旱期出现在6月中旬,最晚在9月中下旬。近40 a来,伏旱期高温(≥35.0°C)日数大幅提高,伏旱期高温发生率从1980年代的30%左右上升到近年的50%以上,对人体健康构成威胁。长(强)伏旱期的副热带环流特征为西太平洋副热带高压稳定控制长江中下游区,而短(弱)伏旱期西太平洋副热带高压多偏东或偏南,入秋伏旱与副热带高压稳定西伸有关。伏旱期内高温日数的演变受到双重人类活动的制约,它不仅与人类活动导致的全球海陆升温有关,并且因地而异,即受到局地生态环境演变和城市热岛效应增减的人为作用影响。印度—太平洋季风区陆海温差对长江中下游伏旱期长度年代际变化有重要调制作用。

豫东平原‘郑麦1860’不同播期播量试验的超高产效应

为了实现豫东平原小麦主产区超高产栽培良种良法配套,充分发挥优质中强筋小麦新品种‘郑麦1860’的超高产潜力,运用裂区试验及统计分析方法,研究确定‘郑麦1860’在豫东平原小麦主产区超高产条件下适宜的播期播量指标。结果表明‘:郑麦1860’的适宜播期为10月10日-10月15日,最佳播期为10月15日;适宜播量为187.5~225.0 kg/hm^(2),最佳播量为225.0 kg/hm^(2)。最佳播期播量组合为10月15日与187.5 kg/hm^(2),折合单产12326 kg/hm^(2)。本项试验所取得的适宜播期播量技术指标,可在豫东平原小麦主产区‘郑麦1860’超高产栽培条件下推广应用。

场地地震动特征周期对高层建筑结构工程材料用量和破坏状态影响的研究

建立了2个典型高层建筑结构(剪力墙结构和框架剪力墙)的分析模型,在层间位移角满足规范规定限值的前提下,研究了混凝土用量和钢筋用量随着抗震设计地震动特征周期改变的变化规律;另外,保持两个结构模型不变,通过改变设计地震动特征周期的取值,分析了不同特征周期下两个结构模型的层间位移角最大值,给出了特征周期取值不同时两个结构模型的破坏状态。研究结果表明:层间位移角满足规范要求时,随着特征周期的变大,剪力墙结构和框架剪力墙结构中混凝土和钢筋的用量在增大,特征周期越大,混凝土和钢筋用量增量越大;特征周期增值相同时,框架剪力墙结构中混凝土用量增量和钢筋用量增量要小于剪力墙结构中的混凝土用量和钢筋用量;结构模型保持不变,特征周期变大时会影响结构的破坏状态,但基本都处于轻微破坏状态下,除了剪力墙结构在T_(g)=0.9 s时达到了中等破坏的状态。