High-order harmonic generation of ZnO crystals in chirped and static electric fields

High harmonic generation in ZnO crystals under chirped single-color field and static electric field are investigated by solving the semiconductor Bloch equation(SBE). It is found that when the chirp pulse is introduced, the interference structure becomes obvious while the harmonic cutoff is not extended. Furthermore, the harmonic efficiency is improved when the static electric field is included. These phenomena are demonstrated by the classical recollision model in real space affected by the waveform of laser field and inversion symmetry. Specifically, the electron motion in k-space shows that the change of waveform and the destruction of the symmetry of the laser field lead to the incomplete X-structure of the crystal-momentum-resolved(k-resolved) inter-band harmonic spectrum. Furthermore, a pre-acceleration process in the solid four-step model is confirmed.

Tailoring OAM spectrum of high-order harmonic generation driven by two mixed Laguerre–Gaussian beams with nonzero radial nodes

The extreme ultraviolet(XUV)light beam carrying orbital angular momentum(OAM)can be produced via high-order harmonic generation(HHG)due to the interaction of an intense vortex infrared laser and a gas medium.Here we show that the OAM spectrum of vortex HHG can be readily tailored by varying the radial node(from 0 to 2)in the driving laser consisting of two mixed Laguerre-Gaussian(LG)beams.We find that due to the change in spatial profile of HHG,the distribution range of the OAM spectrum can be broadened and its shape can be modified by increasing the radial node.We also show that the OAM mode range becomes much wider and its distribution shape becomes more symmetric when the harmonic order is increased from the plateau to the cutoff when the driving laser has the nonzero radial nodes.Through the map of coherence length and the evolution of harmonic field in the medium,we reveal that the favorable off-axis phase-matching conditions are greatly modified due to the change of intensity and phase distributions of driving laser with the radial node.We anticipate this work to stimulate some interests in generating the XUV vortex beam with tunable OAM spectrum through the gaseous HHG process achieved by manipulating the mode properties of the driving laser beam.

Floquet spectrum and universal dynamics of a periodically driven two-atom system

We investigate the Floquet spectrum and excitation properties of a two-ultracold-atom system with periodically driven interaction in a three-dimensional harmonic trap.The interaction between the atoms is changed by varying the s-wave scattering length in two ways,the cosine and the square-wave modulations.It is found that as the driving frequency increases,the Floquet spectrum exhibits two main features for both modulations,the accumulating and the spreading of the quasienergy levels,which further lead to different dynamical behaviors.The accumulation is associated with collective excitations and the persistent growth of the energy,while the spread indicates that the energy is bounded at all times.The initial scattering length,the driving frequency and amplitude can all significantly change the Floquet spectrum as well as the dynamics.However,the corresponding relation between them is valid universally.Finally,we propose a mechanism for selectively exciting the system to one specific state by using the avoided crossing of two quasienergy levels,which could guide preparation of a desired state in experiments.

Influence of the initial electronic state on minima of high-order harmonic spectrum radiated from hydrogen molecular ion

We theoretically investigate the high-order harmonic generation of the one-dimensional hydrogen molecular ion at fixed intermediate internuclear distance, driven by a multicycle laser field. Our results show that the initial electronic state of the hydrogen molecular ion affects the modulation of the high-order harmonic spectrum, especially the positions of the minima. Based on the two-state model, the underlying physical mechanism of the minimum is analyzed and discussed. Further analysis shows that the different positions of the minima in the different initial electronic states can be understood via the different interferences of the two phase-adiabatic states at the ionization times.

Cross-Band Spectrum Prediction Based on Deep Transfer Learning

Spectrum prediction is a promising technology to infer future spectrum state by exploiting inherent patterns of historical spectrum data.In practice,for a given spectrum band of interest,when facing relatively scarce historical data,spectrum prediction based on traditional learning methods does not work well.Thus,this paper proposes a cross-band spectrum prediction model based on transfer learning.Firstly,by analysing service activities and computing the distances between various frequency points based on Dynamic Time Warping,the similarity between spectrum bands has been verified.Next,the features,which mainly affect the performance of transfer learning in the crossband spectrum prediction,are explored by leveraging transfer component analysis.Then,the effectiveness of transfer learning for the cross-band spectrum prediction has been demonstrated.Further,experimental results with real-world spectrum data demonstrate that the performance of the proposed model is better than the state-of-theart models when the historical spectrum data is limited.

SINGULAR CROSS SPECTRUM APPLIED ON DIAGNOSTIC ANALYSIS OF AIR-SEA INTERACTION

The Singular Cross Spectrum Analysis (SCSA) method was employed to investigate the coupledperiods of air-sea/mid-low circulation interaction using 1951-1993 500-hpa geopotential heights and the seasurface temperature (SST) in the Northern Hemisphere. Results show that air-sea correlation is noticeable on the3-7 year scale which is similar to ENSO circle. In this sense, ENSO is a Strong signal. Quasi-ten/quasi-four yearperiods are prominent in the mid-low interaction, and quasi-if our year oscillation is uniform with the ENSO circle.Studies indicate that the mid-low interaction possess quasi-ten year oscillation beside being affected by ENSO.

GPR信号去噪的变分模态分解

为了进一步提高探地雷达(ground penetrating radar,GPR)数据的信噪比,压制由随机扰动引起的随机绕射能量,将二维变分模态分解(two-dimensional variational mode decomposition,2D-VMD)引入二维GPR数据的噪声压制处理中。首先,对GPR数据进行2D-VMD处理,并分析各阶本征模态函数(intrinsic mode function,IMF)分量及其对应的频率-波数域谱来确定雷达剖面中的各回波类型。然后,计算IMF分量与原始数据的互相关系数来确定信号模态和噪声模态,并对信号模态进行重构得到降噪后的数据。理论数据和实测数据测试表明,相比于传统的1D-VMD法,2D-VMD滤波后的含噪正演记录峰值信噪比由6.44 dB增加到7.72 dB;经2D-VMD降噪处理后的雷达剖面在保留有效信号的基础上,可以有效压制随机扰动带来的噪声,并且得到的雷达剖面同相轴连续性更好。

声波法在输油管道上的可检测泄漏率分析

为了确保输油管道出现泄漏时能够及时检测到泄漏,对声波法在输油管道上的可检测泄漏率进行了分析。首先,建立了声波产生和传播衰减模型,根据模型分析了声波在管道内的衰减性,得到管道两端声波衰减幅值的计算方法;其次,采用基于互信息优化的自适应噪声完备集合经验模态分解(complete ensemble empirical mode decomposition with adaptive noise,简称CEEMDAN)和互谱分析相结合的去噪算法对采集的信号进行去噪,并实验验证了衰减模型可准确估计声波的衰减幅值,去噪算法可以有效消除信号中的噪声;最后,分析了管道特性与声波法在输油管道上的可检测泄漏率之间的关系。研究表明,管道特性在一定程度上决定了声波法的可检测泄漏率,声波法在输油管道上的最小可检测泄漏率可达0.45%。

Frequency Domain Fatigue Evaluation on SCR Girth-Weld Based on Structural Stress

The Steel Catenary Riser(SCR)is a vital component for transporting oil and gas from the seabed to the floating platform.The harsh environmental conditions and complex platform motion make the SCR’s girth-weld prone to fatigue failure.The structural stress fatigue theory and Master S-N curve method provide accurate predictions for the fatigue damage on the welded joints,which demonstrate significant potential and compatibility in multi-axial and random fatigue evaluation.Here,we propose a new frequency fatigue model subjected to welded joints of SCR under multiaxial stress,which fully integrates the mesh-insensitive structural stress and frequency domain random process and transforms the conventional welding fatigue technique of SCR into a spectrum analysis technique utilizing structural stress.Besides,a full-scale FE model of SCR with welds is established to obtain the modal structural stress of the girth weld and the frequency response function(FRF)of modal coordinate,and a biaxial fatigue evaluation about the girth weld of the SCR can be achieved by taking the effects of multi-load correlation and pipe-soil interaction into account.The research results indicate that the frequency-domain fatigue results are aligned with the time-domain results,meeting the fatigue evaluation requirements of the SCR.

基于Back-n白光中子实验装置的SRAM翻转截面测量

本文主要研究静态随机存取存储器(static random-access memory,SRAM)的单粒子效应翻转截面的测量方法。基于宽能谱白光中子束流,采用了一种操作更方便的SRAM翻转截面测量方法。在SRAM前面放置聚乙烯中子慢化材料改变入射到SRAM表面上的中子能谱,利用模拟计算得到改变后的中子能谱。利用奇异值分解法求解翻转率的矩阵方程得到SRAM的翻转截面。结果表明在4~15 MeV的能量范围内,使用反角白光中子源测试的SRAM翻转截面信息和参考文献中使用单能中子源测试拟合的SRAM翻转截面信息基本吻合。

海南省东方市社区居民疾病谱分析

目的探讨2021年海南省东方市社区居民的疾病构成及流行病学特征。方法采用横断面研究设计,分析了2021年1月1日—12月31日海南省东方市疾病监测数据库196761例社区居民的病历资料。对该人群进行了年龄标准化后,分析和描述了前十位疾病构成。结果196761例社区居民中,男性91621例(占46.56%),女性105140例(占53.43%),男女比为0.87;以≥45岁居民居多,占46.13%(90763/196761)。疾病谱前3位依次是呼吸系统疾病[32.69%(64327/196761)]、循环系统疾病[7.86%(15470/196761)]和消化系统疾病[7.21%(14191/196761)];前10位系统疾病中,男性损伤、中毒和外因的某些其他后果的年龄标准化患病率高于女性(3.11%vs 2.30%),女性泌尿生殖系统疾病的年龄标准化患病率高于男性(3.01%vs 1.20%)。呼吸系统疾病以急性上呼吸道感染(5.17%)、急性支气管炎(1.84%)和慢性支气管炎(1.44%)为主,其在<5岁组的患病率(38.59%)和伤残损失寿命年率(656.01/10万)最高;循环系统疾病以高血压病(1.72%)、脑梗死(0.71%)和慢性缺血性心脏病(0.49%)为主,其在≥60岁组的患病率(17.13%)和伤残损失寿命年率(2175.49/10万)最高;消化系统疾病以胃炎和十二指肠炎(1.10%)、其他非感染性胃肠炎和结肠炎(0.43%)以及龈炎和牙周疾病(0.38%)为主,其在≥60岁组的患病率(6.98%)和伤残损失寿命年率(1710.75/10万)最高。结论2021年海南省东方市社区居民以呼吸系统疾病、循环系统疾病和消化系统疾病为主,且疾病负担主要归因于需要长期管理的疾病。东方市应针对疾病谱特征优化和调配医疗资源,加强高危人群的健康促进和防治工作。

面向小型载荷的宽带通信信号随机共振检测方法

随着人工智能等领域的飞速发展,无人机在各个领域,尤其是现代战争中发挥着越来越重要的作用。无人机凭借其强大的灵活性和高隐蔽性优势,对现代作战具有颠覆性影响。在无人机执行战场侦察、监视、通信、电子干扰、雷达诱骗、激光制导等任务的过程中,及时探测敌方辐射信号尤为重要。随机共振方法利用特定非线性系统可以使输入其中的信号与噪声在一定条件下产生协同作用的特点,将噪声的能量转移到信号中来放大信号,是常用的微弱信号检测方法之一。窄带信号的随机共振检测技术已趋于成熟。而对小型载荷无人机监视、通信等场景下的微弱宽带通信信号,现有随机共振检测方法直接采用窄带信号的时域能量检测方法,并未基于宽带信号特点设计专门的检测方法,低信噪比条件下检测性能受限。为了满足对微弱宽带通信信号的检测要求,本文以常用的宽带通信信号为研究对象,对于单通道接收场景,提出基于宽带信号局部频谱的随机共振信号检测方法,该方法与传统的随机共振信号检测方法相比性能提升约6 dB。其次,对于更低信噪比的情况,提出一种随机共振与互相关检测相结合的双通道协同检测方法,并提出基于随机共振后两路互相关系数最大化的系统参数优化准则。典型场景下的仿真分析表明,对于单通道接收的低信噪比微弱宽带通信信号,局部频谱法可以有效提升检测性能;对于更低信噪比宽带通信信号,采用随机共振处理后的双通道互相关检测可以进一步提升检测性能。

基于拉曼光谱鉴定书写/打印文件的朱墨时序

朱墨时序鉴定是司法鉴定文件检验研究与实践领域中的热点和难点问题。朱墨时序的鉴定可证明文件真伪,推断文件物证是否与案件事实相符,从而为司法机关的侦查破案和案件判定提供法律依据。常见的检验方法有显微镜法、荧光法、文检仪法、计算机软件辅助法等,其鉴定结论易受检验人员主观意识的影响,因不够客观而备受质疑。研究在制备3类签字笔和9种印油组合54个书写样本、激光打印和9种印油组合18个打印样本的基础上,运用共焦显微拉曼光谱仪对其进行检验。以10#百乐签字笔和1#印之秀印油为样本测试得到最佳实验条件,将制备的72个样本放入储存箱内避光保存1年,依次测定样本中先字后印、先印后字、印油和字迹4个位点的拉曼光谱,每个样本测试3次。实验结果基本不受储存环境、纸张本底等因素的干扰,无论是书写样本还是打印样本,先字后印交叉点显示出了印油的部分特征,先印后字交叉点显示出了字迹的部分特征,交叉位点的拉曼光谱图显示大部分为上层相应的字迹或印油的拉曼位移,这与20倍物镜观察的μView图显示结果相互印证。通过比对印文交叉部位的拉曼位移,进而鉴定出样本的朱墨时序。结果表明:相比常规检验方法,此新颖方法简便快捷、无损、灵敏度和准确率高,对文件的朱墨时序鉴定具有指导意义。

陕西省2021年极端秋雨成因分析

利用NCEP/NCAR 2.5°×2.5°的逐月再分析资料和NOAA提供的逐月海温资料等从海温异常、大尺度环流异常、水汽输送异常等角度分析2021年秋季陕西降水异常偏多的原因,结果表明:(1)小波交叉谱分析显示Ni?o3.4区海温距平与陕西秋季降水存在2~4 a的共振周期,前期冬季海温与次年秋季降水关系呈正相关的共振周期。(2)受双峰型La Nina事件影响,2021年9-11月西太平洋副热带高压(简称“副高”)控制的范围明显偏大,副高脊线位置较气候态偏北1°,西伸脊点较气候态偏西23°,造成主雨带偏西偏北。(3)中低层西北太平洋到我国华北北部的异常反气旋,与孟加拉湾北侧异常的气旋中心,造成气流在我国西北地区东部异常辐合。(4)从水汽收支来看,陕西2021年秋季水汽净流入较气候态偏多66.7%,尤其是东边界的异常水汽输送,还增强了低层冷垫,利于暖湿气流在冷垫上爬升导致陕西降水较常年异常偏多。

基于宽频认知层叠网的海上跨域协同组网通信技术

随着海上多平台远洋部署和强敌对抗形势日益严峻,依托于短波(HF)、超短波(V/UHF)、微波等单频段海上无线通信技术已不能满足海上跨域协同作战对多链融合组网、动态资源分配和体系化抗干扰的通信保障需求。文中提出一种基于宽频认知层叠网的新型多链融合网络通信架构,采用综合射频一体化前端进行宽频带射频资源管理,通过统一带宽、统一时隙、统一时空形成数据链融合元模型,在此基础上采用软件通信体系后端进行多链融合和层叠组网,通过宽带频谱感知、认知频谱接入和任务驱动自适应组网等技术实现对空海潜等海上多域通信环境的动态适应和强敌对抗环境的认知决策,可提升海上通信网络扩展、跨域传输和抗干扰通信能力。文中对宽频认知层叠网的关键技术进行了深入研究,给出了宽频认知层叠网的可行技术途径,可为本领域的体系架构设计和技术研究提供参考借鉴。

Interference of harmonics emitted by different tunneling momentum channels in laser fields

By numerically solving the semiconductor Bloch equation(SBEs),we theoretically study the high-harmonic generation of ZnO crystals driven by one-color and two-color intense laser pulses.The results show the enhancement of harmonics and the cut-off remains the same in the two-color field,which can be explained by the recollision trajectories and electron excitation from multi-channels.Based on the quantum path analysis,we investigate contribution of different ranges of the crystal momentum k of ZnO to the harmonic yield,and find that in two-color laser fields,the intensity of the harmonic yield of different ranges from the crystal momentum makes a big difference and the harmonic intensity is depressed from all k channels,which is related to the interferences between harmonics from symmetric k channels.

拟合楼层反应谱的三向地震动时程生成方法研究

为获得满足规范要求的拟合楼层反应谱的三向地震动时程,采用六阶常微分方程的本征函数为基函数,分解并重构天然地震动时程;通过构建影响矩阵,迭代计算逐步调整本征函数的幅值系数,最终实现预设拟合精度;引入正交化方法保证三向地震动时程两两之间的统计独立性,并通过两组算例提供验证。研究结果表明,迭代所得时程反应谱与目标楼层反应谱的拟合精度较高,迭代过程中时程反应谱均匀一致地向目标谱逼近,迭代过程收敛,所得三向地震动时程均无零飘现象,满足规范要求。

强干扰下基于三阶互累积量的FMCW雷达信号DOA估计算法

针对车载调频连续波雷达存在相互干扰的实际情况,本文提出了一种基于三阶互累积量的多目标波达方向超分辨估计方法.该方法利用同步采集的阵列信号并根据回波信号和干扰信号的非相关特性,构造三阶互累积量矩阵,随后根据子空间方法获得二维空间谱,实现距离和方位的联合估计.计算机仿真结果表明,本文所提方法能够在抑制强干扰影响的同时实现3°以内多目标识别,与现有超分辨法和重构法相比,具有更高的角度分辨率和估计精度.

非完美频谱感知下认知超密集网络的资源分配

针对实际认知超密集网络场景中认知无线电存在非完美频谱感知的情况,提出了一种基于非完美频谱感知的资源分配方案,目标是在考虑跨/同层干扰约束、保障用户服务质量下,最大化非完美频谱感知下认知超密集网络中次级网络的能效。为此,依据网络模型构建能效优化问题,其为混合整数非凸规划问题,先通过分时共享松弛法和丁克尔巴赫法将其转换成等价的凸优化问题,再使用拉格朗日对偶法求其最优解,以此获得最优能效时的子信道和功率分配策略。基于此,提出了一种迭代的子信道和功率分配算法;为权衡计算复杂度,还提出了一种实用的子信道和功率分配算法。仿真结果表明,所提算法都有效地提升了网络能效。

应用屏栅电离室测量^(16)O(n,α)^(13)C反应的模拟研究

基于Geant4建立了一套利用屏栅电离室测量^(16)O(n,α)^(13)C反应的模拟方法,能预测不同入射中子能量、不同屏栅电离室结构、不同工作气体和样品参数等实验条件下屏栅电离室测量到的阴极-阳极幅度2维谱。为了验证该模拟方法的可靠性,在北京大学4.5 MV静电加速器上开展了实验,实验所用的中子能量为5.06 MeV,工作气体为0.3 MPa的Ar与体积分数为3.45%CO 2组成的混合气体(其中的氧作为待测样品)。实验测得了屏栅电离室的阴极阳极幅度2维谱,且模拟与实验结果吻合较好。基于此模拟方法提出了一套实验条件的优化方案,可为下一步在宽能区精确测量^(16)O(n,α)^(13)C反应截面提供参考。