Generation of Broadband Attosecond Pulse via Controlling Quantum Path by Two Color Field in Long Wavelength Driving Regime

In this paper, we have investigated theoretically the high harmonic generation form helium atom in long wavelength driving regime at 2000 nm through solving time-dependent Schr6dinger equation. By adding a second harmonic pulse （1000 nm） and a UV attosecond pulse （200 nm） to the driving field, an efficient method for picking out and enhancing ionization path to generate high-yield supercontinuum harmonics is realized, and then an isolated sub-100 as pulse with a bandwidth of 190 eV is significantly obtained.

Single-pinhole diffraction of few-cycle isolated attosecond pulses with a two-color field

The spatio-temporal characterization of an isolated attosecond pulse is investigated theoretically in a two-color field. Our results show that a few-cycle isolated attosecond pulse With the center wavelength of 16 nm can be generated effectively by adding a weak controlling field. Using the split and delay units, the isolated attosecond pulse can be split to the two same ones, and then single-pinhole diffractive patterns of the two pulses with different delays can be achieved. The diffractive patterns depend severely on the periods of the attosecond pulses, which can be helpful to obtain temporal information of the coherent sources.

Pattern formation in particle systems driven by color field

The structural evolution of systems with two kinds of particles driven in opposite directions, i.e., driven by a color field, is investigated by molecular dynamics simulations. Gaussian thermostat, a common treatment to restrict the thermal velocity of the particles in the systems, has been used so as to account for the dissipation of heat and allow the system to reach a steady state. It has been found that with the increase of the strength of driving force （F）, the system undergoes an obvious structural transition from an initially random mixing state to a state characterized by separate lanes and in each lane only one kind of particles exists. The analysis shows that the reason for the formation of lane structure is not only the increase of F but also the variation of particle friction coefficient. While using Gaussian thermostat the particle friction coefficient becomes a function of F. Increasing F leads to high particle friction coefficient and inevitably results in lane formation for strong enough driving force. When lifting the effect of F on friction coefficient and choosing a constant friction coefficient, our results show that for a given F there always exists a critical value of friction coefficient higher than which the system will develop into lane structure.

Dependence of above-threshold ionization spectrum on polarization directions of two-color laser fields

Using the frequency-domain theory, we investigate the above-threshold ionization（ATI） process of an atom in twocolor laser fields. When both photon energies of the two-color laser fields are much smaller than the atomic ionization threshold, the ATI spectrum depends on the angle between the two lasers＇ polarization directions. While when the photon energy of one laser is comparable with or larger than the atomic ionization threshold, the ATI spectrum is independent of the angle, and only several dips appear at certain angles. By analyzing the contributions of different quantum channels, we find that, for the case that both frequencies of the two color lasers are low, the quantum interferences between the channels are strong, and hence the spectrum changes with the angle between the two lasers＇ polarization directions. While for the case that the frequency of one of the two color lasers is high, the contributions of the channels to the ATI spectrum decrease dramatically with increasing channel order, hence the interferences between the channels disappear, and the ATI spectrum has a step-like structure, which is independent of the angle between the two lasers＇ polarizations. These results can shed light on the study of the corresponding relation between classical and quantum mechanisms of the matter–laser interaction in high-frequency laser fields.

High-order harmonic generation in a two-color strong laser field with Bohmian trajectory theory

We theoretically study the high-order harmonic generation （HHG） in a two-color laser field using the Bohmian mechanics. Our results show that, for tile case of a weak second-color laser field, the simulation of the HHG with only one central Bohmian trajectory is in a good agreement with the ab initio time-dependent Schrodinger equation （TDSE） results. In contrast, with the increase of the amplitude of the second-color laser field, the HHG spectra from the single central Bohmian trajectory deviate from the TDSE results more and more significantly. By analyzing the Bohmian trajectories, we find that the significant deviation is due to the fact that the central Bohmian trajectory leaves the core quickly in the two-color laser field with the breaking of inversion symmetry. Interestingly, we find that another Bohmian trajectory with different initial position, which keeps oscillating around the core, could qualitatively well reproduce the TDSE results. Furthermore, we study the HHG spectrum in a two-color laser field with inversion symmetry and find that the HHG spectrum in TDSE can be still well simulated with the central Bohmian trajectory. These results indicate that, similar to the case of one color laser field, the HHG spectra in a two-color laser field can be also reproduced with a single Bohmian trajectory, although the initial position of the trajectory is dependent on the symmetry of the laser field. Our work thus demonstrates that Bohmian trajectory theory can be used as a promising tool in investigating the HHG process in a two-color laser field.

Controlling the contributions to high-order harmonic generation from different nuclei of N_2 with an orthogonally polarized two-color laser field

The generation of high-order harmonic and the attosecond pulse of the N2 molecule with an orthogonally polarized two-color laser field are investigated by the strong-field Lewenstein model.We show that the control of contributions to high-order harmonic generation（HHG） from different nuclei is realized by properly selecting the relative phase.When the relative phase is chosen to be φ= 0.4π,the contribution to HHG from one nucleus is much more than that from another.Interference between two nuclei can be suppressed greatly; a supercontinuum spectrum of HHG appears from 40 e V to125 e V.The underlying physical mechanism is well explained by the time–frequency analysis and the semi-classical threestep model with a finite initial transverse velocity.By superposing several orders of harmonics,an isolated attosecond pulse with a duration of 80 as can be generated.

High-order harmonic generation of N2molecule in two-color circularly polarized laser fields

The generation of high-order harmonics and the attosecond pulse of the N2 molecule in two-color circularly polarized laser fields are investigated by the strong-field Lewenstein model. We show that the plateau of spectra is dramatically extended and a continuous harmonic spectrum with the bandwidth of 113 eV is obtained. When a static field is added to the x direction, the quantum path control is realized and a supercontinuum spectrum can be obtained, which is beneficial to obtain a shorter attosecond pulse. The underlying physical mechanism is well explained by the time-frequency analysis and the semi-classical three-step model with a finite initial transverse velocity. By superposing several orders of harmonics in the combination of two-color circularly polarized laser fields and a static field, an isolated attosecond pulse with a duration of 30 as can be generated.

Imitation design of temperature field in coloring hot dip galvanization process

Colors were generated by preferential oxidation of the metals in coloring hot dip galvanization process, and the evolution of temperature field during cooling after hot dip would be responsible for the coloration results directly. The influences of bath temperature and velocity of steel strip moving on temperature field of the strip were calculated in continuous coloring hot dip galvanization process by means of the ANSYS/Thermal module. The factors were considered including convection heat transfer, latent heat during cooling, and heat radiation in calculation. The effects of temperature field on coloration of sheet steel samples were investigated.

Asymmetric structure of atomic above-threshold ionization spectrum in two-color elliptically polarized laser fields

According to the frequency-domain theory, we investigate the asymmetric structure of above-threshold ionization(ATI) spectrum of an atom in two-color elliptically polarized(EP) laser fields. When both laser fields are linearly polarized(LP), the spectrum shows that the multi-plateau structure is symmetric about the emitted angle of electron at π/2, while the spectrum becomes asymmetric and shifts rightwards with the increase of the EP degree of the IR laser field. Since the total ATI process is regarded as including direct ATI and the rescattering ATI, we analyze the spectrum structure of direct ATI and rescattering ATI separately. Using the saddle-point approximation, we find that for direct ATI, the fringes on the spectrum are mainly attributed to the fact that the ionization probability becomes very small when the direction of emitted electrons is perpendicular to the direction of the XUV laser polarization;while for the rescattering ATI, the interference fringes on the spectrum mainly come from the superposition of the waist structures on the spectra of all sub-channels.

Relative phase effect of nonsequential double ionization in Ar by two-color elliptically polarized laser field

We investigated the nonsequential double ionization(NSDI)in Ar by two-color elliptically polarized laser field with a three-dimensional(3D)classical ensemble method.We study the relative phase effect of NSDI and distinguish two particular recollision channels in NSDI,which are recollision–impact ionization(RII)and recollision-induced excitation with subsequent ionization(RESI),according to the delay-time between the recollision and the final double ionization.The numerical results indicate that the ion momentum distribution is changed and the triangle structure is more obvious with the decrease of the relative phase.We also demonstrate that the RESI process always dominates in the whole double ionization process and the ratio of RESI and RII channels can be influenced by the relative phase.

超广角视网膜激光扫描成像系统在DR筛查中的应用评价

目的评价超广角视网膜激光扫描成像(UWFSLO)在糖尿病视网膜病变(DR)筛查中的应用价值。方法采用横断面研究,选取2016年10—12月在河北省唐山市开滦康复医院行DR筛查的2型糖尿病患者1288例2304眼,收集患者一般信息,完成UWFSLO和45°水平两视野数字彩色眼底照相检查,比较DR的检出及分级水平。不同眼底检查对DR分级的一致性采用Cohen Kappa检验。结果去除成像不清晰及缺失的图像,最终1857眼纳入分析。彩色眼底照相、UWFSLO复合彩色图像及绿通道图像对DR的检出率分别为31.3%(581/1857)、44.5%(826/1857)和43.4%(807/1857),差异有统计学意义(χ^(2)=85.547,P<0.001),其中UWFSLO复合彩色图像、绿通道图像对DR的检出率均高于彩色眼底照相,差异均有统计学意义(均P<0.001)。45°水平两视野数字彩色眼底照相与UWFSLO复合彩色图像在DR检出分级之间的一致性一般,Kappa值为0.375(P<0.001),完全一致性为64.08%。UWFSLO绿通道图像与复合彩色图像对DR分级的一致性极好,Kappa值为0.953(P=0.008),完全一致性为98.17%。UWFSLO复合彩色图像对微动脉瘤(MA)、视网膜内微血管异常(IRMA)、视网膜新生血管(NV)病变的检出水平均低于绿通道图像,差异均有统计学意义(Z=-16.489、-4.198、-2.111,均P<0.05)。结论UWFSLO较45°水平两视野数字彩色眼底照相对DR的检出及分级评估水平更高,其中绿通道图像在MA、IRMA和小的NV病变检出上更为出色。在大规模人群DR的筛查及诊断中,UWFSLO具有较好的应用价值。

基于FPGA的仿造数码迷彩生成系统研究

为了解决传统仿造数码迷彩生成算法开发平台灵活性不足的问题,提出了一种基于现场可编程门阵列(FPGA)的仿造数码迷彩生成算法。采用直方图统计提取主要颜色信息,将背景图像马赛克化获得颜色分布信息,依据色差进行主色替换得到仿造数码迷彩的方法,取得了不同主色数量、色差计算权重、颜色量化数量下的数据。结果表明,颜色量化为3600种后,提取出4种主色作为仿造数码迷彩主色,根据色差主色替换时色彩权重3∶6∶1,生成的仿造数码迷彩具有良好的伪装效果,且基于FPGA实现该算法时系统占用资源少,处理800 pixels×480 pixels分辨率的单帧彩色图像所需时间仅为22.21 ms,满足动态场景下生成仿造数码迷彩的实时性需求。该研究为主动伪装技术提供了参考。

基于Color Lines先验的高阶马尔科夫随机场去雾

传统的一阶马尔科夫随机场在图像先验信息表达和对图像整体的约束上能力有限,同时基于暗通道的去雾算法在天空等大片白色区域处理效果存在偏差。针对以上问题,该文提出一种基于Color Lines的高阶马尔科夫随机场去雾算法。该算法通过引入对颜色失真具有很好鲁棒性的Color Lines先验条件,初步校正经暗通道获取的传输图,然后利用高阶马尔科夫随机场优化传输图,获取最终精确的去雾图像。实验结果表明,与已有算法相比,该文算法具有更强的普适性,可提高雾天图像的清晰度,同时恢复更多的图像细节等信息。

基于改进TextSnake的印章字符检测算法

针对字符模糊、宽高比多变、与背景文字重叠等因素导致的印章字符易被漏检、误检等问题,提出基于TextSnake的印章字符检测算法,记作TextSnake-CR算法。首先在特征融合中嵌入手工设计的感受野增强模块,使浅层特征拥有更大感受视野,从而有效地降低误检率;其次提出颜色特征提取模块,用于提取印章字符中颜色特征,增强模糊印章字符的检测精度;最后改进模型的分类损失函数,抑制背景噪声对模型的干扰,进一步提高模型检测性能。实验结果表明,TextSnake-CR在公开印章数据集与自制数据集上的F值分别达到90.71%和81.79%,与其他算法相比,有效地提高了印章字符检测准确率。

平行偏振三色场对原子非次序双电离的调控

本文利用三维经典系综模型研究了平行偏振三色场中He原子的非次序双电离.利用强度相同的1600 nm和800 nm脉冲组成驱动场,低强度的400 nm脉冲作为控制场.研究结果表明,第一次返回碰撞轨道、奇数次返回碰撞轨道(不含第一次返回碰撞)和偶数次返回碰撞轨道释放的电子对和离子分别处在电子关联动量谱和离子动量谱上的不同区域.通过改变控制场的相位,可以很好地控制不同返回次数碰撞轨道在双电离中的占比,进而实现对电子动量分布和离子动量分布的控制.另外,多次返回碰撞导致的双电离以碰撞电离机制为主,而第一次返回碰撞导致的双电离以碰撞激发电离机制为主,所以通过改变控制场的相位也能实现对双电离机制的调控.

基于双色法的碳烟火焰三维温度场计算层析成像测量

建立了一种基于多角度火焰光学投影信号获取的三维层析重建成像算法,用于测量不规则形态火焰的三维结构。通过搭建可旋转的射流燃烧装置建立不规则轮廓非预混稳定乙烯碳烟火焰,采用单一像增强相机(ICCD)从28个不同角度依次进行火焰自发光的全波段、550 nm和650 nm窄波段投影图像采集,结合自编制的三维重建程序与修正双色法,实现了碳烟火焰三维温度场测量。结果表明,三维重构算法能够再现火焰结构与温度场三维空间分布特性,温度测量值符合层流乙烯扩散碳烟火焰温度范围,采用任意不同角度投影图像数据的数目高于15个时,重构效果最好。

反旋双色椭圆偏振激光场中Ar原子的非序列双电离

利用经典系综方法研究了不同椭偏率的反旋双色椭圆偏振(two-color elliptically polarized,TCEP)激光场中Ar原子非序列双电离(nonsequential double ionization,NSDI)的电子关联特性和再碰撞动力学.不同于反旋双色圆偏振激光场,反旋TCEP激光场不再具有空间对称性,返回电子主要从一个方向返回母离子,从而导致电子动量分布表现出很强的不对称性.数值结果显示随着椭偏率的增大,Ar原子NSDI的产量逐渐减小,并且电子对在椭圆偏振激光场长轴方向上的关联电子动量分布,从主要位于第一和第三象限的正相关逐渐演变为主要位于第二和第四象限的反相关.通过对不同特征时间的统计分析表明,随着椭偏率的增大,旅行时间和返回电子的重碰撞能量逐渐减小,而延迟时间却增大,这是电子对关联特性发生变化的主要原因.此外,进一步分析发现,无论是“短轨迹”还是“长轨迹”,椭偏率的增大都会使两个电子由同向出射逐渐转变为反向出射,这表明椭偏率和旅行时间都影响着电子的出射方向.

快速响应的NV色心微波传感器实现及测量

高精度微波快速测量技术是电磁频谱分析监测的核心,微波测量技术为微波传感器的高精度测量提供了可能。通过建立NV色心空间磁场能级响应频率精密调谐模型,结合NV色心光学宽场成像技术,实现了高精度、响应快可调的微波测量。首先,通过连续光探测磁共振(ODMR)技术,结合宽场成像技术,通过像素级磁场强度-微波频率转化关系,实现宽场成像的微波测量;进而将连续扫频技术转变为施加定频进行检测,通过同步监测频点不同的响应特性,验证了测试结果的有效性。以上结果为高精度固态量子频谱分析仪、高瞬时带宽频谱微波传感器提供了重要技术基础。

彩色叠焦显微颜色空间聚焦评价算法

聚焦评价是叠焦扩展显微景深的关键,为了准确快速地获取叠焦图像序列像素点聚焦位置,生成高质量全聚焦图像,提出了一种基于颜色向量空间的聚焦评价算法。该算法直接在RGB向量空间中计算彩色图像梯度,充分利用了颜色通道间的相关性,避免了传统聚焦评价算法将彩色图转化为灰度图时造成的信息损失,且相较于彩色分量梯度的简单叠加具有更高的准确度;将中心像素与邻域像素在RGB空间的曼哈顿距离均值作为聚焦评价权值,可增强聚焦部分的敏感度,降低离焦部分的评价值,使聚焦评价曲线特性趋向理想化。选取空域、频域和统计学中7种聚焦评价算法与所提算法进行性能对比实验,结果表明:所提算法在仿真图像和真实显微图像中,具有更好的灵敏度、聚焦分辨力和抗噪声能力,曲线特性提升显著,应用于显微镜景深扩展可进一步提升叠焦大景深成像的质量。

双色激光产生太赫兹辐射的调控机理研究

采用双色激光在气体介质中诱导产生太赫兹波的数值模拟方法,深入分析了各种激光参数对双色激光场产生太赫兹波的影响,目的是优化参数以实现太赫兹波辐射能量的最大化。模拟计算表明,双色激光的波长、相对相位、激光脉冲宽度等对瞬态电流和太赫兹能量都有调控作用,并且具有不同的规律性。此外,还通过分析激光电场、电子密度、光电流等相关因素,对双色激光场产生太赫兹波的物理机理进行了解释。该研究为在不同激光激发条件下提高太赫兹波辐射强度,提供了详尽的参数优化分析和理论支持,对后续大幅增强太赫兹辐射效率具有重要意义。