一维多原子分子离子与超短激光场作用产生的高次谐波

从经典方程出发，采用克兰克-尼科尔森中心差分法对一维多原子分子离子与一维超短强激光场相互作用产生的高次谐波进行了数值模拟计算，发现激光场的初始相位对高次谐波辐射谱有明显影响；通过进一步研究，给出了初步解释。从而，可以通过调节适当的激光初始相位和选取适当的系统来改善高次谐波的相位匹配，产生清晰的高次谐波辐射谱。     

强场高次谐波时间特性的理论分析

通过用单电子近似求解含时薛定谔方程,用加窗傅里叶分析的方法,分析了单原子产生的高次谐波的时间特性。理论分析表明,基态粒子数消耗对不同级次谐波辐射的影响程度不一样;谐波辐射随着入射激光场强度的增加会出现饱和;随着谐波级次的增加,波辐射越来越趋向于集中在更短的时间间隔内,相应的线宽胡着谐波级次的增加而增加。     

产生短波长高次谐波条件的优化计算

 从电离的几率方程出发,得到了适用于任意激光脉冲宽度下原子的饱和激光强度和最高次光谐波光子能量与各激光参数的关系表达式,并由此给出了产生高次谐波的优化条件。解决了当超短脉冲激光的脉冲宽度小于皮秒时,产生高次谐波（HHG）的隧道电离理论中的Ammosov－Delone－Krainov方法对产生高次谐波的饱和激光强度以及最高次谐波光子能量的估算不可靠的问题。     

势函数对强激光辐照下原子高次谐波辐射的影响

理论上研究了中红外强激光分别与长程库仑原子和短程势模型原子相互作用产生的高次谐波辐射.发现在相同激光参数条件下,与长程库仑原子的谐波辐射相比,短程原子具有更低的辐射效率,但在高频区域(接近cutoff位置),二者效率相似.通过对谐波辐射的时间频率分析发现,在短程模型原子谐波辐射中,长轨道发挥更重要的作用.利用其产生的高次谐波辐射,可以产生孤立阿秒脉冲.     

基于百太瓦级激光系统驱动的高能量13 nm波段高次谐波产生

利用百太瓦级激光系统在氖气(Ne)中得到基于高次谐波产生的极紫外脉冲。通过松聚焦结构在13 nm波段产生单级次单脉冲能量为13.5 nJ (13.1 nm波长)和11.1 nJ (13.5 nm波长)的高次谐波辐射,转换效率为3.6×10^-7和3.0×10^-7,谐波发散角的半高全宽为0.32 mrad和0.33 mrad。对含时薛定谔方程进行数值求解,得到单原子偶极发射谱,结合麦克斯韦方程组模拟传播效应,同时考虑气体对谐波的吸收效应,理论模拟得到的信号强度随气压和光强的变化趋势与实验结果基本符合。实现相位匹配的谐波光束质量很好,纵向空间分布为高斯型。结合相位匹配条件和空间分布的分析得到了目前激光参数下的最优相位匹配条件。这种基于高次谐波机制的高能量相干极紫外光源在作为自由电子激光的种子光源以及超快非线性实验和半导体工业检测等方面具有广阔的应用前景。     

少周期非均匀场下原子空间位置对高次谐波发射的影响

本文通过数值求解含时薛定谔方程,研究了少周期非均匀激光脉冲作用下氦原子选取不同的空间位置时高次谐波的发射情况.通过对原子在纳米结构空隙中心附近区域不同空间位置上的高次谐波发射情况的研究发现,非均匀激光场中,即使原子处在中心附近的很小的区域内,原子的空间位置也直接影响着高次谐波发射.随着空间内原子坐标的增大,高次谐波的截止得到扩展,且谐波平台区并非单向扩展,而是向两端同时的扩展.通过时频分析和半经典三步模型研究了高次谐波发射的物理机制,并对获得的物理现象给出合理的解释.     

双色场中氦离子高次谐波的保结构算法研究及应用

 通过数值求解含时Schr ö dinger方程，研究了氦离子在不同双色场下的高次谐波。结果显示，尽管倍频光强度仅为基频光的1/10，高次谐波辐射却发生了极大变化。虽然谐波级次推进不多，但效率大大提高了，平台区平均提高103倍，高倍频光对应的高次谐波产生效率提高104~105倍。氦离子在添加倍频光的双色场作用下高次谐波产生效率极大提高的原因是：倍频光极大地加快了电子电离到连续态及返回基态这一过程，使辐射出的高次谐波光子数大大增加，该初步阐释可为高次谐波真正应用到实际中提供一些理论启示。     

高次谐波辐射光子的能量-激光相位关系研究

原子在强激光电场中高次谐波辐射的理论与实验研究是当今科学前沿之一.利用量子力学理论和鞍点方法,细致地研究了高次谐波辐射光子的能量与激光相位的关系.对于时间宽度无限长激光,在一个激光周期内特定相位处产生的高阶辐射(X射线)有特定的能量.能量分布在7180°处成峰,有高斯形函数的对称形状.给出了这种分布的参数化公式.对于不同宽度的飞秒激光,能量分布的成峰位置、最大值和带宽等参数会发生变化.计算表明,三个振荡周期(半高宽)的飞秒激光,当载波-包络相位为175°和105°时,可以分别得到纯净的阿秒单脉冲和双脉 关键词： 超短脉冲激光 高次谐波产生 鞍点方法 能量相位关系     

H2+和H3+系统的电子频谱与高次谐波谱

针对单电子H2+和双电子H3+系统,采用对称分裂算符法求解波函数的时间演化。计算了电子的含时频谱和体系的高次谐波谱,结果表明:电子频谱和高次谐波谱呈互补规律,即电子频谱峰值出现在基波的偶数倍频率位置,高次谐波则出现在奇数位置。对高次谐波谱随软化库仑参数的变化规律进行了数值研究,发现只在软化库仑参数的一定范围内才产生高次谐波,对该现象作了解释。     

双色激光场对氢原子高次谐波的影响

为了深入研究高次谐波产生的多光子过程,使用劈裂算符结合拟谱的方法对氢原子在双色强激光场中的高次谐波辐射作了数值模拟计算,结果显示,当附加一适当双色激光场时,高次谐波谱的平台区大幅提高和展宽.根据计算结果提出的混合跃迁道理论,成功解释了高次谐波谱所呈现的细节特征.     

超晶格量子阱作为光学双稳态器件的相平面特征及其稳定性

辐射强度与量子阱沟道的接受度有关,而接受度的大小则直接取决于量子阱宽度、深度以及电子束流的品质等因素.电子束可以用离子注入机或加速器提供,也可以用电压偏置的方法来获得.利用加速器理论中的相平面分析方法,分析了这种双稳态系统的相平面特征及其稳定性.引入正弦平方势,在经典力学框架内和电压偏置情况下,把粒子运动方程化为具有固定力矩的摆方程,用Jacobian椭圆函数和第一类椭圆积分解析地给出了无扰动系统的解和振动周期,并用数值方法分析了扰动情况下的相平面特征和系统的稳定性.为超晶格量子阱光学双稳态器件的设计提供了基本的理论分析.     

Exact solution for second harmonic generation in XFELs

The generation of harmonic radiation via a non-linear mechanism, driven by electron bunching at the fundamental frequency, is an important option in the operation of high-gain Free-Electron Lasers (FELs). By utilizing harmonic generation at a large scale facility, the production of intense radiation at shorter wavelengths for the same electron beam energy is feasible. This paper describes a theory of second harmonic generation in planar undulators with particular attention to X-Ray FELs (XFELs). Our study is based on an exact analytical solution of Maxwell’s equations, derived with the help of a Green’s function method. Up-to-date theoretical understanding of second harmonic generation is limited to the estimation of the total radiation power, which is based on a comparison of the right hand side of the wave equation for the first harmonic with the right hand side of the equation for the second harmonic, the latter being incorrectly modified. The exact solution should be obtained by solving the wave equation itself. Our work yields correct parametric dependencies and specific predictions of additional properties such as polarization, angular distribution of the radiation intensity and total power. The most surprising prediction is the presence of a vertically polarized part of the second harmonic radiation, whereas current knowledge predicts a horizontally polarized field.     

Collective generation of higher optical harmonics by dipole molecules

An analysis is made of the collective resonant generation of higher harmonics by a spatially extended system of two-level molecules possessing an intrinsic electron dipole moment in the excited state. Frequency and angular dependences of the scattered radiation are studied. It is shown that for moderately small numbers of harmonics their intensity depends comparatively weakly on the number (plateau) and the intensity of the emitted harmonics then falls sharply as the number increases. The angular distribution of the harmonics is strongly anisotropic. It is also shown that collective effects significantly change the time profile of the generated harmonic pulse. In addition, as a result of cooperative effects harmonics of a certain parity are suppressed in the low-frequency part of the radiation spectrum.     

Squeezed harmonic generation in cooperative resonance scattering of intense radiation by dipole molecules

We examine the processes of resonance Raman scattering of intense electromagnetic radiation by a lumped system of two-level atoms with a constant dipole moment. We calculate the intensity of the sth generated harmonic in a saturating electromagnetic field and the statistical characteristics of this harmonic. Finally, we show that the sth harmonic is squeezed at saturating field intensities. Zh. éksp. Teor. Fiz. 113, 1206–1212 (April 1998)     

转动定律实验拓展问题研究

准确地画出了转动系统各物理量间关系与矢量方向;论述了转动惯量实验方程组的建立与求解的完备性;证明了只有传动滑轮质量m0必须非常小时,才会对实验结果影响忽略不计;论证了转动系统平衡条件为满足质心坐标和惯量系数同时为零。     

Dynamics of non-linear optical processes

We study the statistical properties of parametric processes (parametric amplification, frequency conversion, splitting of frequency, etc.) described by the trilinear Hamiltonian and of the second harmonic generation including the lossy mechanism and rotating terms. In a short-time approximation some conservation laws are derived and the existence of the Glauber-Sudarshan weighting function is discussed. It is shown in the second harmonic generation case that good coherence of the incident radiation is lost proportionally to the intensity in interaction while the second harmonic radiation has tendency to be coherent. The effects of the spontaneous emission of the medium and of the physical vacuum are also discussed. In this approximation the statistics is generally described by the superposition of coherent and chaotic fields with correlated components.     

空气中激光等离子体通道的三次谐波光谱特性研究

对不同条件下强激光在空气中形成等离子体通道的三次谐波光谱特性进行了研究。单脉冲能量12 mJ,脉宽30 fs,重复频率10 Hz,中心波长795 nm的飞秒激光脉冲经0.5 m焦距的凹面镜聚焦,在空气中形成了等离子体通道, 并在前 向观测到谱线半峰全宽（FWHM）为15 nm的三次谐波。随着脉冲啁啾的变化,三次谐波的光谱出现红移或兰移,当激光脉冲附带+1.3×105fs2的二阶色散时,三次谐波谱线红移且谱峰强度增长了两倍。同时 ,通过改变可编程声光色散滤波器（AOPDF）光谱调制的位置（Hole position）,三次谐波的光谱也发生频移。     

Efficient second-harmonic generation by scattering from porous gallium phosphide

We experimentally study second-harmonic generation by femtosecond Cr: forsterite-laser radiation scattered on the surface of porous gallium phosphide with characteristic pore sizes and distances between the pores comparable with the second-harmonic wavelength. The intensity of the second-harmonic signal from samples with initial crystallographic surface orientations (110) and (111) is more than an order of magnitude higher than the intensity of the second harmonic generated in reflection from single-crystal gallium phosphide. The efficiency of second-harmonic generation by macroporous gallium phosphide substantially increases as the pump wave-length becomes shorter. The influence of light localization and scattering effects on the enhancement of second-harmonic generation and polarization properties of the second-harmonic is discussed.     

Photon statistics of nonclassical radiation in second harmonic generation

The Fokker-Planck equation approach to the second harmonic generation is adopted to obtain a set of moment equations for this process. This set of equations is truncated in consistence with a model of generalized superposition of coherent fields and quantum noise, which provides equations of motion for unknown quantities in the model. Numerical solution of these equations provides at once the photon number distribution and its factorial moments including fully quantum features of radiation such as its sub-Poisson behaviour and squeezing of vacuum fluctuations. Limits of this approximation are discussed and numerical results are provided.     

低频电磁辐射与脑神经细胞微管的相互作用

研究了低频电磁辐射场与脑神经系统的相互作用规律及其机理。电磁辐射为非电离低频电磁场, 将脑神经细胞骨架微管（MT）中的两态物理系统进行量子化,用密度矩阵描述脑神经系统中信息位的状态, 建立并求解脑神经系统中信息位的动力学方程。 结果表明: 当非电离低频电磁辐射射向大脑时, 脑神经系统中信息位的密度矩阵非对角元在任意时刻都不为零, 能够保持较好的量子相干性, 脑神经系统的功能不会受到破坏。