基于群速度失配的超声兰姆波二次谐波的时域测量方法

考虑到发射和接收换能器对超声兰姆波时域二次谐波信号所带来的不可避免的影响,提出一种基于基频与二倍频兰姆波群速度失配的超声兰姆波二次谐波的时域测量方法。当基频与二倍频超声兰姆波的相速度匹配而群速度失配时,在超声兰姆波传播过程中所发生的二次谐波信号,在时域上可与源于斜劈换能器的二次谐波信号相分离。采用仅源自于基频兰姆波的时域二次谐波的积分振幅,定量描述兰姆波二次谐波的发生效率。以铝板中传播的兰姆波为例,给出了时域二次谐波的具体测量过程。本文提出的测量方法放宽了超声兰姆波二次谐波的测量条件,且扣除了换能器对二次谐波信号所带来的影响,所测得的二次谐波信号完全来自于基频兰姆波时域信号的二次谐波发生效应。      

高次谐波发射的亚原子尺度研究

激光与原子、分子相互作用的高次谐波是产生超短阿秒脉冲和相干高频XUV光源的重要手段之一.为了产生高强度的XUV光源,需要对谐波产生机制深入研究.本文通过数值求解含时薛定谔方程,计算了不同空间位置的含时偶极矩进而得到不同空间位置的高次谐波发射.对不同空间位置的谐波发射谱的分析发现,谐波发射的主要空间位置在核区附近,不同空间位置的谐波中奇次和偶次谐波均能被观察到,整数阶谐波能量辐射强度较大.进一步研究不同空间位置的谐波相位发现,在x=0左右两侧发射的奇次谐波相位相同,偶次谐波相位相反.通过滤波方法分析了不同空间位置的相同次谐波的含时偶极矩信息,发现该相位特征导致了奇次谐波的增强,偶次谐波的消失.     

超谐波声场及其生物组织成像的理论及实验研究

理论及实验研究了平面活塞换能器激发的超谐波(三次、四次及五次谐波的线性幅度叠加)的轴向和径向声场特性,讨论了声源强度及传播距离对超谐波的强度及指向性的影响。结果表明在合适的声源强度及传播距离条件下,超谐波的幅度可大于二次谐波,同时-3dB波束宽度变窄,为基波和二次谐波的59%和77%。对生物组织进行了超谐波成像,并与基波、二次谐波图像对比。结果表明超谐波可提高超声非线性成像的灵敏度及分辨率。     

双波长双脉冲激光三次谐波的产生方法

提出双波长双脉冲激光产生三次谐波的方法。波长不同的两脉冲激光束由反射使它们合拢后通过两块非线性的BBO晶体 ,激光脉冲第一次通过BBO晶体产生两波长的二次谐波 (SHG) ,它们由各自的反射镜反射再次通过BBO又产生了二次谐波 ,这两次产生的二次谐波和基波通过用于产生三次谐波的BBO晶体可产生双波长的三次谐波 ,它们由 45°斜置的耦合镜输出。文中分析了满足二次谐波和三次谐波的四个相位匹配的条件 ,该方法也可用于腔内双波长双脉冲的三次谐波激光的产生 ,给出了相应的实验结果。     

核运动对非奇次谐波产生的影响

理论研究了H2+、D2+、T2+辐射高次谐波的特点.结果表明,在核运动影响下,不仅奇次谐波呈现红移现象,而且谐波光谱呈现非奇次谐波.随着核质量增大,奇次谐波频移和非几次谐波强度都减弱.但是随着激脉宽增大,非奇次谐波的产生明显被增强.理论分析表明,谐波频移是由谐波辐射在激光上升和下降区间的非对称性导致的;非奇次谐波是由于分子对称性在较大核间距离处遭破坏产生的.     

双折射吸收非线性介质薄膜中倍频的产生

从二次谐波电场满足的波动方程出发,考虑到介质对二次谐波的吸收以及把完全边界条件应用于基频波和二次谐波在介质的入射面和出射面之间的反射效应,推导出了单光轴非线性薄膜介质中的二次谐波输出功率的计算式.结果表明：当采用复折射率的概念时,可以把非线性无吸收介质的二次谐波输出功率计算式应用到非线性吸收介质二次谐波输出功率计算式中去. 关键词： 非线性 倍频 二次谐波功率     

自生磁场对相对论谐波辐射的影响

分析了自生磁场对相对论谐波辐射的影响，得出结论，自生磁场对强激光在欠稠密等离子 产生的相对论相干波辐射有重要作用，自生磁场激发偶次谐波辐射，并对奇次谐波辐射产生影响，对二次，三次谐波作了详细分析，发现，自生磁场激发二次谐波辐射，而对三次谐波辐射有削弱作用，并且它还使谐波的失相时间延长。     

谐波回旋速调管的效率优化

推导出了由两个无量纲量群聚系数与相位系数构成的非线性运动方程,并对其进行了数值计算分析。数值计算结果表明:通过优化设计参数,当谐波回旋速调管工作在放大区时,其基波、二次谐波、三次谐波、四次谐波的纵向互作用效率分别可达到55%,40%,30%,15%;当其工作在振荡区时,其基波、二次谐波、三次谐波、四次谐波的纵向互作用效率最高分别可达到93.9%, 88.2%, 81.8%, 62.7%。     

正交偏振双色激光场作用下高次谐波的椭偏特性研究

通过数值求解二维含时薛定谔方程，理论研究了正交偏振双色激光场作用下H2+分子高次谐波发射和孤立阿秒脉冲的产生. 数值结果显示，改变激光场的强度比，可以调控高次谐波强度以及谐波椭偏率. 结果表明随着场强比的增加，y方向的谐波强度不断提高. 当场强比为1:2.5时，x方向和y方向分别仅有奇次谐波和偶次谐波产生，并且偶次谐波的强度比奇次谐波强度高2-3个数量级. 此时，奇次谐波的椭偏率最大可以达到0.3，偶次谐波的椭偏率接近于0. 此外，通过改变分子的准直角可以控制谐波的椭偏率，基于这一现象，利用椭圆偏振高次谐波的平台区合成了椭偏率为0.05的椭圆偏振阿秒脉冲链.     

兰姆波的非线性研究（Ⅱ）

当兰姆波相速等于板材纵波声速时,兰姆波的二次谐波表现出空间积累性质,本文绘出了该情形下兰姆波二次谐波的解析表达式,并对几个阶次的兰姆波二次谐波作了数值计算。对于S_0、A_0模式,二次谐波一定不具有空间积累性质。     

Gluon cascades and amplitudes in light-front perturbation theory

We construct the gluon wave functions, fragmentation functions and scattering amplitudes within the light-front perturbation theory. Recursion relations on the light-front are constructed for the wave functions and fragmentation functions, which in the latter case are the light-front analogs of the Berends–Giele recursion relations. Using general relations between wave functions and scattering amplitudes it is demonstrated how to obtain the maximally-helicity violating amplitudes, and explicit verification of the results is based on simple examples.     

Siebenmann-type cobordisms with borders and topology changes by quantum tunneling

Siebenmann-type cobordisms are constructed to describe topology changes with the Seifert fibered homology spheres in in- and out-states. We study the problem of determining of topology-changing amplitudes for these quantum tunneling processes. The calculations are performed in the stationary phase approximation for Kodama wave functions. In this approximation the amplitudes are expressed in terms of Chern-Simons invariants of flatSU(2)-connections over the cobordism boundary components. The topology-change amplitudes found are factorized into the Kodama wave functions for the lens spaces. The results are compared with those for Fintushel-Stern-type cobordisms which have been previously investigated.     

Observation of the nonlinear dispersion relation and spatial statistics of wave turbulence on the surface of a fluid

We report experiments on gravity-capillary wave turbulence on the surface of a fluid. The wave amplitudes are measured simultaneously in time and space by using an optical method. The full space-time power spectrum shows that the wave energy is localized on several branches in the wave-vector-frequency space. The number of branches depends on the power injected within the waves. The measurement of the nonlinear dispersion relation is found to be well described by a law suggesting that the energy transfer mechanisms involved in wave turbulence are restricted not only to purely resonant interaction between nonlinear waves. The power-law scaling of the spatial spectrum and the probability distribution of the wave amplitudes at a given wave number are also measured and compared to the theoretical predictions.     

Development of ultrahigh-precision coherent control and its applications

Coherent control is based on optical manipulation of the amplitudes and phases of wave functions. It is expected to be a key technique to develop novel quantum technologies such as bond-selective chemistry and quantum computing, and to better understand the quantum worldview founded on wave-particle duality. We have developed high-precision coherent control by imprinting optical amplitudes and phases of ultrashort laser pulses on the quantum amplitudes and phases of molecular wave functions. The history and perspective of coherent control and our recent achievements are described.     

Gravity wave turbulence in a laboratory flume

We present an experimental study of the statistics of surface gravity wave turbulence in a flume of a horizontal size 12 x 6 m. For a wide range of amplitudes the wave energy spectrum was found to scale as Eomega approximately omega(-nu) in a frequency range of up to one decade. However, nu appears to be nonuniversal: it depends on the wave intensity and ranges from about 6 to 4. We discuss our results in the context of existing theories and argue that at low wave amplitudes the wave statistics is affected by the flume finite size, and at high amplitudes the wave breaking effect dominates.     

Wannier-stark ladders in one-dimensional elastic systems

The optical analogues of Bloch oscillations and their associated Wannier-Stark ladders have been recently analyzed. In this Letter we propose an elastic realization of these ladders, employing for this purpose the torsional vibrations of specially designed one-dimensional elastic systems. We have measured, for the first time, the ladder wave amplitudes, which are not directly accessible either in the quantum-mechanical or optical cases. The wave amplitudes are spatially localized and coincide rather well with theoretically predicted amplitudes. The rods we analyze can be used to localize different frequencies in different parts of the elastic systems and vice versa.     

Stimulus dependencies of the gerbil brain-stem auditory-evoked response (BAER). I: Effects of click level, rate, and polarity

Three experiments evaluating the effects of various stimulus manipulations on the click-evoked gerbil brain-stem auditory-evoked response (BAER) are reported. In experiment 1, click polarity and level were covaried. With increasing click level, there is a parallel decrease in the latency of the first five BAER peaks (i-v) and an increase in BAER peak amplitudes. Mean wave i amplitude was greater for rarefaction than condensation clicks at high click levels; mean wave v amplitude was greater for condensation clicks at higher click levels. Experiment 2 covaried click rate and polarity. The latency of the BAER peaks increased with increasing click repetition rate. This rate-dependent latency increase was greater for the later BAER peaks, resulting in an increase in the i-v interval with increasing click rate. As rate increased, the amplitudes of waves i and v decreased monotonically, whereas the amplitudes of waves ii-iv were largely uninfluenced by click rate. As in experiment 1, mean wave i amplitude was greater for rarefaction clicks, whereas mean wave v amplitude was greater for condensation clicks. The magnitude of these polarity dependencies on waves i and v amplitude decreased with increasing click rate. Experiment 3 evaluated the effects of click polarity on BAERs to high-intensity (100 dB pSPL) clicks presented at a rate of 10 Hz. In eight of ten gerbils evaluated, wave i amplitude was greater to rarefaction clicks, and, in all ten animals, wave v amplitude was greater to condensation clicks. The effects of click level and rate on BAER peak amplitudes, latencies, and interwave intervals are reminiscent of stimulus dependencies reported for the human BAER. The effects of click polarity on the amplitudes of waves i and v of the gerbil BAER have also been reported for the human BAER.     

Acoustic waves in random fields

The effect of space- and time-dependent random mass density, velocity, and pressure fields on frequencies and amplitudes of acoustic waves is considered by means of the analytical perturbative method. The analytical results, which are valid for weak fluctuations and long wavelength sound waves, reveal frequency and amplitude alteration, the effect of which depends on the type of random field. In particular, the effect of a random mass density field is to increase wave frequencies. Space-dependent random velocity and pressure fields reduce wave frequencies. While space-dependent random fields attenuate wave amplitudes, their time-dependent counterparts lead to wave amplification. In another example, sound waves that are trapped in the vertical direction but are free to propagate horizontally are affected by a space-dependent random mass density field. This effect depends on the direction along which the field is varying. A random field, which varies along the horizontal direction, does not couple vertically standing modes but increases their frequencies and attenuates amplitudes. These modes are coupled by a random field which depends on the vertical coordinate, but the dispersion relation remains the same as in the case of the deterministic medium.     

A possible approach to breakup in nuclear reaction calculations

It is proposed to use the multi-channel Kato identity for estimating breakup amplitudes when an approximate scattering wave function is known from a variational calculation. Lines of connection are drawn with more familiar expressions for the breakup amplitudes.     

关于π-π作用

本文从下面拉格朗日函数出发,用Tamm-Dancoff近似计算π-π的P波和S波散射。P波的结果和Frazer-Fulco的解相同,但S波的共振行为也相当复杂。