测量液体非线性参量的光声方法

本文描述了一种用光声脉冲测量液体非线性参量Ｂ／Ａ的新方法。用聚焦脉冲激光束通过光击穿机制在液体中激发光声脉冲。在测量距离范围内，光声脉冲声压幅值随传播距离的变化接近球面扩展衰减规律。根据光声脉冲传播一定距离后前沿上升时间的变化，测定了液体的Ｂ／Ａ。和有限束宽的平面脉冲声波相比，用球面脉冲声波可以避免衍射的影响。用此法测量了水的非线性参量，得到了满意的结果。     

钕玻璃脉冲激光击穿陶瓷片时产生的电信号

聚焦普通脉冲钕玻璃激光于未电极化的锆钛酸系(PZT)陶瓷薄片上,在激光击穿样品之前,样品两表面银电极间无电信号出现.然而,一旦样品被光击穿,电极间就会出现脉冲电信号.在电极接线保持不变的情况下,光从前后两表面入射而击穿样品时,所产生的电信号极性正相反.在穿孔较小时所产生的电信号极性是逆光方向,但在穿孔大到适当值时信号极性变为顺光方向.     

0.18 μm部分耗尽绝缘体上硅互补金属氧化物半导体电路单粒子瞬态特性研究

利用脉冲激光入射技术研究100级0.18 μm部分耗尽绝缘体上硅互补金属氧化物半导体反相器链的单粒子瞬态效应, 分析了激光入射器件类型及入射位置对单粒子瞬态脉冲传输特性的影响. 实验结果表明, 单粒子瞬态脉冲在反相器链中的传输与激光入射位置有关, 当激光入射第100级到第2级的n型金属-氧化物-半导体器件, 得到的脉冲宽度从287.4 ps增加到427.5 ps; 当激光入射第99级到第1级的p型金属-氧化物-半导体器件, 得到的脉冲宽度从150.5 ps增加到295.9 ps. 激光入射点靠近输出则得到的瞬态波形窄; 靠近输入则得到的瞬态波形较宽, 单粒子瞬态脉冲随着反相器链的传输而展宽. 入射器件的类型对单粒子瞬态脉冲展宽无影响. 通过理论分析得到, 部分耗尽绝缘体上硅器件浮体效应导致的阈值电压迟滞是反相器链单粒子瞬态脉冲展宽的主要原因. 而示波器观察到的与预期结果幅值相反的正输出脉冲, 是输出节点电容充放电的结果.     

c向切割掺杂LiNbO3晶体中的光耦合

实验研究了三种c向切割的不同掺杂的LiNbO3：Fe晶体中入射光与反射光的耦合情况.发现沿-c轴方向入射时透射光与反射光强的准周期振荡行为是光生伏打机制占优势的瞬态能量转移和扩散机制引起的相反方向的能量转移所致，不是光生伏打场的表面击穿引起的.当入射光沿+c方向入射时，由于两种机制引起的能量转移方向相同，因而不会产生这种振荡. 关键词： 准周期振荡 反射光栅 瞬态能量转移 扩散机制     

厚非线性介质瞬态热光非线性效应的研究

瞬态热光非线性效应是指由脉冲宽度可以和非线性效应的建立时间相比拟的纳秒脉冲激光入射产生的热光非线性效应.用Zscan法测量了不同厚度、不同束腰半径碘的酒精溶液的瞬态热光非线性效应曲线.从声波的传播方程和光波的非线性传播方程出发,用数值计算方法模拟了这一非线性过程.并对实验结果和数值模拟结果进行了比较 关键词： 瞬态热光非线性效应 非线性光学 数值计算     

超短脉冲激光与微小水滴相互作用中电子密度和光场的时空分布

为了研究超短激光脉冲和液滴相互作用过程中电子密度和光场的变化,基于非线性麦克斯韦方程组和电离速率方程,构建了激光等离子体非线性瞬态时域耦合模型,对飞秒激光脉冲击穿微米量级水滴时的电子密度和光场的时空分布进行了计算.结果显示水滴的击穿阈值最小可达2 TW/cm~2,为同等条件下无边界水介质击穿阈值的1/4.随着脉冲能量增强,水滴内自由电子密度峰值区域逆着激光入射方向移动,且入射光越强,水滴对光传播的屏蔽越明显.光束在水滴出射端外部汇聚,汇聚点的光功率密度可达入射光的5倍,且时域波形出现压缩和变形.另外,水滴对激光能量的吸收系数随光强增大而增大,并最终趋于饱和.     

二维平行桨涡干扰气动噪声特性分析

深入理解桨-涡干扰脉冲噪声特性对其噪声的控制具有重要意义。采用气动噪声直接法对低马赫数条件下的二维平行桨-涡干扰气动噪声进行了数值计算,分析了噪声的产生机理和传播、衰减规律。结果表明:当旋涡接近和经过翼型前缘时,翼型前缘附近压强发生强度不同的两次突变,导致翼型气动力变化的同时,向外辐射产生具有偶极子指向性的脉冲声波,其中较弱的一次压强突变能更有效率地辐射声波;通过对4种不同来流速度的声场进行分析,发现上、下远场声压峰值和传播距离成反比,和来流速度的三次方与升力系数波动幅值之积成正比,由此得出了远场声压峰值计算公式,为桨-涡干扰远场声压的预测提供了另一种途径。      

透射型光折变体全息光栅对超短脉冲激光光束衍射的特性

在Kogelnik耦合波理论的基础上,考虑光栅记录介质的色散效应的影响,研究了光折变体全息光栅对不同偏振状态的超短脉冲激光光束衍射的性质,讨论了高斯型入射脉冲激光光束的谱宽与光栅的有效衍射谱宽之比不同时,衍射和透射光束的光谱宽度、时间宽度、波形和衍射效率的变化。结果表明,光栅的有效衍射谱宽受光栅参量及入射条件的影响,对衍射性质的影响很大,且在考虑光栅记录介质的色散效应时减小。当入射脉冲的偏振方向垂直于入射面时,光栅的有效衍射谱宽大于偏振方向平行于入射面的情形,衍射效率在入射脉冲宽度较大时小于偏振方向平行于入射面的情形;谱宽比较大时,衍射光束的时间分布曲线产生展宽和变形,且比偏振方向平行于入射面的情形展宽和变形得更加明显。     

反射型体全息光栅对超短脉冲激光光束衍射的性质

在光栅记录介质色散效应的影响下,拓展Kogelnik的耦合波理论研究了反射型体全息光栅对偏振方向垂直和平行于入射面的超短脉冲激光光束衍射的性质。结果表明对于记录在LiNbO3光折变晶体中的体全息光栅,在色散效应、光栅参量和入射条件的共同影响下,光栅对偏振方向垂直于入射面的超短脉冲激光光束衍射的光谱带宽大于对偏振方向平行于入射面的超短脉冲激光光束衍射的光谱带宽,在不考虑光栅介质色散效应的影响时,它们都变大。进一步给出了衍射光的光谱带宽及光栅的衍射效率随入射脉冲的光谱带宽与光栅的光谱带宽比值的变化情况。     

碳纳米管悬浮液的光限幅特性实验研究

 通过超声波降解法制备了多壁碳纳米管的水-表面活性剂悬浮液，测量了其对于1 064 nm，脉宽10 ns Nd:YAG调Q脉冲激光的光限幅曲线。实验发现：入射激光能量密度较低时，出射能量密度随入射能量密度的增加而线性增加；当入射能量密度为160 mJ/cm²时，出射能量不再线性增加并且逐渐趋近于光限幅器的嵌位输出值，约16 mJ，同时，对激光的透过率从71%下降到15%。通过Z扫描和探针光实验以及45°散射角下散射能量、散射率随入射激光能量变化曲线的测量，对碳纳米管悬浮液的光限幅机理进行了研究。结果表明：其限幅机理可能源于碳纳米管吸收激光能量后升华产生的膨胀的碳气泡对入射激光产生的非线性散射；另外，非线性折射对光限幅效果也有一定的作用。     

激光引致激波演化数值研究

在高重复频率激光推进的研究中,激波的合并是发生在激波演化后期的,同时由于脉冲间隔短,脉冲宽度对流场演化的影响也需要详细研究。考虑了激光辐照过程对流场演化的影响,通过数值计算对激波演化特性进行了研究。结果表明,初期波阵面的椭球形状逐渐转化为一个球形,球心与击穿点的距离随时间逐渐减小并最终趋于稳定。基于激波合并的应用,当激波马赫数在1~2之间时,给出了激波波阵面半径随时间的变化规律,以及激波高压区长度和波峰压强随激波波阵面半径变化规律的经验公式。     

Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Algorithm

The anomalous wave propagation imaging (AWPI) method is proposed for the laser ultrasonic propagation imaging system using a Q-switched laser and a laser mirror scanner to highlight the anomalies in complex structures. The AWPI algorithm was developed based on the observation that the waves from two adjacent scanning points are very similar, and that the propagation direction of the incident wave is different from that of the anomalous wave caused by structural anomalies including damage. The structural anomaly is highlighted by suppressing the incident waves and exaggerating the anomalous wave through adjacent waves subtraction after arrival time and amplitude matching. The variable time window amplitude mapping (VTWAM) method was also developed, based on the difference in arrival time between the residual incident wave and anomalous waves. The VTWAM method enhances anomaly visualization and sizing, notably for composite damages, by mapping the amplitudes of the confining wave within the damage. Our results showed that the AWPI increased the signal-to-noise ratio of a back-surface hole damage in a steel plate by 13.76 dB, while in another inspection of a composite wing with two impact damages, the AWPI results enhanced by the VTWAM compared favorably with the results of the immersion ultrasonic C-scan. The AWPI and VTWAM adopt implicit spatial referencing wherein all necessary data can be obtained through a single-time scan, therefore circumvent the disadvantages of conventional temporal baseline referencing.     

Reflection–refraction of attenuated waves at the interface between a thermo-poroelastic medium and a thermoelastic medium

Phenomenon of reflection and refraction is considered at the plane interface between a thermoelastic medium and thermo-poroelastic medium. Both the media are isotropic and behave dissipative to wave propagation. Incident wave in thermo-poroelastic medium is considered inhomogeneous with deviation allowed between the directions of propagation and maximum attenuation. For this incidence, four attenuated waves reflect back in thermo-poroelastic medium and three waves refract to the continuing thermoelastic medium. Each of these reflected/refracted waves is inhomogeneous and propagates with a phase shift. The propagation characteristics (velocity, attenuation, inhomogeneity, phase shift, amplitude, energy) of reflected and refracted waves are calculated as functions of propagation direction and inhomogeneity of the incident wave. Variations in these propagation characteristics with the incident direction are illustrated through a numerical example.     

Coherent control of THz wave generation in ambient air

Our study of THz wave generation in the pulsed laser induced air plasma with individually controlled phase, polarization, and amplitude of the optical fundamental wave (omega) and its second harmonic (2omega) indicates that the third-order nonlinear optical process mixing the omega and 2omega beams in the ionized plasma is the main mechanism of the efficient THz wave generation. The polarity and the strength of the emitted THz field are completely controlled by the relative phase between the omega and 2omega waves. The measured THz field amplitude is proportional to the pulse energy of the fundamental beam and to the square root of the pulse energy of the second-harmonic beam once the total optical pulse energy exceeds the plasma formation threshold. The optimal THz field is achieved when all waves (omega, 2omega, and THz waves) are at the same polarization in the four-wave-mixing process.     

Edge wave on axis behind an aperture or disk having a ragged edge

Diffraction by a circular aperture or disk having a ragged edge is investigated. Theory and measurements are reported. The ragged edge is modeled as N arcs, of differing radii a(i), each of which contributes a scattered signal to the edge wave on axis behind the aperture or disk. The amplitude of each scattered signal is proportional to the angle of the arc, and the corresponding time delay is square root of [(ai)2 + (s0)2/c0], where s0 is the axial distance from the aperture plane and c0 is the sound speed. Kirchhoff theory is used to make the calculation. A formula is derived for the rms pressure of the edge wave in terms of the rms pressure and autocorrelation function of the incident wave. The formula can be evaluated for incident waves that are sinusoidal, random (e.g., noise), or transient. Predictions agree reasonably well with underwater measurements made with a spark-generated pulse incident on various apertures. The main result is that making the edge ragged reduces the rms pressure of the edge wave. Indeed, an edge profile is presented that, for a given frequency and axial observation point, eliminates the edge wave completely.     

Features of the scattering of focused terahertz radiation from the probe of a terahertz near-field microscope

The scattering of pulsed terahertz radiation from metallic probes in the form of thin cylinders and cones with a small opening angle, which are used in apertureless terahertz near-field microscopes, has been investigated. The extrema of the waveform of pulsed terahertz radiation scattered from a free probe are linearly shifted with a change in the vertical position of the probe, and the spectral distribution is characterized by an inversely proportional frequency dependence. In the presence of a reflecting surface under the probe, when new excitation and detection directions appear, the spectrum of scattered terahertz radiation does not differ from the spectrum of the incident radiation. The experimental data are in mutual agreement with the theoretical results obtained within the model of the generation of diffraction edge waves at the interface of inhomogeneous excitation between the excitation region and shadow region.     

二维三组元声子晶体中的半狄拉克点及奇异特性

设计了一种二维三组元声子晶体结构, 利用偶然简并的方法在布里渊区中心实现了半狄拉克点, 探究了随着组元几何参数的改变半狄拉克点的演变过程. 利用有限元方法研究发现在半狄拉克点频率附近沿着ΓX 方向声子晶体表现出与零折射率材料相似的行为, 许多奇异特性如单向透射等均可以观察到. 另外还发现在半狄拉克点频率附近声子晶体是各向异性的, 沿着不同方向声波的传播现象是不同的, 这种特性是狄拉克点及类狄拉克点所不具备的. 这种各向异性的声波传播特性有许多重要的应用, 如单方向完美透射和单方向波前整形等.     

远距离探测拉曼光谱特性

为了发展远距离探测未知或危险物质的方法, 设计并建立了近同轴可见光远距离拉曼光谱探测实验装置, 对硝酸盐固体样品进行了距离为2-10 m的拉曼光谱测量, 初步研究了拉曼信号强度与激发光功率、探测距离、样品浓度及样品表面方向之间的关系. 实验观察到三种硝酸盐在1050 cm^-1附近的拉曼谱线, 其微小的差异可作为识别特征. 硝酸铵的特征拉曼谱线强度正比于激发光功率, 近似平方关系; 与探测距离之间趋向于二次反比关系; 与样品浓度接近指数关系; 与样品表面朝向有近似余弦函数的关系.     

Traveling waves in a nonlinear medium with cubic nonlinearity

Unlike linear nondispersive media, which allow propagation of wave packets of arbitrary forms, nonlinear media admit only certain profiles of traveling waves. Here we examine media with Duffing oscillators, i.e., with bound electrons for which an equilibrium disturbance causes forces proportional to the first and third powers of deviation. We show that the linearly polarized traveling plane waves such media can transmit have profiles modulated as Jacobi elliptic functions. When discussing propagation across an interface between different media, only incidence from the side of the linear medium is considered. Even in this case, to launch a traveling wave in the nonlinear medium, a severe restriction must be imposed on the incident wave’s amplitude.