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.     

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。     

二次谐波共焦成像的分辨率

通过与双光子荧光共焦成像过程比较,研究了非线性二次谐波的强度脉冲响应函数并对它进行频域分析,获得了横向、纵向截止频率和通频带.研究结果表明：二次谐波共焦成像具有更高的分辨率.     

基于开口环阵列结构的表面晶格共振产生及二次谐波增强

理论研究了二维周期排列的金开口环谐振器的磁共振模式与周期阵列的衍射模式发生强耦合所需满足的条件及其对二次谐波产生效率的影响.通过控制阵列结构在x和y方向的周期大小,使得衍射模式只在其中一个方向产生,当衍射模式的电场方向与入射光电场偏振方向一致时,衍射模式才会与开口环谐振器的磁共振模式发生强耦合作用,产生表面晶格共振进而实现近场场增强.在此基础上,进一步计算了金开口环谐振器阵列的二次谐波产生效率,随着阵列周期逐渐增大,即开口环谐振器的数密度减小,二次谐波强度呈现先增加后降低的趋势,当开口环谐振器数密度降为原来的1/4左右时,二次谐波强度可以增强2倍以上.本文的研究为金属超表面二次谐波产生效率的提高提供了一种新的可能途径.     

全硅光子晶体波导中二次谐波产生及影响因素

硅材料是半导体微细加工工艺中的常用材料,属于m3m对称点群,通常无法实现二次谐波产生,导致光子系统芯片中的非线性组件集成困难.提出一种在硅材料中可以实现二次谐波产生光子晶体波导结构.首先给出该波导结构的组成及其基本原理,然后讨论谐波产生的计算模型和计算方法,最后给出针对10.6 μm波长而设计的全硅二维光子晶体波导具体结构参数,以有限时域差分算法为基础,计算分析了谐波产生情况.研究结果表明:该结构利用光子晶体带隙边缘效应增强了硅材料的电四极极化强度从而实现二次谐波产生,在完全相位匹配条件下,当抽运波强度为1.3 MW/mm2时,转换效率为0.2%.最后,对影响谐波转换效率的因素进行了初步分析.     

1.053μm激光打靶产生的二次谐波

测量了基频激光打靶条件下不同角度的二次谐波光谱,得到了谐波光强随发射角度的变化关系以及能量转换效率随激光强度变化的曲线.实验现象可能与参量衰变级联造成的Langmuir湍流有关 关键词： 二次谐波发射 激光等离子体 共振吸收 离子声衰变不稳定性     

直流电场诱导LB膜中分子重新取向研究

用旋转二次谐波产生方法研究了半花菁/花生酸Y型交替LB多层膜中由外加法向直流电场诱导的分子的重新取向.外加法向直流电场能够有效地改变LB膜中光学活性半花菁分子的取向,迫使其长轴转向电场的方向.随着极化电场的增大,分子在基板平面内的各向异性逐渐减小,而二次谐波强度的增量则随外加电场的增大而呈现平方增长的规律.在室温下,用1.3×10⁶V/m的电场极化10min,可以使半花菁LB多层膜的二次谐波强度增大一个量级.     

基于二次谐波产生技术的BaTiO3薄膜对称性研究

光学二次谐波产生技术是研究材料极化特性和对称性的重要工具,利用搭建的显微二次谐波产生光路,对生长在SrTiO_3衬底上的BaTiO_3薄膜样品进行了二次谐波产生测试。结果表明,BaTiO_3薄膜在面内方向具有二重旋转对称性。在四种偏振模式下测量了样品的二次谐波信号,结果表明,随着基频光入射功率的变化,二次谐波强度的图样也发生了变化。当入射光功率为7.5～7.7 mW时,信号发生突变;当入射光功率大于7.5 mW时,水平偏振出射光的二次谐波信号图样主峰减弱,次峰增强。     

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     

非平行偏振双色场驱动产生脉宽稳定的单个宽谱阿秒脉冲

提出一种直接得到脉宽稳定的单个宽谱阿秒脉冲的新方法.利用波长为800 nm脉宽为5 fs的超短脉冲叠加上偏振方向与主脉冲成π/3,脉宽同样为5 fs的二次谐波脉冲驱动氦原子,可以得到宽度达到50 eV的超连续谱.当二次谐波的强度大于10¹⁴ W/cm²时,超连续谱的位置以及谱宽几乎不会随着二次谐波脉冲的强度的改变而改变.对85—125 eV的超连续谱进行滤波可以直接得到100 as左右的单个阿秒脉冲,这个性质对于实验上获得单个宽谱阿秒脉冲而言是非常有利的. 关键词： 阿秒脉冲 超连续谱     

Selective corneal imaging using combined second-harmonic generation and two-photon excited fluorescence

A multiphoton microscope employing second-harmonic generation (SHG) and two-photon excited fluorescence (TPF) is used for high-resolution ex vivo imaging of rabbit cornea in a backscattering geometry. Endogenous TPF and SHG signals from corneal cells and extracellular matrix, respectively, are clearly visible without exogenous dyes. Spectral characterization of these upconverted signals provides confirmation of the structural origin of both TPF and SHG, and spectral imaging facilitates the separation of keratocyte and epithelial cells from the collagen-rich corneal stroma. The polarization dependence of collagen SHG is used to highlight fiber orientation, and three-dimensional SHG tomography reveals that approximately 88% of the stromal volume is occupied by collagen lamellae.     

Transform-limited spectral compression due to self-phase modulation in fibers

We demonstrate near-transform-limited pulse generation through spectral compression arising from nonlinear propagation of negatively chirped pulses in optical fiber. The output pulse intensity and phase were quantified by use of second-harmonic generation frequency-resolved optical gating. Spectral compression from 8.4 to 2.4 nm was obtained. Furthermore, the phase of the spectrally compressed pulse was found to be constant over the spectral and temporal envelopes, which is indicative of a transform-limited pulse. Good agreement was found between the experimental results and numerical pulse-propagation studies.     

Tomographic Imaging of Collagen Fiber Orientation in Human Tissue Using Depth-Resolved Polarimetry of Second-Harmonic-Generation Light

We propose a nonlinear optical probe method to image the distribution of collagen fiber orientation in human tissue by measuring the polarization of collagen-induced second-harmonic-generation (SHG) light (SHG polarimetry). Depth-resolved SHG polarimetry, with a depth resolution of 14 μm, was used to evaluate the cross-sectional profile of collagen fiber orientation in Achilles tendon and dentin, revealing a characteristic distribution of collagen orientation along the depth direction. We evaluated the two-dimensional (2D) lateral distribution of collagen fiber orientation in back reticular dermis and anklebone by polarization-resolved SHG imaging, and confirmed an appreciable difference in the distribution profiles of the two samples. We further extended the method to a depth-resolved measurement of the three-dimensional (3D) distribution of collagen orientation in anklebone. The proposed system promises to be a powerful tool for in vivo measurement of collagen fiber orientation in human tissue.     

Monitoring the thermally induced structural transitions of collagen by use of second-harmonic generation microscopy

The thermal disruption of collagen I in rat tail tendon is investigated with second-harmonic generation (SHG) microscopy. We investigate its effects on SHG images and intensity in the temperature range 25 degrees-60 degrees C. We find that the SHG signal decreases rapidly starting at 45 degrees C. However, SHG imaging reveals that breakage of collagen fibers is not evident until 57 degrees C and worsens with increasing temperature. At 57 degrees C, structures of both molten and fibrous collagen exist, and the disruption of collagen appears to be complete at 60 degrees C. Our results suggest that, in addition to intensity measurement, SHG imaging is necessary for monitoring details of thermally induced changes in collagen structures in biomedical applications.     

Optical second-harmonic generation in ferroelectric nanowires

Optical second-harmonic generation in ferroelectric nanowires of potassium dihydrogen phosphate (KDP) and sodium nitrite embedded into chrysotile-asbestos nanotubes from a melt or solution has been studied. The second-harmonic radiation appears to be polarized, and its intensity is unexpectedly high in comparison with bulk materials. Analysis of the polarization characteristics of the observed signal reveals the possibility of the formation of ferroelectric nanocrystals having the same orientation along the matrix nanotubes and randomly orientated in the plane perpendicular to this direction.     

Dynamic properties of second-harmonic intensity of a nanosecond-order laser pulse

Numerical and experimental results on the dynamic properties of the second-harmonic generation of Q-switched nanosecond-order laser pulses are described by reexamining the influence of the imperfect phase-matching condition. For a type-I noncritical phase-matching scheme, the fluctuation in the second-harmonic intensity owing to the change in environmental temperature can be effectively avoided by using the conversion region in the second-harmonic generation. In addition, the instability of the second-harmonic intensity owing to the variation in the fundamental intensity might be suppressed by introducing an optimized phase-mismatching condition to the reconversion region if the fundamental laser pulse has a flat-top profile. These approaches are expected to be applied for the peak-intensity stabilization of the pump laser used in optical parametric chirped pulse amplification.     

Nondestructive evaluation of incipient corrosion in a metal beneath paint by second-harmonic tomography

A second-harmonic optical scanning imaging method for nondestructive evaluation of corrosion of painted metals is demonstrated. Two-dimensional images of the sectional structure from a sample of painted metal with corrosion were obtained by detection of second-harmonic generation (SHG). The second-harmonic signals generated from paint, corrosion, and metal can be spatially imaged in ~10-mum sliced subsurface layers. Corroded metal layers covered with paint are found to have more intensity variation than normal polished metal. The spatial mapping of the second-harmonic signals shows depth differentiation of paint, corrosion, and metal surfaces. The depth of corrosion beneath the paint can be measured from the SHG images.     

Two-photon autofluorescence spectroscopy and second-harmonic generation of epithelial tissue

A spectroscopy system is developed for studying the two-photon excited fluorescence (TPEF) and second-harmonic generation (SHG) of epithelial tissue in backscattering geometry. Our findings show that TPEF signals from epithelial and underlying stromal layers exhibit different spectral characteristics, providing information on the biomorphology and biochemistry of tissue. The SHG signal serves as a sensitive indicator of collagen to separate the epithelial layer from underlying stroma. The polarization dependence of the SHG signal reveals a well-ordered orientation of collagen fibers in the stromal layer. The results demonstrate the potential of depth-resolved TPEF and SHG in determining the pathology of epithelial tissue.     

Refractive index dispersion properties of collagen for microscopic second-harmonic generation

We theoretically simulate second-harmonic generation (SHG) in collagen under linearly polarized focused laser beam. With this model, the effects of numerical aperture (NA) and refractive index dispersion on SHG emission have been investigated. Dispersion properties of collagen are significant under incident wavelength in the visible range. Our results show that the efficient SHG is obtained by controlling the NA, and the higher NA is a necessity when the dispersion effect is considered. Our theoretical simulation results provide useful clues for experimental study of microscopic SHG emission in collagen excited by focused beam.     

Nonlinear spectral imaging microscopy of rabbit aortic wall

Employing nonlinear spectral imaging technique based on two-photon-excited fluorescence and second- harmonic generation （SHG） of biological tissue, we combine the image-guided spectral analysis method and multi-channel subsequent detection imaging to map and visualize the intrinsic species in a native rabbit aortic wall. A series of recorded nonlinear spectral images excited by a broad range of laser wavelengths （730- 910 nm） are used to identify five components in the native rabbit aortic wall, including nicotinamide adenine dinucleotide （NADH）, elastic fiber, flavin, porphyrin derivatives, and collagen. Integrating multichannel subsequent detection imaging technique, the high-resolution, high contrast images of collagen and elastic fiber in the aortic wall are obtained. Our results demonstrate that this method can yield complementary biochemical and morphological information about aortic tissues, which have the potential to determine the tissue pathology associated with mechanical properties of aortic wall and to evaluate the phaxmacodynamical studies of vessels.