大非线性相移下光学非线性Z扫描特性的研究

采用高斯分解法(GD)对大非线性相移下的Z扫描特性进行了分析,通过对数值算法的优化,将GD推广到对脉冲入射激光下大非线性相移下的Z扫描理论分析.对不同条件下大非线性相移Z扫描曲线峰谷结构的比较,发现在大非线性相移的情况下,Z扫描曲线的峰和谷随透过光阑或入射光强变化表现出某些新的特性.随着透过光阑孔径的增加,Z扫描曲线峰的变化要明显快于谷的变化,而且在谷明显存在的情况下,峰很快消失.采用皮秒脉冲激光下的纯二硫化碳实验对理论结果加以验证,实验结果和理论分析相一致.我们的分析结果对大非线性相移下Z扫描测量有一定的指导性意义,避免在大非线性相移下对Z扫描结果产生错误的分析.     

大非线性相移下光学非线性Z扫描特性的研究

采用高斯分解法(GD)对大非线性相移下的Z扫描特性进行了分析，通过对数值算法的优化，将GD推广到对脉冲入射激光下大非线性相移下的Z扫描理论分析.对不同条件下大非线性相移Z扫描曲线峰谷结构的比较，发现在大非线性相移的情况下，Z扫描曲线的峰和谷随透过光阑或入射光强变化表现出某些新的特性.随着透过光阑孔径的增加，Z扫描曲线峰的变化要明显快于谷的变化，而且在谷明显存在的情况下，峰很快消失.采用皮秒脉冲激光下的纯二硫化碳实验对理论结果加以验证，实验结果和理论分析相一致.我们的分析结果对大非线性相移下Z扫描测量有一定的指导性意义，避免在大非线性相移下对Z扫描结果产生错误的分析. 关键词： 大非线性相移 高斯分解法 Z扫描     

基于衍射模型的Z扫描理论

以菲涅耳－基尔霍夫衍射理论为基础，建立了非线性光学介质对高斯光束的衍射模型，从一种新的角度解释了Z扫描现象，这种理论不仅在小非线性相移的情况下与传播Z扫描理论的结果完全一致，而且还适用于大非线性相移的情况。数值计算表明，随着非线性相移的增大，原来近似对称的Z扫描曲线发生畸变，谷的透过率被饱和抑制而峰的透过率被增强。通过开孔的光功率随着入射激光功率的增加会出现振荡衰减现象。     

可溶性碳纳米管Z扫描实验研究

以可溶性碳纳米管为研究对象,进行了Z扫描实验.实验中通过改变样品的参量,研究了材料在不同浓度和不同厚度下的Z扫描曲线特性;实验分别在532nm和1064nm的激光下进行,并分别进行了开孔和闭孔实验.材料的闭孔 Z扫描曲线没有明显的峰谷对称结构,而开孔Z扫描曲线呈现关于原点对称的单一谷形状,表明样品存在较强的非线性吸收效应,非线性吸收可能是其主要的光限幅机理.     

测量慢弛豫介质的非线性折射率Z扫描方法

对非线性折射率慢弛豫介质的Z扫描理论进行了详细推导,得到了透射光场的表达式.分析表明:对于慢弛豫介质,其Z扫描过程也是非线性折射的累积过程.实验结果与理论分析一致. 关键词：     

可溶性碳纳米管Z扫描实验研究

以可溶性碳纳米管为研究对象，进行了Z扫描实验.实验中通过改变样品的参量，研究了材料在不同浓度和不同厚度下的Z扫描曲线特性；实验分别在532nm和1064nm的激光下进行，并分别进行了开孔和闭孔实验.材料的闭孔Z扫描曲线没有明显的峰谷对称结构，而开孔Z扫描曲线呈现关于原点对称的单一谷形状，表明样品存在较强的非线性吸收效应，非线性吸收可能是其主要的光限幅机理.     

任意光束形状和介质厚度的Z扫描技术

利用分步傅里叶算法结合坐标变换理论,建立了在光致折射率改变介质中任意形状光束的传输、聚焦模拟方法,能够用于计算任意光束形状、任意介质厚度的Z扫描开孔和闭孔拟合曲线。在薄介质和高斯光束条件下,该方法的拟合曲线与经典算法相吻合。在平顶光束情况下,该方法的拟合结果与实验数据基本一致。利用此方法测量了LiTaO3晶体的非线性吸收和非线性折射率系数,并对高斯光束和涡旋光束在厚介质中的Z扫描过程进行了分析。     

反饱和吸收激发态非线性材料的Z扫描分析

本文用速率方程理论分析了激发志光学非线性材料非线性吸收和折射,通过数值模拟讨论了激发态和基态吸收截面、折射度相互关系等参数对非线性吸收和折射性质的影响。并从理论上分析了C₆₀甲苯溶液的Z扫描实验曲线,它具有强非线性吸收和弱非线性折射。     

以衍射理论为基础考虑双光子吸收的Z-扫描理论

以菲涅尔-基尔霍夫衍射理论为基础,建立了非线性介质对高斯光束的衍射模型,对高斯光束通过非线性介质后的传输行为进行了详细的理论推导和数值计算,从一种新的角度解释了Z-扫描现象.理论推导出了考虑双光子吸收的闭孔和开孔Z-扫描曲线的统一公式,是计算非线性折射率和双光子吸收系数的一种新方法.数值模拟计算表明,对于考虑双光子吸收的闭孔Z-扫描曲线,其结果与经典的Z-扫描理论完全一致.而对于已给定的开孔Z-扫描曲线,用该理论计算出的双光子吸收系数是经典开孔Z-扫描理论计算值的1/3,其它结论与传统Z-扫描理论完全吻合.该理论的近似条件只要求薄样品和小非线性吸收,比传统理论具有更好的准确性.     

基于衍射理论的大非线性相移Z扫描研究

以衍射理论为基础,建立了强非线性吸收下,非线性介质对高斯光束的衍射模型.数值计算表明,折射相移的增大使峰的透过率增强而谷的透过率趋于饱和;而吸收相移的增大使峰的透过率被抑制而谷的透过率增强.相移比值是决定谷-峰结构存在与否的关键.在一定的输入功率下,远场输出功率并不是完全随着非线性吸收系数的增大而增大.该理论的近似条件只要求是薄样品,具有更好的适用性.     

A precise data processing method for extracting χ from Z-scan technique

We present a precise data processing method, in the Z-scan experiments using a Gaussian beam and trimmed Airy beam, for directly extracting nonlinear refraction from the closed aperture Z-scan trace with the aid of the open aperture Z-scan trace when the materials exhibit nonlinear refraction and nonlinear absorption simultaneously. This method is still applicable when the nonlinear absorption is dominant and the closed aperture Z-scan curve degenerates into a single valley configuration, which is a salient advantage over other methods. In addition, we give gracefully empirical formulas with very high precision, which have good practicability for characterizing the optical nonlinearities of materials by use of the Gaussian beam and trimmed Airy beam Z-scan technique, respectively.     

Third-order NLO properties of solution grown methyl-p-hydroxy benzoate single crystals

The third-order nonlinear optical properties of solution-grown methyl-p-hydroxy benzoate (MHB) single crystals were studied by Z-scan technique using a He-Ne (632.8 nm, 30 mW) laser. From the closed aperture Z-scan data, the valley followed by peak on the normalized transmittance indicates the sign of the nonlinear refractive index is positive and shows a self focusing nature. From the open aperture Z-scan curve, it is found that the nonlinear absorption is due to saturation. The order of magnitude of third order susceptibility was estimated to be 10^?6 esu. UV-Vis-NIR spectrum of MHB single crystal reveals a very low cutoff wavelength (310 nm) and transparency in the entire visible region. Also the material has direct allowed transition and it possesses a band gap of 3.7 eV. The dissipation factor is low and SHG efficiency is high. The higher magnitudes of second and third order NLO parameters make the material suitable for photonic applications like frequency conversion and eye/sensor protection.     

Z-扫描参量对薄膜材料三阶非线性光学特性的影响

采用数值计算方法对薄膜材料的Z-扫描参量进行研究.对不同激光脉宽下Z-扫描的三阶非线性折射系数和吸收系数进行分析,结果显示不同等级的脉宽激光器对计算结果的数量级影响很大.对不同小孔光阑半径的Z-扫描闭孔曲线进行仿真,结果显示光阑小孔越小,获得特征曲线的峰谷值越明显.对不同光阑离透镜焦点的距离值进行Z-扫描闭孔曲线仿真,结果显示距离为1/2透镜焦距值时闭孔Z-扫描曲线能得到相对较大的峰谷差值.数值分析结果与相关文献报道的实验结果基本一致.     

偶氮分散红聚合物薄膜的Z扫描研究

采用单光束Z扫描技术对新型光学存储材料偶氮分散红聚合物的薄膜样品进行了测量。实验结果表明 ,该样品的非线性折射率为负值。为消除非线性吸收的影响 ,将实验所得有孔Z扫描曲线除以开孔Z扫描曲线进行了处理。从Z扫描曲线的实验数据处理得到这种新型偶氮分散红聚合物薄膜材料的非线性折射率为 :- 5 .5× 10 -6cm2 /W ,其值比常见无机非线性光学材料大 8～ 9个数量级 ,是一种很有应用前景的新型光学存储材料。最后 ,对这种偶氮聚合物薄膜具有大的非线性折射率的物理机制进行了分析。初步认为 :在线偏振光照射下 ,偶氮分子变为各向异性。这些类似液晶相的分子很好地关联在一起 ,分子的取向对外界扰动的响应是一种集体的效果 ,所以他们具有很大的非线性响应。     

Nonlinear responses and optical limiting behavior of Basic Violet 16 dye under CW laser illumination

Nonlinear optical properties of Basic Violet 16 dye solution in water are studied employing different optical techniques. Experiments are performed using the second harmonic of a continuous Nd-Yag laser beam at 532 nm wavelength and 100 mW power. The effect of nonlinearity of Basic Violet 16 dye in broadening the laser beam is observed. The optical limiting behavior is investigated by measuring the transmission of the samples. The third-order nonlinearity, χ³ of Basic Violet 16 dye, is measured using Z-scan data. The nonlinear absorption coefficient is calculated using the open aperture Z-scan data, while its nonlinear refractive index is measured using the closed aperture Z-scan data. All experiments are done for different concentrations and thicknesses of Basic Violet 16 dye solution. The effect of intensity of input laser beam on the nonlinear susceptibility is studied experimentally. Results indicate that Basic Violet 16 dye is a potential candidate for low-power optical limiting applications.     

Determination of nonlinear absorption and refraction by single Z-scan method

We report a simplified Z-scan technique based on a study on the symmetric features of a typical Z-scan curve. The contributions from the two-photon absorption (TPA) and the nonlinear refraction (NLR) are easily separated from a closed-aperture Z-scan curve using this method. And the determination of the two nonlinearities is simplified and unambiguous. We demonstrate this method on ZnSe, CdS, and ZnTe semiconductors with 120-fs laser pulses. And the influence from the uncertainty of the focal plane (Z=0) position is discussed. It is also found that the TPA coefficient can be obtained independently without knowing the exact location of the focal point. Received: 8 July 1999 / Published online: 3 November 1999     

Investigation of astigmatic Gaussian beam Z scan with simultaneous third- and fifth-order nonlinear refraction

Based on Gaussian decomposition method, we present a theoretical investigation on the astigmatic Gaussian beam Z-scan when materials possess the simultaneous third- and fifth-order nonlinear refraction effects, and obtain an analytic expression of normalized transmittance of the Z-scan trace for the case of pinhole. The influences of ellipticity and waist separation on the Z-scan curves are analyzed. We find that normalized transmittance difference between the peak and the valley in Z-scan curves decreases gradually with the increment of the waist separation or with the decrement of the ellipticity.     

Nonlinear optics of nontoxic nanomaterials

We studied the linear and nonlinear optical properties of halloysite nanotubes using Z-scan technique. Halloysite is alumina silicate clay rolled into 50 nm diameter hollow cylinders, where the silica layer is at the outer surface of the tube and alumina layer is in the inner surface. Optical absorption spectra show an absorption peak around 600 nm. Open aperture Z-scan measurements using 3 ns laser pulses at 532 nm reveal two-photon induced absorption. The closed aperture Z-scan indicates a positive nonlinear refractive index. As these nanotubes are nontoxic and biocompatible, they have advantages over conventional carbon nanotubes for biomedical applications.