Dependence of second-harmonic generation in ferroelectric liquid crystals with a defect on the optical gap

Second-harmonic generation (SHG) in ferroelectric liquid crystal with a twist defect is studied by de Vries' solution. It is shown that the width of the photonic band gap for the SH wave is an important factor to determine the saturation length. SHG is effectively enhanced by using narrow photonic band gaps.     

Some features of second harmonic generation in the nematic liquid crystal 5CB in the pulsed-periodic electric field

Second harmonic generation (SHG) induced by an external pulsed-periodic electric field in a 5CB nematic liquid crystal (NLC) is investigated. Deformation of the NLC under this field is analyzed, and a possibility of its accumulation is established. The observed SHG features are due to the domains with quite different director orientations emerging in the NLC.     

Influence of the flow on the anchoring of nematic liquid crystals on Langmuir-Blodgett monolayers studied by optical second-harmonic generation

The influence of capillary flow on the alignment of the nematic liquid crystal 5CB on fatty acid Langmuir-Blodgett monolayers was studied by optical second-harmonic generation (SHG). The surface dipole sensitivity of the technique allows probing the orientation of the first liquid crystal monolayer in the presence of the liquid crystal bulk. It was found that capillary flow causes the first monolayer of liquid crystal molecules in contact with the fatty acid monolayer to be oriented in the flow direction with a large pretilt (78 degrees), resulting in a quasi-planar alignment with splay-bend deformation of the nematic director in the bulk. The large pretilt angle also suggests that the Langmuir-Blodgett film itself is affected by the flow. The quasi-planar flow-induced alignment was found to be metastable. Once the flow ceases, circular domains of homeotropic orientation nucleate in the sample and expand until the whole sample becomes homeotropic. This relaxation process from flow-induced quasi-planar to surface-induced homeotropic alignment was also monitored by SHG. It was found that in the homeotropic state the first nematic layer presents a pretilt of 38 degrees almost isotropically distributed in the plane of the cell, with a slight preference for the direction of the previous flow. Received 8 November 2000 and Received in final form 12 March 2001     

Enhancement of optical nonlinearity in a guest-host system by nematic ordering

Second-harmonic generation (SHG) has been studied for understanding the enhancement mechanism for the second-order optical nonlinearity by the nematic (or axial) ordering in a liquid crystal doped with one-dimensional nonlinear optical (NLO) organic molecules. An extended version of the Maier-Saupe mean-field theory for nematic liquid crystals was developed to obtain analytical expressions for the second-order NLO coefficients in terms of the axial order, the polar order and the effective nematic potential. From the SHG data in a guest-host system composed ofN,N'-dimethylaminonitrostilbene molecules (0.5% by weight) and a liquid crystal, the enhancement of the second-order NLO coefficient,d ₃₃, by nematic ordering becomes almost 3, which agrees well with our theoretical predictions.     

4-戊基-4'-氰基联苯液晶的分子二阶非线性光学极化率测量方法

在玻璃衬底上制得单层硬脂酸的Langmuir-Blodgett(LB)膜,做为4-戊基-4''-氰基联苯(5CB)液晶的取向层,在取向层上旋涂一层5CB液晶,用透射二次谐波方法测量出5CB液晶分子二阶非线性光学极化率,其值为β=7×10-29esu.对比文献值,测量得到的结果符合较好.     

Switching behaviour of ferroelectric liquid crystals probed by optical second-harmonic generation

Ferroelectric properties of thin (1.5–4 μm planar cells of a ferroelectric liquid crystal (FLC) mixture are studied using electro-optic measurements, second-harmonic generation (SHG) and SHG interferometry. A switching behaviour of the FLC cells in external dc electric fields is observed. It is characterised by rotation of the polarisation plane of the transmitted light and by changes in the SHG intensity, phase and anisotropy dependences, which are attributed to a collective motion of the system as a ferroelectric uniform state with C₂ symmetry. Received: 16 October 2001 / Revised version: 18 March 2002 / Published online: 6 June 2002     

Automatically tunable second-harmonic generation of dye lasers

A new technique which realizes the phase-matching of second harmonic generation (SHG) over a wide spectral range without mechanical or temperature tuning of the SHG crystal is developed. The phase-matching is achieved by changing the angle of the laser beam incident on the SHG crystal using dispersive components such as gratings or prisms. Over the entire spectral range of a Rhodamine-6G dye laser, the phase-matching is realized with a KDP crystal using this technique. This system is capable of producing tunable coherent ultraviolet light from 2300 Å to 3500 Å with an extremely fast scanning rate, if the dispersion of the prism and the SHG crystal is carefully matched.     

Second harmonic generation due to quadrupole interaction in a photonic crystal slab: angle dependence and symmetry of the unit cell

We investigate second harmonic generation (SHG) from a photonic crystal slab consisting of centrosymmetric materials. The SHG signal is observed in the transmission direction when the incident laser excites the quasiwaveguide mode. As the SHG frequency approaches the exciton level, the SHG intensity increases resonantly. When the incident angle is exactly 0, the SHG signal vanishes even if the transmission dip is excited. This fact is readily explained by a quadrupole theory based on the Lorentz oscillator model, where the source of the nonlinearity is the Lorentz force. When the unit cell in the photonic crystal lacks inversion symmetry, the SHG signal is expected even for the normal incidence. It is experimentally demonstrated for a square array of triangular semiconductor slabs.     

应用级联倍频方法提高倍频系统输出稳定性研究

提出应用级联倍频方法提高倍频系统输出稳定性,并就该方法的有效性进行了理论分析和模拟计算.分析和计算结果不但证明级联倍频方法能实现倍频系统稳定输出,而且还表明可以通过仔细调节第一块倍频晶体中波矢方向 k 与光轴间夹角、两块倍频晶体间的间隔,能调节改变实现倍频系统最稳输出时所需第二块晶体的理论计算长度,使之与第二块倍频晶体的实际加工长度一致,最终实现系统稳定倍频输出.级联倍频方法在实现高输出稳定性的同时能实现高的倍频转换效率,对应用于光参量啁啾脉冲放大系统的高稳定抽运源系统的设计建造具有重要参考意义. 关键词： 级联倍频 稳定倍频输出 光参量啁啾脉冲放大     

Investigation on intracavity second-harmonic generation of a new Li-doped GdCa4O(BO3)3 crystal

Efficient intracavity second-harmonic generation (SHG) has been realized in a new Li-doped GdCa₄O(BO₃)₃ (GdCOB) crystal. 2.55 W CW SHG output power was obtained with an optical conversion efficiency of 19.6%, while the effective intracavity SHG efficiency was determined to be 44%. Compared to that of undoped GdCOB crystal, the effective SHG efficiency of the Li-doped GdCOB crystal was found to increase by 12.5%.     

晶体球中非相位匹配二次谐波产生及最佳聚焦条件

晶体球法是近年来在国际上发展起来的一种测量非线性系数的新方法。根据聚焦高斯光束二次谐波产生的孔径方程理论,分析了晶体球中的第Ⅰ类非相位匹配二次谐波产生过程,讨论了晶体球中第Ⅰ类非相位匹配二次谐波最佳聚焦参数的选择。报道了LiNbO3晶体球中皮秒激光脉冲抽运的相位匹配二次谐波实验。所得结果与理论预计相符。     

二倍频晶体温升分布对输出光场分布的影响

 利用分步快速傅里叶变换和四阶龙格-库塔法,对具有一定温升分布的倍频晶体的二次谐波转换过程进行了研究。综合考虑了谐波转换过程中的离散、衍射、二阶、三阶非线性等效应,着重讨论了倍频晶体吸收光能后,晶体内温升分布对晶体内o光和e光的折射率分布的影响,定量分析了温度分布引起的相位失配量、输出光场分布、二次谐波转换效率随倍频晶体温度分布变化的规律。结果表明:在高功率倍频系统中,倍频晶体温升分布引起基频光、倍频光的相位失配,相位失配导致输出光场光强分布的变化以及谐波转换效率的降低。     

BBO晶体在CPM非稳腔Nd：YAG激光器中的腔内倍频效应

本文报道用BBO晶体作为腔内倍频元件,在带抗共振环的对撞脉冲锁模非稳腔Nd:YAG激光器中实现腔内倍频激光输出,获得倍频能量转换效率为50.4%.理论和实验均证明了该激光器的BBO腔内倍频转换效率高于腔外倍频转换效率.     

Efficiency of second harmonic generation for partially coherent light in anisotropic crystals

This paper analyzes the second harmonic generation (SHG) efficiency of light with partial temporal coherence due to depolarization effects in birefringent media. It discusses relations between SHG efficiency fading, light source spectrum, crystal birefringence, and phase matching conditions. The efficiency of SHG pumped by the partially coherent light beam that may depolarize light in nonlinear birefringent crystal is also analyzed. The basic theory of SHG with its modification for partially coherent light with depolarization and some numerical calculations of the SHG process are described. Presented at 9-th International Workshop on Nonlinear Optics Applications, NOA 207, May 17–20, 2007, Świnoujście, Poland     

粉末倍频系数的模拟研究

根据Kurtz粉末倍频理论,使用光纤光谱仪自行设计搭建一个粉末倍频测试系统,该系统可在基频光波入射方向不变的情况下,通过转动旋转臂在不同方向探测二次谐波,并且具有较高的精确度和灵敏度。使具有位相匹配特性的颗粒尺寸范围扩大为46μm～250μm 。使用波长为1.064μm的基频光,对KIO3晶体粉末的倍频效应进行了测试。测试结果表明,KIO3晶体粉末透射倍频光具有位相匹配特性。这一测试结果与已报道结果相一致。通过数值模拟实验测试曲线,获得了KIO3晶体粉末的倍频系数d2ω的值为9.7×10-12m/V。这一模拟值与已报道的KIO3晶体对该基频光波产生的倍频系数d32的值相吻合。由此可见,该方法切实可行。并且可推广应用到其他晶体材料的非线性特性测试研究。     

Size effect on the SHG properties of Cu-doped CdI2 nanocrystals

Because the optically induced second harmonic generation (SHG) is prevented by symmetry in a centrosymmetric material, one needs to form noncentrosymmetric processes in order to observe the SHG. However, one of the efficient ways to enhance the noncentrosymmetricity of a material is to dope it with an appropriate impurity and amount. We grow Cu-doped CdI₂ layered nanocrystal structures from the mixture of CdI₂ and CuI using the standard Bridgman-Stockbarger method and investigate the nano-confined effects by studying the second-order optical effect via the measurements of SHG. The second-order susceptibility for the nanocrystals is calculated and the values at liquid helium temperature range from 0.38 to 0.83 pm V⁻¹ for the thicknesses of 10-0.8 nm respectively. The size dependence demonstrates the nano-sized quantum-confined effect with a clear increase in the SHG with decreasing the thickness of the nanocrystal or crystal temperature. Since the local electron-phonon anharmonicity is described by third-order rank tensors in disordered systems, the SHG is very similar to that one introduced for the third-order optical susceptibility. It has been confirmed by observing the large photoluminescent yield of the pure crystals. The Raman scattering spectra taken for thin nanocrystals confirm the phonon modes originating from interlayer phonons crucially responsible for the observed effects. The obtained results show that the Cu-doped CdI₂ layered nanocrystals are promising materials for applications in optoelectronic nano-devices.     

激光热效应对PPLN晶体倍频效率的影响

从准相位失配关系出发,研究了PPLN晶体倍频效率与温度的关系,针对温度对折射率与极化周期的影响,分析了激光热效应对准相位匹配系统谐波转换过程的影响。采用有限元分析法计算了PPLN材料内部温度分布状况,并对热效应影响下晶体内部倍频效率变化情况进行了计算。计算结果表明:在不同基频光功率密度下,在倍频过程中晶体温度及通光方向截面折射率的分布不同,在倍频晶体出射面倍频效率的分布也不同;随基频光功率密度增大,晶体整体温度与折射率增大,出射端面倍频效率分布随基频光功率密度变化而变化;与理想条件倍频系统相比,激光热效应对晶体倍频效率有一定的影响。     

晶体和基团倍频系数间的普遍变换公式

本文使用倍频系数的不可约张量表示法,得到了离子基团的微观倍频系数与晶体宏观倍频系数间的一般变换公式,从而为晶体中不同取向的基因对宏观倍频系数的贡献提供了普遍的计算公式。应用这一计算方法可方便地计算各种碘酸盐的倍频系数,结果相当满意。本文对于探索新型倍频材料,提供了明确的空间结构条件。 关键词：     

KTP晶体中一种特殊缺陷的研究

研究发现,在KTP晶体中有一特殊的区域,在这一区域内,KTP晶体的倍频效率明显增加,本文借助于透射同步辐射(SR)白光形貌方法,对这一区域进行了研究,发现在这区域内有一面缺陷,在这面缺陷处由于杂质的积累,使晶格发生了微小畸变,从而使KTP晶体的倍频效率增加,并根据消光规律确定了该缺陷的最大应变方向R=<010>。 关键词：     

Li2B4O7晶体生长及其二次谐波系数

用恰克拉斯基（Ｃｚｏｃｈｒａｌｓｋｉ）法特殊工艺生长出尺寸为Φ３０×３０ｍｍ的优质Ｌｉ２Ｂ４Ｏ７单晶，讨论了工艺参数对晶体宏观缺陷的影响。用Ｍａｋｅｒ条纹测量了晶体的二次谐波系数ｄ３１和ｄ３３，结果表明，Ｌｉ２Ｂ４Ｏ７的ｄ３３是石英晶体ｄ１１的三倍。