光折变晶体几何结构对自泵浦相位共轭特性影响的研究

基于Cu∶KNSBN晶体的自泵浦相位共轭实验,研究了光折变晶体的几何结构对自泵浦相位共轭特性的影响,得到在入射角不变的情况下,随着入射位置的变化相位共轭光输出有最大值,几何结构不同最大值输出对应的位置不同.并从理论上分析了晶体结构在双作用区自泵浦相位共轭机制中的作用,指出几何结构、入射位置与自泵浦光通道、耦合系数的关系及在自泵浦相位共轭效应中存在一个最佳入射光工作位置.     

原子系统中远失谐脉冲光束对的群速度操控

基于远失谐的四波混频过程, 在实验上得到了放大的探针光脉冲和产生的共轭光脉冲的同时慢光传输, 并通过改变抽运光和探针光之间的双光子失谐实现了群速度的同时操控. 首先在连续光模式下, 研究了入射探针光和新产生共轭光的增益与单光子失谐之间的变化关系. 随着单光子失谐在一定范围内加大, 探针光和共轭光的增益均表现出先增加后减小的变化趋势. 在具有增益特性的基础上, 分别采用6 μs和365 ns探针光脉冲, 研究了慢光的延迟时间和双光子失谐的关系. 对6 μs的探针光, 得到探针和共轭光脉冲的最大延迟分别为2.1 μs 和1.9 μs, 对应的群速度分别约为0.000119 c和0.000132 c, 相应延迟比分别为0.35和0.32. 对365 ns探针光, 探针和共轭光脉冲的最大延迟分别为756 ns和670 ns, 对应的群速度分别约为0.00033 c和0.00037 c, 相应延迟比提高到2.07和1.83.     

晶体几何结构及入射角度对自抽运相位共轭特性影响的研究

基于两块不同尺寸的Cu∶KNSBN晶体自抽运相位共轭实验,研究了光折变晶体的几何结构及入射角度对自抽运相位共轭特性的影响,得到在入射位置不变化的情况下,随着入射角度的变化相位共轭光输出有最大值,几何结构不同对应的最大值不同。并从理论上分析了晶体结构及入射角度在双作用区自抽运相位共轭机理中的作用,指出几何结构、入射角度与自抽运光通道、耦合系数的关系,及在自抽运相位共轭效应中存在一个最佳入射角度,这时相位共轭光输出最大。最后,对理论上的相位共轭反射率公式进行了修正。对自抽运相位共轭实际应用中,选择最佳入射角度提供了理论和实验依据。     

基于相位共轭的动态体全息衍射特性的实时非破坏性测量

基于相位共轭技术,提出了一种动态体全息衍射特性的实时测量方法.在光学系统设计中,通过调整使得物光和参考光较强,而与参考光共轭的再现光非常弱（约为参考光的1/1 000）,借助非常微弱的共轭再现光实现了动态体全息的非破坏性实时测量.三束光的强度和偏振态可通过1/4波片、偏振片和衰减片进行调节和组合,可记录光强调制型或偏振态调制型体全息.本方法适合于光致折射率变化和光致变色材料体全息的测量.     

苯乙炔分子电子激发态超快动力学研究

采用飞秒时间分辨质谱技术结合飞秒时间分辨光电子影像技术研究了苯乙炔分子电子激发态超快非绝热弛豫动力学.用235 nm光作为泵浦光,将苯乙炔分子激发到第二激发态S2,用400 nm光探测激发态的演化过程.时间分辨的母体离子的变化曲线用指数和高斯函数卷积得到不同的两个组分,一个是超快衰减组分,时间常数为116 fs,一个是慢速组分,时间常数为106 ps.通过分析时间分辨的光电子影像得到光电子动能分布,结合时间分辨光电子能谱数据发现,时间常数为116 fs的快速组分反映了S2态向S1态的内转换过程.实验还表明S1态通过内转换被布局后向T1态的系间窜跃过程为重要的衰减通道.本工作为苯乙炔分子S2态非绝热弛豫动力学提供了较清晰的物理图像.     

KNSBN晶体猫式互抽运相位共轭光特性

在KNSBN∶Ce晶体中,利用二波耦合作用,在单一光束无法获得相位共轭光的条件下,实现了“猫”式互抽运相位共轭光输出,获得较高的共轭光反射率.实验结果表明,晶体的二波耦合作用可以使晶体的自抽运相位共轭光的阈值光强降低、入射光的入射角范围增大、响应时间缩短 关键词： 钾钠铌酸锶钡晶体 互抽运相位共轭 二波耦合 自抽运相位共轭 响应时间     

双光折变晶体半线性相位共轭器的自脉动

在由两块光折变BaTiO3晶体(其中一块晶体为全内反射式自抽运相位共轭镜工作方式,起外反射镜作用)构成的半线性自抽运相位共轭器中观察到了相位共轭光的自脉动现象。自脉动的平均频率f与入射光功率I0的关系为f∝I01.43;相位共轭光的平均维持时间ΔT与入射光功率I0近似成反比。观察到光折变BaTiO3晶体中扇形散射光具有动态的空间结构,扇形散射光的这种特性是引起上述半线性自抽运相位共轭器中相位共轭光脉动的原因     

Cu∶KNSBN晶体自泵浦与互泵浦相位共轭共存特性的实验研究

利用Cu∶KNSBN晶体,研究了在两束泵浦光对称入射的条件下,自泵浦与互泵浦相位共轭共存时相互竞争和相互影响的特性,以及共存时的相位共轭特性与两束泵浦光的入射位置、入射夹角和泵浦比之间的关系,并对自泵浦相位共轭反射率和两束泵浦光共同存在条件下的自泵浦相位共轭反射率进行了比较.     

Cu:KNSBN晶体自泵浦与互泵浦相位共轭共存特性的实验研究

利用Cu:KNSBN晶体,研究了在两束泵浦光对称入射的条件下,自泵浦与互泵浦相位共轭共存时相互竞争和相互影响的特性,以及共存时的相位共轭特性与两束泵浦光的入射位置、入射夹角和泵浦比之间的关系,并对自泵浦相位共轭反射率和两束泵浦光共同存在条件下的自泵浦相位共轭反射率进行了比较.     

Ce∶KNSBN光折变类光纤简并四波混频特性研究

研究了Ce∶KNSBN光折变类光纤在泵浦光和信号光夹角分别为小角度和大角度两种情况下简并四波混频的基本特性 .当夹角较大时 ,在光折变类光纤内部形成了两个四波混频作用区域 ,获得了比小角度情况提高 4倍的大相位共轭反射率 .给出了Ce∶KNSBN光折变类光纤中 ,在入射夹角分别为小角度和大角度两种情况下 ,相位共轭光反射率分别随信号光光强、两束泵浦光光强比变化的实验结果 ,并用理论公式进行了拟合 ,理论分析和实验结果相符 .还研究了Ce∶KNSBN光折变类光纤四波混频光栅模式和相位共轭光时间响应特性 .光折变类光纤的相位共轭响应时间较快 ,可为秒量级 .     

在聚合物表面刻写方格子的新方法及理论研究

为了提高制作偶氮苯微结构的实验技术,在偶氮苯聚合物的光致异构和光致取向特性的基础上,利用两激光束的新实验方案,运用比较简便的分步制作的方法,成功的在偶氮苯功能化聚合物薄膜表面“刻写”出二维的正方格子结构。提出了一个新的光异构取向场理论：在两束偏振方向互相垂直的偏振光照射下,偶氮苯聚合物表面光栅的形成是光异构取向场的干涉的结果,并对实验现象进行了解释。用计算机对实验结果进行了模拟,将模拟结果与实验结果进行了比较,结果表明计算机模拟将会更加有利于偶氮苯聚合物微结构的设计和制作。     

Photoelectrochemical switch based on cis-Azobenzene chromophore modified TiO2 nanowires

Carboxylated-azobenzene chromophore modified TiO₂ nanowire composites were prepared and characterized. Photocurrent measured with monochromatic incident light irradiation results showed that azobenzene modified TiO₂ nanowire electrode had obviously higher photocurrent and broader visible light response covering range of 350-650 nm, and the wavelength position corresponding to the maximum photocurrent was red shift to about 470 nm. After alternate irradiation with UV and visible light, the azobenzene modified TiO₂ nanowire electrode exhibited obvious photoelectrochemical switching properties. Furthermore, the photocurrent under visible light irradiation was much higher than that under UV irradiation due to the cis-to-trans isomerization transformation of azobenzene chromophore.     

Isomerization and fluorescence characteristics of sterically hindered azobenzene derivatives

We report synthesis and isomerization behaviors of sterically hindered azobenzene derivatives (1 and 2) with decyloxy and hydroxy groups, respectively, and their fluorescence enhancement under UV light irradiation characterized by means of absorption and fluorescence spectroscopy measurements. Upon irradiation of as-prepared solution (1) with UV light (∼200 mJ/cm²) a cis-rich photostationary state was reached. Obviously different from 2 showing very fast thermal cis-to-trans isomerization within 2 min, slow cis-to-trans thermal back isomerization of 1 with a long alkyl chain at ambient temperature was observed on the time scale of weeks. In contrast to no striking changes in absorption and fluorescence spectra of compound 2, the azobenzene 1 showed green fluorescence upon prolonged irradiation with UV light (about 3-8 J/cm² exposure doses), although both the initial trans-rich and cis-rich states of azobenzene molecules were not fluorescent in solution. The stability of fluorescence efficiency caused by drying and redissolving processes was examined.     

Photonic applications with photoanisotropic nanomaterials

Linearly polarized light beam can rotate the trans-azobenzene molecules into cis-form resulting in inducing photoanisotropy. This property is used to control the transmission of nanosecond and picosecond laser pulses and also for holographic data storage applications. Studies were performed using four commercially available azobenzene dyes in different solvents and also in custom prepared nanoporous azobenzene Dendron films. Nanopores are created in a silica matrix and azobenzene molecules are attached to the inner walls of these pores to mimic solution like environment for the azobenzene molecules such that they can undergo photoinduced trans-cis-trans isomerization process at a faster rate.     

Photochemical behaviour of an acid-terminated azopolymer in solution and in Langmuir–Blodgett films

Trans–cis–trans isomerization processes of an acid-terminated azopolymer have been investigated in mono and multilayered Langmuir–Blodgett (LB) films and the results compared with the behaviour of this azopolymer in solution. UV–vis spectroscopy was used to determine the apparent extent of photoisomerization at the photostationary state, as well as the order and rate constant of the cis → trans thermal isomerization, whilst structural changes of the LB films during photoisomerization have been monitored by AFM microscopy. From these studies it was concluded that the trans → cis photoisomerization extent largely depends on the free volume around the azobenzene chromophores, with the smaller apparent extent of photoisomerization obtained for a well-organized 10-layer LB film. Data also revealed a first-order kinetics for the thermal cis → trans isomerization process of the azopolymer both in solution and in LB films. The cis → trans isomerization process depends on the balance between the stress produced by the larger cross-section of cis-isomers inside the system and the interactions between cis-isomers and neighbouring molecules. The slowest cis → trans isomerization process takes place in a 35-layer LB film, whilst LB films with well-packed azobenzene layers show faster cis → trans isomerization processes.     

Photophysical characteristics of polyaniline with photochromic azobenzene side groups

Photochromic characteristics and optical molecular reorientation in conducting polymer such as polyaniline derivatives containing photochromic azobenzene moieties (PAPNPAPOA) in side chain are studied. Changes in the UV—vis absorption, refractive index, thickness, contact angle and morphology of these films after irradiation of a linearly polarized light with a wavelength of 365nm are measured. The trans-cis isomerization of PAPNPAPOA is proved irreversible even after withdrawing the UV light for a long time. That the structure of main-chain attends by the trans-cis isomerization of side-chain is confirmed by the absorption and the solution colour during the UV irradiation. These effects are discussed by taking the trans-cis isomerization of azobenzene into consideration.     

Molecules with multiple switching units on a Au(111) surface: self-organization and single-molecule manipulation

Three different molecules, each containing two azobenzene switching units, were synthesized, successfully deposited onto a Au(111) surface by sublimation and studied by scanning tunneling microscopy at low temperatures. To investigate the influence of electronic coupling between the switching units as well as to the surface, the two azo moieties were connected either via π-conjugated para-phenylene or decoupling meta-phenylene bridges, and the number of tert-butyl groups was varied in the meta-phenylene-linked derivatives. Single molecules were found to be intact after deposition as identified by their characteristic appearance in STM images. Due to their mobility on the Au(111) surface at room temperature, the molecules spontaneously formed self-organized molecular arrangements that reflected their chemical structure. While lateral displacement of the molecules was accomplished by manipulation, trans-cis isomerization processes, typical for azobenzene switches, could not be induced.     

Conformational molecular switch of the azobenzene molecule: a scanning tunneling microscopy study

We propose to utilize azobenzene as a nanomolecular switch which can be triggered by transmitting electrons above threshold biases. The effect is explained by an electron impact trans-cis conformational change of the isolated azobenzene molecules. The molecular electronic states of both isomers have been measured with spatially resolved scanning tunneling microscopy or spectroscopy, leading to suggested transition pathways of the electron-induced isomerization.     

Effect of fluorine groups and different terminal chains on the electro-isomerization of azobenzene liquid crystals

A series of azobenzene liquid crystals with one or two terminal acrylate groups were synthesized and their polymers were fabricated.The azobenzene liquid crystals and their polymers achieved the photoisomerization from the liquid crystalline trans-isomer to the isotropic cis-isomer with UV irradiation.Then,the cis to trans isomerization induced by an electric field was studied,the time required for electro-isomerization was measured,the texture change and absorption variation from cis to trans form induced by the electric field were observed clearly,and the time required for electro-isomerization was much shorter than that for thermal relaxation.The influence of the polar group(fluorine),terminal acrylate group,and flexible alkyl chain on the time of electro-isomerization was studied.The results show that the compounds with polar fluorine group require shorter time for electro-isomerization and the polymerization of terminal acrylate group delays the electro-isomerization.     

Theoretical study on the isomerization mechanisms of phenylazopyridine on S0 and S1 states

In this work, some important structures tied closely to the isomerization of 2‐(phenylazo)pyridine (2‐PAPy) and 4‐(phenylazo)pyridine (4‐PAPy) on the S₀ and S₁ states were characterized in detail by using the complete active space SCF (CASSCF) theory. The isomerization mechanism was discussed on the basis of the mapped potential energy surfaces (PESs) and conical intersections (CIs). A comparison of PAPy with azobenzene was carried out to stress the effect of molecular structure on the photoisomerization details. The results indicate that the thermal isomerization for both 2‐PAPy and 4‐PAPy are mainly attributed to the inversion of CNN angle on the side of the pyridine ring. In view of the energy, an optimized CI_rot with a twisting structure supports the rotation mechanism in the photoisomerization of PAPy on the S₁ state. However, it was found that another conical intersection (CI_inv) with a planar structure is higher in energy than the corresponding trans‐FC structure, via which only the PAPy excited on S₂ state or vibrationally hot S₁ state can relax their excitation energy. Minimum energy paths (MEPs) showed that the relaxation process of cis‐PAPy being excited on the S₁ state is characterized by a smoothly falling curve, which is very similar to that of azobenzene. Furthermore, a S₁ minimum and a transition state (TS₁) were found to exist on the MEP starting from the trans‐FC point to the CI_rot for 4‐PAPy, but these two typical structures were not found on the MEP of 2‐PAPy. Compared with azobenzene, 4‐PAPy exhibits a very similar photoisomerization PES, but a subtly different one can be predicted in the case of 2‐PAPy. The present results are expected to provide useful information for the design of photoresponsive materials based on the PAPy units. Copyright © 2009 John Wiley & Sons, Ltd.