【作者】 范贺良；

【导师】 许东； 赵显；

【作者基本信息】 山东大学 ， 材料物理与化学， 2010， 博士

【摘要】 全光开关可以突破限制电、声、热、机械等光开关单通道传输速率的瓶颈,实现数据从源节点到目的节点的传输过程都在光域内进行。因而,研究解决可实现全光开关的关键材料与器件是当今国际光电科学技术发展的前沿问题,受到了人们高度关注。全光开关的类型有多种,其中最具代表性的Mach-Zehnder干涉型全光开关主要利用了材料的三阶非线性光学效应。用一束控制光引起材料的折射率变化,信号光在其中通过时就会带来相位的变化,从而实现光开关的开关动作。其非线性相移与2πn2IL／λ成正比,其中I是光强,L是光波相互作用长度,n2是非线性折射率。全光开关的交换速度、能耗、对偏振态的敏感性、插入损耗等性能,均取决于用于器件制备的三阶非线性光学材料。长期以来,国际上对三阶非线性光学材料的研究主要集中在基于分子取向的铁电液晶、基于带隙共振吸收的半导体、金属氧化物类、硫系玻璃、有机染料、富勒烯衍生物及以聚乙二炔、聚乙炔、聚苯乙炔为代表的聚合物等材料上。其中,液晶的响应速度偏慢,有机染料和聚乙二炔等导电聚合物的非线性折射率较小,半导体材料由于其带隙共振作用导致非线性吸收较大、响应速度也变慢,总之,以上几类材料都难以满足全光开关器件化的要求。本论文基于全光开关应用的需要,致力于探索集高三阶非线性光学效应和高迁移率性能于一体的配合物型材料,即从有机配合物超导体、导体和半导体一类材料中探索高迁移率的材料,从而避开线性吸收区域工作并响应速度极快。特别是当配合物型材料在有机共轭体系中引入金属离子以后,金属与有机体系之间的电荷转移使得整个共轭体系的电子离域性更强,可以进一步增强有机化合物的三阶非线性光学特性并提高迁移率,产生具有快速响应的非共振三阶非线性光学效应。早期,1,3-二硫杂环戊烯-2-硫酮-4,5-二巯基(简称dmit)类有机配合物材料导电特性被人们广泛关注,对它们的三阶非线性光学特性以及其它相关性能进行深入研究。近年来,人们渐渐发现这类材料的分子具有优良的π电子共轭体系,离域π电子很容易带来大的三阶非线性光学效应。本文将研究对象集中于dmit这一类有机／无机配合物型半导体材料,对它们的三阶非线性光学特性以及其它相关性能进行深入研究。在前期工作中,本课题组已经对dmit类材料在溶液中的三阶非线性光学特性做了较为深入地研究,筛选出了几种具有较大非线性折射和较小非线性吸收的材料。本论文在前人工作基础上,有针对性的对几种dmit类材料在薄膜中的三阶非线性光学特性做了系统性的研究,从内部机理探讨了影响材料非线性光学特性的各项因素,并对影响薄膜非线性响应和光学质量的热致损耗和光传输损耗性质做了初步探索。目的是为了探索到适用于全光开关器件应用的非线性光学材料和提高材料三阶非线性折射,降低线性和非线性吸收的理想途径,为材料的最终器件化提供实验和理论依据。本论文的研究工作主要体现在以下几个方面：1、Dmit/PMMA聚合物复合薄膜三阶非线性光学特性研究。筛选出了几种具有较大非线性折射和较小非线性吸收,能够被考虑用在全光开关器件制作上的候选材料,并发现了其三阶非线性光学性质与中心金属离子及结构密切相关的规律。全光开关器件要求材料不仅具有大的非线性折射率,而且需要具有小的线性和非线性吸收。对于采用Z-扫描方法得到的材料非线性折射率、非线性吸收以及二阶分子超极化率等光学参量,我们用两个品质因子W=n2I／α0λ和T=βλ/n2来分析dmit材料是否适合于全光开关器件的研制,当|W|>>1而且|T|<<1时,就认为这种材料符合全光开关器件研制要求。对金属离子分别为Au和Cu的dmit类材料的三阶非线性光学特性研究发现,两类材料都具有较大的非线性折射效应,而Cu(dmit)2类材料相对于Au(dmit)2类材料则具有更大的非线性吸收。得到的两种Au(dmit)2类材料[C16H33(CH3)3N][Au(dmit)2] (CTAu)和[(CH3)4N][Au(dmit)2] (MeAu)的品质因子W和T都满足|W|>>1而且|T|<<1的要求,而Cu(dmit)2材料的|T|>>1。由此可断定,CTAu和MeAu两种材料满足全光开关器件对非线性光学材料的要求,在全光开关器件化方面具有较好的应用前景,而Cu(dmit)2类材料则不适合用在全光开关应用上。通过提高BFDT／PMMA聚合物复合薄膜中非线性基元的掺入量,薄膜的非线性光学响应得到显著提高,其中非线性折射增强的同时非线性吸收的变化很小。并且该类材料除了表现出较强的非线性折射效应以外,它们的非线性吸收效应很小以致于可以忽略不计,其品质因子W的值在1.05～3.87之间,而T≈0,基本满足|W|>>1和|t|<<1的要求,所以该类材料也是将来可以用在全光开关器件制作上的良好候选材料。2、dmit类材料的超快时间响应特性研究。采用飞秒时间分辨光克尔技术得到了dmit类材料的光克尔信号的响应时间都在200 fS左右,其响应速度比当前通用的电光开关的响应速度约快4～6个数量级。其中一些材料的响应时间是首次报道。以dmit类材料作为研究对象,除了由于它们具有较大的由π共轭结构引起的三阶非线性光学效应之外,更多的是因为其相对于其它材料具有超快响应速度的特性。3、Dmit／PMMA聚合物复合薄膜热效应因素研究。得出了随着薄膜体系内非线性基元掺入量的增加,薄膜非线性光学效应增强的同时,热效应影响也相应增大的规律。利用带有温度控制装置的棱镜耦合系统得到的dmit／PMMA薄膜的热光系数在10-5/℃量级。由热效应引起的热致折射率变化是阻碍全光开关器件实现开关状态和超快响应速度的主要因素。通过降低薄膜体系的线性吸收和非线性吸收,可以有效减小热效应的影响。4、BFDT/PMMA聚合物复合薄膜的光传输损耗研究。降低波导薄膜中的光传输损耗,可以提高薄膜光学稳定性和实用性,进而优化波导器件开关性能。通过比较不同掺杂浓度下BFDT/PMMA聚合物复合薄膜的光传输损耗系数,得出随着掺入量的增加,薄膜的传输损耗也呈近似线性增加的趋势。改善薄膜的制备工艺和表面处理技术,减少薄膜中的散射点,是有效降低光传输损耗的途径之一。采用聚合物包覆法制备的dmit/PMMA聚合物复合薄膜,以表面活性剂作为包覆剂,所制备的薄膜在扫描探针显微镜的观察下发现其表面均匀分布着dmit材料析出的纳米晶颗粒,颗粒的尺度约在20～50 nm之间。Dmit材料被均匀地掺入进了聚合物PMMA基质中去。用此种方法制备的BFDT/PMMA聚合物复合薄膜的光传输特性得到明显改善。5、Cu(dmit)2类材料在纳秒脉冲激发条件下的光限幅特性研究。通过一个五能级模型分析,得到Cu(dmit)2类材料在18ns脉冲激发条件下会发生反饱和吸收的结论,然后对其由反饱和吸收引起的光限幅特性做了详细研究。获得了的丙酮溶液样品在纳秒激光通过三个不同光学厚度后的光限幅阈值,其高的线性透过率和低的非线性透过率性质揭示了Cu(dmit)2类材料在激光防护等光限幅领域良好的应用前景。并对这两种材料之间光限幅效应的差异从其内部分子不同阳离子结构的角度做了阐述,明确了阳离子半径更小是导致EtCu的光限幅特性强于BuCu的原因。综上所述,本论文对dmit类材料的三阶非线性光学特性以及时间响应特性做了分析,研究了含金属离子和不含金属离子材料、具有同种金属离子和不同阳离子的材料、不同金属离子的材料在不同脉宽和波长下的非线性光学现象,揭示了阳离子和金属离子对材料非线性光学响应的影响和非线性吸收与不同脉宽之间的关系。另外,通过对不同掺杂浓度的dmit／PMMA聚合物复合薄膜的热效应和光传输损耗的研究,揭示了薄膜中非线性基元的掺入量对其热致损耗和光传输损耗的影响。通过对材料的筛选,获得了一些可以在全光开关及光限幅等领域有着良好应用前景的非线性光学材料。在现有材料的探索和性能研究的基础上,通过揭示材料分子结构以及外部因素与材料功能特性之间的内在联系,找到了一些解决问题的有效方法和途径,重要的是为将来进一步的深入研究指明了方向。本论文的工作将为下一步的波导器件制作与研究工作,提供重要的实验基础。更多还原

【Abstract】 All-optical switching can break through the bottleneck of single-channel transmission velocity of other switching such as electro-optical switching, acousto-optical switching, thermal-optical switching and mechanical-optical switching and make the data transmit from source node to aim node within optics field. Therefore, research of key materials and devices which can achieve all-optical switching is a frontier issue of the development of international photoelectric science and technology in today and to be paid high attention by people.There are various type of all-optical switchings, in which the most representative Mach-Zehnder interferometric all-optical switching makes use of the third-order nonlinear optical (NLO) effect of materials. A controlled beam is irradiated into material resulting in variety of refractive index. When a signal beam is irradiated to the material, its phase changes, then achieve the "on" and "off" of switching. The change of phase is proportional with 2πn2IL/λ, where I is intensity of irradiance, L is the interaction length of light and n2 is the nonlinear refractive index. The properties of all-optical switching, such as switch velocity, energy loss, sensitivity for polarization, insert loss and so on, are all determined by third-order NLO materials which are used in fabrication of devices.For a long time, the research about third-order NLO materials both at home and abroad are almost focused on ferroelectric liquid crystal basing on molecular orientation, semiconductor based on band gap resonance absorption, metallic oxide, chalcogenide glass, organic dye, fullerene derivative and polymers such as PDA, PA, PPV and the like. Among above-mentioned materials, the response velocity of liquid crystal is too slow, the nonlinear refractive indices of organic dye and polymers are too small, the nonlinear absorption of semiconductor is too large because of the action of resonance absorption in band gap as well as its slower response velocity. In a word, all above-mentioned materials are not suitable for the demand of all-optical switching device.In the dissertation, basing on the demand of all-optical switching application, we devote ourselves to explore a series of complexes which have properties of larger third-order NLO effects and higher mobility. Especially when metal ion is introducted into the conjugated system of complex, the charge transfer between metal and organic system makes the electron delocalization property of total conjugated system stronger, then further enhances the third-order NLO response and electron mobility, and induce off-resonant third-order NLO effects with ultrafast response time. In the earlier works, the conductive properties of 4,5-dithiolato-1,3-dithiole-2-thione (abbreviated as dmit) complexes were widely concerned. Recently, It has been gradually found that the molecules of these materials have excellentπelectron conjugated system, and thus the beginning of their third-order NLO properties. In this dissertation, we will make investigations focused on these classes of organic/inorganic complex semiconductors and study their third-order NLO properties as well as other relative characteristics.In previous works, our group has done in-depth studies on the third-order NLO properties of dmit complex in solution and had selected several materials which have larger nonlinear refraction and smaller nonlinear absorption. In this paper, we aimed at several typical dmit complex and did some systemic studies for their third-order NLO properties in PMMA film. The factors that impact the NLO properties of materials were discussed from their inherent mechanism. The thermal-induced loss and light transmission loss which can affect the NLO response and optical quality of the film were also explored preliminarily. The purpose is to get NLO materials which are applicable for all-optical switching device and ideal methods that can enhance nonlinear refraction and weaken nonlinear absorption, and provide experimental and theoretical evidence for the final device of the materials. The works of the dissertation are mainly reflected in the following aspects:1. Investigations of third-order NLO properties of dmit/PMMA polymeric complex films. Screened out several candidate materials which have properties of larger nonlinear refraction and smaller nonlinear absorption and can be used in all-optical switching device. Discovered the laws which the third-order NLO properties are closely related to central metal ion and structure.All-optical switching device demands materials have properties of not only larger nonlinear refraction but also smaller linear and nonlinear absorption. For the parameters obtained by Z-scan method such as nonlinear refractive indices, nonlinear absorptive coefficients and so on, two figures of merit W=n2I/α0λ和T=βλ/n2 were used to analyze the suitability of a material for application in all-optical switching devices. For the application purpose, it is necessary to achieve (?)＞＞1 and (?)＜＜1.For the research of the dmit comploexes with the metal ions are Au and Cu we found the two types of materials both show larger nonlinear refraction. Cu(dmit)2 compounds show larger nonlinear absorption compared with Au(dmit)2 compounds. The obtained figures of merit of two Au(dmit)2 compounds: [C16H33(CH3)3N][Au(dmit)2] (CTAu) and [(CH3)4N][Au(dmit)2] (MeAu) both satisfy the demands of (?)＞＞1and (?)＜＜1 . However, the values T of Cu(dmit)2 compounds are much larger than 1. We can conclude from those results that the two materials: CTAu and MeAu both satisfy the demands of all-optical switching devices for the NLO materials and have a promising prospect in the field of all-optical switching device. While for Cu(dmit)2 compounds are not suitable for use in all-optical switching applications.By increasing the doping concentration of nonlinear elements into the BFDT/PMMA polymeric complex films, the nonlinear responses of the films were enhanced obviously. With the nonlinear refraction enhanced, the change of nonlinear absorption was tiny. Otherwise, besides the larger nonlinear refraction effect, the BFDT/PMMA films showed tiny nonlinear absorption so that it can be ignored. The values that W were about at between 1.05～3.87 and T≈0) is up to the demand of (?)＞＞1 and (?)＜＜1. Therefore, BFDT is also a better candidate material which may be applied in fabrication of all-optical switching device in the future.2. Investigations of ultrafast time response properties of dmit complexes.Using femtosecond optical Kerr technology, the response time of OKE signal of dmit materials were all about 200 fs. Which were faster than that of current popular electro-optic switching about 4～6 orders of magnitude. The response time of some materials were first reported in this dissertation. Besides the third-order NLO effect induced by largerπ-conjugated structure, the ultrafast response velocity properties of dmit materials compared with other materials is the main reason that we select the type of materials as our study subject.3. Investigations of thermal effects of dmit/PMMA polymeric complex films.Obtained the law that, with the increase of the nonlinear elements in the film system, the influence of thermal effect is accordingly enhanced as well as the NLO effect. The thermo-optic coefficients of dmit/PMMA films were calculated to be at 10-5/℃magnitude by using a prism-coupler system attached temperature-controlled apparatus. The change of refractive index induced by thermal effect is the major factor that disturb the implementation of switch and response velocity. The influence of thermal effect can be depressed effectively by reducing the linear and nonlinear absorption of film systems.4. Investigations of light transmission loss of BFDT/PMMA polymeric complex films.Reducing the light transmission loss of waveguide film can improve the optical stability and practicality, then optimize the switching performance of waveguide device. By comparing the light transmission loss coefficients of BFDT/PMMA films at different doping concentrations, we obtained the tendency that the transmission loss of the film increases approximate linearly with the increase of doping concentration. Improving the fabrication technique and surface treatment technology of film and reduce scattering points in film is one of the effective methods to reduce the light transmission loss.Dmit/PMMA polymeric complex films were prepared using polymer coated method. We used surfactant as the coating agent. The films were observed using a scanning probe microscopy (SPM) and it was found that nanocrystalline particles separated out from PMMA matrix were distributed uniformly on the surface of the films. The particle scales were about within 20～50 nm. Dmit complex were doped into the polymeric PMMA matrix uniformly. The light transmission properties of BFDT/PMMA polymeric complex film which is prepared by polymer coated method were improved obviously.5. Investigations of optical limiting properties of Cu(dmit)2 compounds at the excitation of nanosecond pulse laser.Analyzed using a five-level mode, the conclusion that Cu(dmit)2 compounds shows the property of reverse saturable absorption at the excitation of 18 ns pulse was obtained. Then the optical limiting induced by reverse saturable absorption was studied in detail. We obtained the limiting thresholds of [(C2H5)4N]2[Cu(dmit)2] (EtCu) and [(C4H9)4N]2[Cu(dmit)2] (BuCu) in acetone solution in three different thicknesses by 18 ns pulse excitation. The larger linear transmittance and lower nonlinear transmittance of Cu(dmit)2 complexes indicated their promising prospects in the field of optical limiting such as laser protection. The difference of the two materials was expatiated from the perspective of different cations in molecular structure. It was confirmed that the smaller cation radius is the reason that the optical limiting effect of EtCu is stronger than that of BuCu.In conclusion, in the dissertation we analyze the third-order NLO and time response properties. The NLO responses of dmit materials which have metal ion and no metal ion, have same metal ion but different cation, have different metal ion were investigated at conditions of different pulse width and wavelength. The influence of cations and metal ions for NLO response of materials and relationship between nonlinear absorption and different pulse width were also indicated in the dissertation. In addition, by investigation on thermal effect and light transmission loss of dmit/PMMA polymeric complex films with different doping concentrations, we indicated the influence of doping concentration of nonlinear elements for the thermal-induced loss and light transmission loss. By screening materials, we obtained some NLO materials which would be promising materials at fields of all-optical switching or optical limiting. Basing on exploration and characterization of current materials, through indicating the internal relationship between molecular structures, external factors and functional features of materials, we found some effective methods of solving problems, and what is important, pointed the way for the further in-depth investigation in the future. The works of the dissertation will provide important experimental foundations for the further fabrication and investigation of waveguide devices.更多还原