【作者】 孙晶；

【导师】 许东； 任诠； 王新强；

【作者基本信息】 山东大学 ， 材料物理与化学， 2009， 博士

【摘要】 现代社会已进入光信息化时代,人们对信息的需求也在急剧增加。在目前使用的光纤通信系统中,由于在信号转换方面存在着较多的光—电、电—光转换器件,大大影响了信息的快速传输,造成了通信系统中的“信息瓶颈”。为了解决这个问题,采用具有超快响应速度的全光开关构建全光网络成为必然。全光开关通常都基于非线性光学原理工作,其中非线性折射型全光开关,主要利用了材料的三阶非线性折射特性,即通过一束控制光引起材料折射率的变化,使得信号光在其中通过时产生相位的变化,从而实现光开关的开关动作。评定材料是否适用于全光开关有两个品质因子:W=n₂I/α₀λ和T=βλ/n₂,其中n₂为非线性光学折射率,I为测试光强,α₀为线性吸收系数,λ为测试波长,β为非线性吸收系数,材料必须满足|W|>>1且|T|<<1才能用于全光开关。因此这类全光开关对材料性能的基本要求是:1、在工作波段有大的三阶非线性折射率,从而可使用光功率密度较低的控制光,降低对器件造成的损伤;2、在工作波段具有小的线性和非线性吸收,进而降低信息传输损耗,减小热效应的影响,提高开关速度,增强系统可靠性和稳定性;3、具有超快的非线性光学响应速度,实现与全光网络的匹配;4、具有稳定的物理化学性质,易于与基质材料复合并可进行波导器件的制备。国际上关于全光开关器件的研究,既包括技术和结构方面的创新与改进,也包括新的作用机理和材料方面的寻找和探索。由于目前还没有找到性能全面优异理想的适用于全光开关器件研制的非线性光学材料,因而,全光开关仍未进入实用化,探索新型的具有优良三阶非线性光学性能、高品质因子的非线性光学材料还是目前国际上最主要的任务。本课题组一直致力于寻找适用于全光开关的三阶非线性光学材料,通过对大量材料设计与实验研究发现,过渡金属的DMIT类材料具有大的平面共轭结构,易发生极化和电荷转移,金属和有机体系之间的电荷转移可以进一步增强材料的三阶非线性光学性质。同时还注意到该类材料分子中的阴离子是富含硫的离子基团,硫原子通过S…S相互作用,可在分子间形成有效的轨道重叠,使得材料的介电常数小,电子迁移率高,响应速度快,在全光开关的应用上有潜在价值。由于全光开关要求材料具有大的非线性折射和小的非线性吸收,如何从影响材料非线性光学性质的众多因素中寻找规律,进一步提高材料的三阶非线性折射率,降低线性及非线性吸收就显得尤为重要。本论文在前期工作基础上,集中选择了具有较小非线性吸收的Cu（dmit）₂和Au（dmit）₂系列材料为研究对象,采用Z扫描方法分别对材料在溶液和薄膜中的三阶非线性光学性质进行了较系统的性能研究,从内部机理探讨了影响材料非线性折射和非线性吸收的各项因素,其目的是为了寻找到提高材料的三阶非线性折射率,降低非线性吸收的方法,为材料的器件化提供理论依据。本论文研究工作主要体现在如下几个方面:第一,研究了Cu和Au的一系列DMIT类配合物材料中,中心金属离子及外部阳离子对材料非线性折射率和非线性吸收的影响,发现其三阶非线性光学性质与中心金属离子及结构密切相关。我们发现Cu（dmit）₂和Au（dmit）₂在全光通信波段（1.3～1.6μm）具有较小的线性及非线性吸收,因此,本论文选择了Cu（dmit）₂及Au（dmit）₂作为全光开关研究的候选对象,其中,TPPPADTA、TPEPADT、TPPPADT等为国际上首次报道的新型光学材料。采用Z扫描方法分别对Cu和Au的DMIT类材料的三阶非线性光学特性进行系统的研究,得到了材料的三阶非线性极化率、非线性折射率、非线性吸收系数、双光子吸收截面和激发态吸收截面等性能参数,并结合测试样品的浓度计算得到了材料的分子二阶超极化率。通过对材料性质的深入研究发现,获得了一些重要的研究结果:A.DMIT配合物材料的非线性折射性质,与中心金属离子有密切关系。具有相同金属离子的材料表现出相同的非线性折射性质,在1064 nm处,浓度为1×10^-3mol/L的Cu（dmit）₂/乙腈材料均表现出自散焦效应。B.中心金属离子对材料的非线性吸收也有很大影响。具有相同外部阳离子和浓度的TMACDT与TMAADT材料,在1064 nm处,TMACDT表现出双光子吸收,而TMAADT的非线性吸收很弱,基本没有测到。C.在同一中心离子形成的配合物中,体积较小的阳离子对材料的三阶非线性折射或非线性吸收有增强的趋势。在Cu（dmit）₂和Au（dmit）₂材料中,均是具有最小阳离子的材料TMACDT和TMAADT的非线性折射率最大。这些结果说明金属形成的配位阴离子是导致材料具有非线性光学性质的主要原因,选择具有较小阳离子的非线性光学材料有可能获得更好的三阶非线性光学性质。第二,研究了各项外部因素（激光的脉宽、波长、样品浓度等）对DMIT配合物的三阶非线性折射和非线性吸收的影响,发现在材料共振及近共振区测得的非线性吸收较大;使用较长脉宽光源测得的材料的非线性光学系数更大;材料的非线性光学效应随溶液浓度的增大而增强。采用Z扫描方法分别在1064 nm和532 nm的脉冲照射条件下对Cu和Au的DMIT类材料的三阶非线性光学特性进行比较,我们发现:A.同种材料在测试波长位于材料共振及近共振区时,测得的非线性吸收比较强,其三阶非线性光学效应也更强。在532 nm处,1×10^-3mol/L的Cu（dmit）₂/乙腈溶液的非线性吸收系数为10^-12 m/W,而在1064 nm处,相同材料的非线性吸收系数为10^-13 m/W,比532 nm处小一个数量级,这是由于532 nm位于Cu（dmit）₂材料的共振区,其线性吸收较大,也更易于发生非线性吸收。B.同种材料在不同脉宽条件下测得的非线性光学性质也不尽相同,TMACDT在皮秒脉宽激光作用下是双光子吸收,而在纳秒脉宽激光作用下则表现为反饱和吸收,运用五能级模型对出现这种差异的原因进行了分析,表明材料在皮秒脉冲作用下的非线性产生机制归因于单重态激发态的态态跃迁,而在纳秒条件下则主要是由于三重态激发态的态态跃迁导致的。由于材料非线性光学效应的产生机制不同,导致采用脉宽较长的光源时,得到的计算结果要相对大一些。C.通过对1064 nm,20 ps条件下,1×10^-3mol/L的Cu（dmit）₂/乙腈溶液及同浓度的Au（dmit）₂/乙腈溶液的三阶非线性光学性质研究,发现Cu（dmit）₂/乙腈溶液的三阶非线性折射率一般为10_-13～10^-12esu,但是其非线性吸收比较大,而相同浓度的Au（dmit）₂配合物表现出了高的非共振三阶非线性光学效应,其中TMAADT的非线性折射率为-4.11×10^-12esu,未测到非线性吸收,进一步计算得到TMAADT的品质因子|W|值为2.87,|T|值约为0,基本满足全光开关对材料品质因子W=n₂I/α₀λ,和T=βλ/n₂（且|W|>>1,|T|<<1）的要求。第三,采用溶胶—凝胶法将金属DMIT配合物制备成复合薄膜,用Z扫描方法研究了薄膜状态下材料的非线性光学性质变化,以及掺杂浓度等条件对Au（dmit）₂/PMMA复合薄膜的非线性折射及非线性吸收的影响,获得了比较理想的实验结果。A.发现Au（dmit）₂/PMMA复合薄膜可以很好满足全光开关对材料品质因子的要求。在1064 nm,20 ps条件下,对Au（dmit）₂/PMMA复合薄膜材料的品质因子计算发现|W|比溶液中的要高2个数量级,而由于未测到非线性吸收,因此可认为|T|近似为0,完全满足全光开关对材料品质因子|W|>>1,|T|<<1的要求。B.掺杂浓度为1%的Au（dmit）₂薄膜的三阶非线性光学系数相对于1×10^-3mol/LAu（dmit）₂/乙腈溶液的要强2个数量级,Au（dmit）₂/PMMA复合薄膜的三阶非线性折射率可达到10^-10 esu,未测到非线性吸收。改变薄膜中的掺杂浓度,也发现材料的三阶非线性光学系数随着掺杂浓度的提高而增大。证明可以通过提高薄膜中DMIT材料的掺杂浓度来提高其三阶非线性光学系数。本论文通过对Cu和Au的DMIT类材料三阶非线性光学性质的探索研究,发现这两类材料具有优良的三阶非线性光学性能,其中Au（dmit）₂材料具有三阶非线性折射大、非线性吸收小和响应速度快的特点,可以满足全光开关对材料品质因子的要求,并通过制备该材料三阶非线性光学薄膜与性能测试工作,证实这一重要结果。它将为下一步波导器件制备与研究工作,提供依据和重要基础。更多还原

【Abstract】 Our society is in the "Information Age".The traditional correspondence technique can not satisfy with people’s demands,which has increased rapidly in the amount of information.In the present communication system,the optical fiber communication system（OFCS） used many electronic components with some disadvantages such as slow switching speed,clock displacement,serious crosstalk and high power loss which can easily lead to the "bottleneck" of the system.Under such conditions,it is necessary to construct the "all-optical network" while information exchange at points adopts all-optical switch（AOS）.In general,AOS is based on nonlinear optical（NLO） effects.Taking advantage of the third-order nonlinear refraction of materials,AOS’ work principle can be described as follows:With a control beam inducing the change of refractive index of material,the other signal beam transmitting in the material will produce an additional phase difference to realize the function "On" or "Off" of optical switches.There are two factors defined as W=n₂I/α₀λ.and T=βλ/n₂,while n₂ is the NLO refraction,I is the laser intensity,α₀ is the linear absorption,λis the wavelength,βis the nonlinear absorption. To make the practicable all-optical switches,NLO materials must simultaneously satisfy |W|>>1 and |T|<<1.Therefore,the materials must satisfy:1.large nonlinear refractive index （n₂） at the operating wavelength which can reduce the power density of control light; 2.minimal linear and nonlinear absorption which lower the information transmission loss,increase the switching speed and then enhance the reliability and stability of the system;3.ultrafast nonlinear refractive index responses and relaxation times （picoseconds or less）;4.easy to make wave-guides.Currently,the investigations of AOS device,include not only the improvement and innovation of technologies and structures but also the exploration of new action mechanism and novel materials.Currently,people still do not discover such materials whose performances are comprehensively suitable to the development of these NLO devices.Therefore,it seems to be particularly important to explore and synthesize novel materials possessing excellent third-order NLO properties and good factors of merits.Our group has studied many materials,and found that the transition metal DMIT complexes possess largeπ-electron delocalization,charge transfer and high electromigration rate in the systems.Hence the characteristics should result in an ultrafast optical response capability and large third-order NLO effects.The AOS require materials with large NLO refraction and small NLO absorption,hence it is important to discover the rules which affect the NLO properties of the materials.This dissertation takes the Cu（dmit）₂ and Au（dmit）₂ as the object of the research. Researches the NLO properties of materials by Z-scan technique,and discussed the factors which affect the NLO refraction and NLO absorption for the purpose of raise the NLO refraction and NLO absorption of the materials.The dissertation mainly comprises the following aspects:Firstly,a series of metal-dmit complexes whose central metal ions are Cu and Au were explored,it is found that the NLO properties come of the central metal ions.The Cu（dmit）₂ and Au（dmit）₂ materials have small linear and NLO absorption at the correspondence wavelength（1.3～1.6μm）,therefore this dissertation takes the Cu（dmit）₂ and Au（dmit）₂ as the object of the research,TPPPADTA,TPEPADT and TPPPADT were reported for the first time as new optical materials.In this dissertation,we investigated systematically the optical nonlinearities of DMIT materials by Z-scan technique and obtained the third-order nonlinear susceptibility,third-order nonlinear refractive index,nonlinear absorption coefficient, molecular second-order hyperpolarizability,absorption cross section of ground and excited states,and two-photon absorption cross section,etc..We also investigated the factors which effact the third-order NLO properties of materials:A.It is found that the central metal ions influence the NLO refraction of DMIT complexes.The materials with same central metal ion exhibited same NLO refraction, the Cu（dmit）₂ materials with the concentration 1×10^-3 mol/L have self-defocus at 1064 nm.B.The central metal ions also influence the NLO absorption.TMACDT and TMAADT with same cations and concentration have different NLO absorption, TMACDT has two-photon absorption and TMAADT without NLO absorption.C.The complex with same central metal ion and smaller cation represents large third-order NLO coefficients.TMACDT and TMAADT have larger NLO refraction than other materials with same central metal ion.It indicates that the central metal ion play an important role on the NLO properties and choose the materials with small cations can obtain the NLO properties with larger coefficients.Secondly,the third-order NLO refraction and absorption of DMIT materials were systematically studied.It is discovered that the materials exhibit larger NLO absorption at resonance region and at broad pulse width.In this dissertation,we investigated systematically the optical nonlinearities of DMIT materials in both 1064 and 532 nm.A.The wavelength and pulse width of the laser affect the NLO parameters.The NLO absorption coefficients of material that obtained at resonant wavelength are larger than those obtained at other range.The NLO absorption of Cu（dmit）₂ is 10^-12 m/W at 532 nm while 10^-13 m/W at 1064 nm.B.The NLO properties for the same material at different pulse width are also distinct.TMACDT represent two-photon absorption at picosecond laser pulses while reverse saturation absorption at nanosecond laser pulsed.This can be explained by the five-level model:the nonlinear absorption was dominated by the singlet-singlet excited state transitions for picosecond laser pulses,while for nanosecond pulses it can be mainly attributed to the triplet-triplet excited state transitions.The different origins induced the results that obtained at nanosecond are larger.C.With 20 ps laser pulses at 1064 nm,Cu（dmit）₂ possesses high third-order NLO refraction which is obtained as 10^-13～10^-12 esu and NLO absorption.Au（dmit）₂ exhibits non-resonant third-order optical nonlinearities,the NLO refraction of TMAADT is -4.11×10^-12 esu and with no NLO absorption which satisfy the requirements of AOS for materials’ figures of merit,W=2.87 and T=0（|W|>>1 and |T|<<1）.Thirdly,we prepared the composite film of DMIT complexes by sol-gel process and investigated the NLO properties of Au（dmit）₂/PMMA film by Z-scan technique and obtained some conclusions.A.It is found the Au（dmit）₂/PMMA film can satisfy the requirements of AOS.With 20 ps laser pulses at 1064 nm,Au（dmit）₂ possesses high non-resonant third-order optical nonlinearities and the figures of merit are two orders of magnitude larger than which are measured in solution.Thus the Au（dmit）₂ can satisfy the requirements of AOS for materials’ figures of merit |W|>>1 and |T|<<1.B.Compared with the results that obtained in solution,the NLO parameters of composite film are two orders of magnitude larger than which are measured in solution, the NLO refraction of Au（dmit）₂/PMMA film can attain as 10^-10 esu.The phenomenon revealed that the concentration of DMIT materials in film is higher than that in solution. It is also found the NLO coefficients are enhanced with the doping concentration increases.Hence we can enhance the NLO properties by enhance concentration of DMIT materials in film.All the outcomes suggest that Cu（dmit）₂ and Au（dmit）₂ materials possess good NLO properties and Au（dmit）₂ materials have large NLO refractions,small NLO absorptions and ultrafast response which can satisfy the requirements of AOS.All the conclusions can provide a basis and an important foundation for the preparation of waveguide devices in the future.更多还原