【作者】 胡志家；

【导师】 张其锦；

【作者基本信息】 中国科学技术大学 ， 高分子化学与物理， 2014， 博士

【摘要】 相对于传统的激光,随机激光的反馈机制是基于无序介质引入的光散射。随机激光根据反馈机制可以分为两种随机激光：(1)基于能量密度或者能量反馈的非相干激光：(2)基于场或者振幅反馈的相干随机激光。在传统随机激光领域中,无论是哪一种随机激光,高的激光阈值和无方向性限制它的应用。为了解决传统随机激光的这两个难题,人们利用光纤一维束缚作用得到低阈值和有一定方向性的随机激光。目前,关于随机光纤激光的研究主要是基于分布式反馈的光纤随机激光。本论文主要研究基于纳米颗粒散射的随机光纤激光。我们进行了不同光纤波导结构和散射体系条件对随机激光的阈值、方向性、以及其机理的研究。具体内容如下所述：1.利用聚倍半硅氧烷纳米颗粒(POSS:18.5wt%)和PM597(1.5×10-2M)激光染料的二硫化碳溶液灌入到空芯光纤中,制作成液芯光纤。通过端向泵浦我们得到相干的随机光纤激光。我们发现泵浦光在液芯光纤中的损耗较小,可以从液芯光纤的一端传输到另一端。这主要是因为POSS纳米颗粒以分子水平(1-3nm)分散在二硫化碳溶剂中,并且这个分散体系很稳定,POSS纳米颗粒不会在二硫化碳溶剂中沉淀出来。超过泵浦阈值时候,在这一弱散射体系中随机光纤激光出现谐振反馈的多模激光。对于只含有同浓度PM597染料而不含有POSS纳米颗粒的液芯光纤,在同样的泵浦条件下,我们只能观察到放大自发辐射现象。2.比较了POSS/PM597的极弱散射体系分别在比色皿中和液芯光纤中的随机激光行为。在比色皿中,由于散射体系是极弱散射体系,我们只能观察到非相干随机激光,而在液芯光纤中则观察到了相干随机激光。基于可控实验和实验结果,我们得出在极弱散射系统中出现相干随机光纤激光的机理：在光纤波导条件限制和全反射作用下,液芯光纤中的纳米颗粒多重散射得到加强。这种相干随机光纤激光有望使随机激光具有低阈值、有方向性及发射波长可调控的特点,具有潜在的应用前景。3.使用Teflon法制作一种纤芯具有无序结构的聚合物光纤。这种无序聚合物光纤的纤芯材料组成为：PM597掺杂的甲基丙烯酸甲酯、甲酯丙烯酸苄酯和甲基丙烯酸甲酯基笼状低聚倍半硅氧烷(MMAPOSS)的共聚物；包层材料组成为：甲基丙烯酸甲酯和丙烯酸丁酯的共聚物。在光纤纤芯的制备过程中POSS基团在聚合反应中原位自组装形成纳米颗粒。利用POSS纳米颗粒的弱散射和光纤波导条件加强散射效应,在聚合物光纤中得到了稳定的相干随机激光。同时聚合物光纤随机激光的阈值比之前报道的液芯光纤光纤随机激光的阈值差不多低一个数量级。我们还研究了激光染料PM597掺杂的这种无序聚合物光纤中的局域化效应。4.金属纳米颗粒的局域表面等离子体共振效应对荧光分子的荧光有选择性增强,从而可以预计金属纳米颗粒在随机系统中可以降低随机激光的阈值。结合光纤波导对随机激光的束缚作用和金属纳米颗粒的局域表面等离子体共振效应,我们制作了金属纳米颗粒掺杂的液芯光纤。研究金属纳米颗粒在光纤波导条件下的局域表面等离子体共振效应。比较了在大块体系和光纤波导体系中金属纳米颗粒的局域表面等离子体共振效应与荧光分子的发射谱重叠度对随机激光阈值的影响。更多还原

【Abstract】 A random laser (RL) is a non-conventional laser, whose feedback mechanismis based on disorder-induced light scattering. Based on the feedback mechanisms, random lasers are classified into two categories:(1) random lasers (RLs) with incoherent (or non-resonant) feedback, also called incoherent RLs,(2) RLs with coherent (or resonant) feedback, also called coherent RLs. The nondirectional and high threshold characters of the traditional RL systems have largely limited their application. The confinement effect of optical fibers on the lasing properties of RL has obtained by introducing random scattering system into the fiber core, bringing about the birth of random fiber lasers (RFLs) with low threshold and directionality. To date, the research about RFLs most based on a distributed-feedback mechanism.This dissertation concerns the research on RFLs based on the scattering of nanoparticles. The influences of the structures of optical fibers and random systems on the threshold, directionality and mechanism of RLs have been studied. All the details are shown as follows:1. RFL is obtained by end pumping a hollow optical fiber filled with a dispersive solution of polyhedral oligomeric silsesquioxanes (POSS) nanoparticles and laser dye pyrromethene597(PM597) in carbon disulfide (CS2), in which the concentration is1.5x10-2M for PM597and18.5wt%for POSS, respectively. It is found that the pump light at the one end of the liquid core optical fiber (LCOF) can pass the whole length of LCOF because the POSS nanoparticles were dispersed in CS2at a molecular level (1-3nm) with high stability and without sedimentation. Above the threshold pump energy the RFL appears coherent and resonant feedback multimode lasing in the weakly scattering system. For the LCOF containing PM597with the same concentration and no POSS nanoparticles, there occurs only amplified spontaneous emission that can be observed under the same experimental condition.2. The RL action of the POSS/PM597medium with the extremely weakly scattering regime in the cuvette system and LCOF system were performed, respectively. Incoherent RL has been observed for the POSS/PM597medium with the extremely weakly scattering regime in a quartz cuvette. Strikingly, however, by end pumping a hollow optical fiber filled with the same solution we have achieved coherent RFLs under controlled conditions. Based on well designed control experiments combined with pertinent analyses, its working mechanism has been well elucidated; i.e., multiple scattering of nanoparticles in the LCOF is greatly enhanced by the waveguide confinement effect under the total internal reflection condition. We envision that the coherent RFL demonstrated in this work may open a window to future RL applications aiming at low threshold, directionality, and wavelength tunability.3. The disordered polymer optical fiber (POF) was fabricated using the Teflon technique. The core material of the disordered POF is PM597-doped poly(methyl methacrylate-co-benzyl methacrylate-co-methacrylisobutyl polyhedral oligomeric silsesquioxanes) copolymer. And the cladding material of the disordered POF is poly(methyl methacrylate-co-butylacrylate) copolymer. The realization of a random polymer fiber laser (RPFL) is based on laser dye Pyrromethene597-doped one-dimensional disordered POF. The stabilized coherent laser action for the disordered POF has been obtained by the weak optical multiple scattering of the POSS nanoparticles in the core of the POF in situ formed during polymerization, which was enhanced by the waveguide confinement effect. Meanwhile, the threshold of our RPFL system is almost one order of magnitude lower than that of the liquid core RFL reported previously. Meanwhile, the localized effect of the one dimensional disorder POF is also discussed in terms of the experimental results.4. The localized surface plasmon resonance (LSPR) of metal nanoparticles as scattering centers will induces selective enhancement of the photoluminescence, which is expected to decrease the threshold of random laser in the random system. In order to combine the effects of one-dimensional confinement on the lasing properties of a classical RL system with LSPR of metal nanoparticles we fabricated a hollow optical fiber filled metal nanoparticles and gain medium. The effect of LSPR of metal nanoparticles on the one-dimensional confinement of the fiber has been researched. The overlap between of the LSPR of the metal nanoparticles and the photoluminescence spectrum has been found to be a factor influencing the threshold the RL.更多还原