【作者】 张波；

【导师】 侯延冰；

【作者基本信息】 北京交通大学 ， 光学工程， 2012， 博士

【摘要】 共轭聚合物材料具有荧光量子效率高、受激发射截面大、吸收光谱与发射光谱交叠区小、制备工艺简单等特点,是非常理想的激光增益介质。近年来,聚合物激光已经成为有机发光领域引人注目的研究方向。然而,实现注入式电泵浦激光器仍然是巨大的挑战。针对聚合物材料的受激辐射阈值较高、金属电极引起光学损耗较大等问题,本论文研究了以共轭聚合物MEH-PPV光波导作为增益介质,围绕着如何降低聚合物光波导的放大的自发辐射(ASE)阈值、减少金属电极损耗、基于聚合物受激辐射进行电场调制应用等方面开展了以下几个方面的工作：1.利用溶剂蒸汽处理、电场生长和纳米掺杂对薄膜形貌、内部分子结构和波导中分布反馈进行了改善,有效地降低了聚合物光波导的受激辐射阈值。(1)利用不同溶剂蒸汽对聚合物薄膜进行处理改善聚合物薄膜的形貌。研究结果表明,溶剂蒸汽改变了有机溶剂的蒸发速率和聚合物分子自组装,薄膜的表面粗糙度明显降低,大约降低了一倍。从薄膜的AFM图中可知,氯仿的作用更为明显,薄膜表面粗糙度变低,光在MEH-PPV波导中的传播损耗降低,薄膜的受激辐射阈值更低,增益更高。(2)利用垂直电场下制备聚合物薄膜改变薄膜内部分子排列。在旋涂过程中,合适电场作用下聚合物分子产生了取向,使聚合物分子链沿水平方向定向排列,减小了光子在链间的传输损耗,提高了净增益系数,受激辐射阈值降低了21%,使电场作用下制备的MEH-PPV薄膜更容易实现受激辐射。(3)掺入适量的TiO2纳米颗粒到MEH-PPV薄膜中有利于薄膜的受激发射光在纳米颗粒间形成随机谐振腔,出现随机激光。增加了单位长度的增益,提高了增益系数,降低了激射阈值。2.选取不同的金属作为电极,探究金属电极影响聚合物光波导受激辐射的因素。(1)研究了以金属银作为衬底的聚合物光波导受激辐射特性。研究发现,Ag电极对受激辐射有猝灭作用,该结构的受激辐射阈值比没有金属电极衬底的聚合物薄膜高6.5倍。研究结果表明聚合物的厚度对受激辐射的影响很大。当聚合物薄膜的厚度较低时,金属电极对发光猝灭很严重,但随着MEH-PPV薄膜厚度增加,猝灭现象减弱。由此可知在发光层与金属电极附近光生激子的猝灭影响了受激辐射。(2)研究了不同金属电极对聚合物光波导受激辐射的影响。当以高折射率的Al金属作为电极时,光谱窄化的现象完全消失,受激辐射被Al电极猝灭。当电极为低折射率的金属层Ag或Cu时,出现受激辐射现象。但由于金属电极对发光的吸收损耗,金属覆层聚合物光波导的受激辐射阈值增大。当在活性层和电极之间镀一层SiO2绝缘层时,高折射率的Al金属作为衬底的聚合物光波导的受激辐射现象复现。由于不同金属电极的对光的吸收系数不同,器件的受激辐射阈值不同。研究结果表明,折射率低、反射率高的Ag电极制备的聚合物光波导受激辐射器件的增益最高、阈值最低。3.设计了用MEH-PPV作为光功能层、以SiO2为辅助层,双电极包裹的光波导器件,研究了电场调制下ASE。(1)研究了器件ITO/SiO2/MEH-PPV/SiO2/Al在电场下的受激辐射的特性。研究显示,MEH-PPV发射光谱除了在620nm处出现受激辐射,在580nm处出现明显的自发辐射肩峰。在电场作用下,受激辐射和自发辐射的电场猝灭现象出现,原因是在电场的作用下,激子离化的作用使发光强度降低。相比之下,受激辐射比自发辐射在电场下猝灭更为明显。我们通过理论分析,给出了自发辐射与放大自发辐射和电场强度的关系。此机制可用于电场调制光放大器。(2)研究了在不同厚度SiO2包覆的MEH-PPV光波导的受激辐射特性。随着SiO2层厚度的增加,MEH-PPV光波导的受激辐射发射强度增强。研究表明,SiO2包裹层从50nm增加到100nm, MEH-PPV光波导的有效折射率提高,100nm厚的SiO2包裹层的器件的受激辐射阈值比50nm厚的SiO2包裹层的器件降低了2.5倍。(3)基于上述的研究结果优化了电场调制聚合物受激辐射的器件结构,以折射率低、反射率高的Ag作为电极。研究结果表明Ag电极降低了光波导传播损耗。在高电场的作用下,激子离化显现,受激辐射发光强度几乎完全猝灭。更多还原

【Abstract】 Conjugated polymers are attractive laser gain media materials due to their high photoluminescence efficiencies, large stimulated emission cross section, low fabrication cost and chemically tunable emission wavelength. This dissertation focuses on the study of the amplified spontaneous emission (ASE) characteristics of poly [2-methoxy-5-(2’-ethyl-hexyloxy)-1,4-phenylenevinylene](MEH-PPV) film in order to solve the high threshold and absorption loss from metallic cathode. Some innovative researches as following were carried out.1. Three ways to reduce threshold of ASE from polymer waveguide was studied. The threshold of ASE can be reduced by changing polymer film morphology, inducing orientation of polymer chains and making resonant feedback structure.(1) The enhancements of ASE from MEH-PPV have been achieved via solvent vapor treatment. Correlations between the morphology of the films and the optical performance of polymer-based ASE are investigated. The results show that solvent-vapor treatment can induce self-organization of MEH-PPV into an ordered structure and a smooth surface, leading to the enhanced optical gain. Thus the solvent-vapor treatment is a good method to improve the ASE optical properties of MEH-PPV.(2) An electric field is applied to induce the polarization of MEH-PPV molecule during the process of spin-coating, which cause the orientation of the MEH-PPV chains. The orientated thin film shows lower threshold and higher gain than the normal thin films.(3) ASE from composition of MEH-PPV and TiO2nanocrystals was observed. The role of TiO2nanoparticles plays resonant feedback in improving performance of amplified spontaneous emission.2. The effect of metallic cladding on amplified spontaneous emission of MEH-PPV films was studied. The low optical loss metallic cathode was picked out.(1) Amplified spontaneous emission from an Ag-backed MEH-PPV film controlled by the effect of MEH-PPV film thickness was investigated. The study of ASE characteristics of Ag-backed MEH-PPV films with different thicknesses shows that increasing the film thickness can reduce the quenching which happened in area near to the Ag cladding. (2) The effect of metallic cladding on amplified spontaneous. emission of MEH-PPV films was studied. The results showed that the metallic layer is critical for the ASE of MEH-PPV. The ASE threshold of MEH-PPV films with Ag cladding is11times greater than that of sample without Ag cladding layer. Furthermore, ASE from the samples with Al cladding completely vanishes. ASE recurred when a thin spacer layer, such as a few nanometers of SiO2, is introduced between the MEH-PPV film and the Al cladding. Compared with the Cu or Al electrode, the Ag is most suited as low loss electrode with due to its low refractive index and high reflectivity.3. Electric-field modulation of the ASE in waveguide based on MEH-PPV was studied.(1) The electric-field induced quenching of the ASE at620nm and the spontaneous emission (SE) at580nm was observed which increased. The quenching increases with the rising electric field. The ASE intensity was more effectively quenched than the SE intensity as the field increased. The field dependence of the ASE and the SE intensity can be attributed to field-induced dissociation of photogenerated excitons in the MEH-PPV thin film.(2) ASE from MEH-PPV waveguide with different thickness of SiO2cladding was investigated. Availability refractive index of MEH-PPV waveguide increases with the rising thickness of SiO2cladding. The study of ASE characteristics of MEH-PPV films with different thicknesses SiO2cladding shows that increasing film thickness can reduce lasing threshold of polymer film.(3) ASE in optically pumped MEH-PPV waveguides with Al or Ag electrode was investigated. The results show that Ag electrode can reduce the ASE threshold compared with Al. The electric-field induced quenching of the ASE has been observed. When the electric field reaches8×106V/cm, the ASE intensity in the device with the Ag electrode is nearly shut off.更多还原