高转换效率的液晶/聚合物光栅分布反馈激光器研究

Distributed feedback (DFB) lasers based on liquid crystal/polymer gratings have the advantages of easy processing, low cost, and ability to be electrically tuned. They have been widely investigated by researchers for the potential in providing ultra-thin, low-cost and wavelength tunable laser sources. However, the laser dyes as original gain medium in this organic laser suffer from concentration quenching which limits its conversion efficiency. We intend to adopt the organic semiconductors to replace the dyes to enhance the laser performance. First, we will design a new kind of DFB working structure with external feedback based on the foundations of coupled wave theory and waveguiding theory; then, we will obtain single mode lasing operation through the optimization of gain medium and structure parameters; after that, we will further improve the laser performances by fabricating second-order DFB lasers or enhancing the refractive index modulation; Finally, we will extend the wavelength tuning range by using high birefringence liquid crystals and perform investigations on the device stability. Our goal is to fabricate a new kind of organic laser with high energy conversion efficiency, singe-mode lasing output and large wavelength tuning range based on liquid crystal/polymer gratings. The results of our research will give theoretical and technical support to the utilization and development of organic DFB lasers. It is a novel laser light source with great potential for everyday use which is ultra-thin, highly efficient, low cost and multifunctional lasers.

液晶/聚合物光栅DFB激光器具备制备简单、价格低廉、电场调谐等优势，被研究者广泛研究，其可为人类带来一种全新的超薄、高效、廉价、可调谐激光光源。但是该激光器的激光染料存在浓度淬灭缺陷，导致光能转换效率低。本项目拟采用有机半导体材料作为增益介质，解决激光染料浓度淬灭问题。为此，首先利用耦合波和波导理论分析激光器的工作机理，引入有机半导体材料，并设计全新的、增益层和光栅层分离的工作结构。然后进行有机半导体材料的优化选取和结构参数的优化设计，获得单纵模激光输出；接着通过制备2级激光器和提高折射率调制量，进一步提高激光器的能量转换效率。最后，利用高双折射液晶和高液晶含量来获得大调谐范围，并进行激光器的稳定性研究。最终获得光能转换效率高、单纵模输出、大调谐范围和工作稳定的、基于有机半导体材料的液晶/聚合物光栅DFB激光器。该研究结果为液晶/聚合物光栅DFB激光器的实用化和快速发展提供理论和技术支持。

在本项目中，将有机半导体薄膜作为增益介质引入到液晶/聚合物激光器中，由于该材料无法和预聚物互溶，所以设计了外反馈DFB腔。液晶/聚合物光栅单独制备在有机半导体薄膜上面，两层通过疏式波进行相互作用。实验表明，增益薄膜厚度在75nm左右时候，器件可以在增益以及反馈两个物理过程之间得到平衡，从而得到最佳的激光性能。与基于染料的激光器相比，这种激光器的性能有大幅度的提高：单模出射，TE线偏振，阈值降到原来十分之一（21μJ/cm2），能量效率上升了六倍（5.9%）。.另外，选取非芳香族极性溶剂四氢呋喃以及三氯甲烷，芳香族弱极性溶剂氯苯、对二甲苯以及甲苯溶解MEH-PPV。探讨不同溶解环境下，MEH-PPV分子旋涂成膜后，分子排列取向以及薄膜发光情况。波导ASE实验结果表明，非芳香族极性溶剂溶解MEH-PPV时，薄膜具有更高的净增益，将有助于器件阈值降低以及转化效率的提升。四氢呋喃溶解MEH-PPV时，激光出射阈值6.7μJ/cm2，转化效率高达9.5%。.最后制备了激光染料DCM掺杂的液晶聚合物光栅，控制MEH-PPV厚度为150nm，使其不发光，引人准波导结构。波导ASE实验表明，相同泵浦条件下，准波导结构的净增益大于波导结构的净增益，亦即光信号在准波导结构传播时，将更加有效被放大，有利于降低激光出射阈值，提升转化。在准波导结构中，通过改变光栅周期，实现了573.2至722.3nm共149.1nm激光出射。在高衍射级次中，利用准波导结构中实现了双波长激光出射，双波长激光出射范围超过40nm。最后，将MEH-PPV薄膜控制在80nm，成功制备了三波长激光器，将温度从室温加热至清亮点以上，其温度调谐范围高达8nm。.本项目的完成将为得到连续的、电场可调谐、电流注入激射的有机半导体DFB激光器提供新的思路和理论依据。