基于悬空石墨烯结构的长程传输等离激元及其性能调控研究

Graphene based plasmons have excellent optical properties such as broadband response, electrical tunability, high local field enhancement and very low intrinsic loss. Therefore, they have great application prospects in waveguide devices, sensors and metamaterials. However, the necessary conditions for the successful construction of the above functional devices are achievement of graphene plasmon with high quality factors and long propagation distance. Due to natural two-dimensional materials of graphene, it is highly susceptible to be strongly affected by the surrounding dielectric environment. Multiple damping pathways contribute to the loss of graphene plasmon, which makes its quality factor limited, thus the corresponding propagation distance is only one hundred nanometers. This project intends to construct a special suspended graphene plasmon structure, which can break through the limit of plasmonic performance in the dielectric environment. Because it allows eliminate multiple loss channels from the substrate, including charge impurity scattering, phonon scattering and dielectric loss, etc. simultaneously, thereby increasing its quality factor and propagation distance (up to micrometer level). Moreover, suspended graphene provides a special pure plasmonic environment (no interference from the substrate), which can be employed as a platform for individually studying the graphene plasmon damping mechanism and obtaining various damping channels with quantitative results. This project will develop a new way for the realization of high-performance graphene plasmon devices, and provide important experimental and theoretical support for the wide applications of graphene plasmons in nanophotonics.

石墨烯等离激元具有响应带宽广、电学可调谐、高局域场增强以及低本征损耗的优异特性，在波导器件、传感器和超材料等领域有巨大应用前景。但是，成功构筑上述功能器件的必要条件是高品质因子和长程传输的石墨烯等离激元性能。然而，石墨烯天然二维材料的特点使其极易受周围介电环境强烈干扰，多重因素促使石墨烯等离激元损耗过快，导致其品质因子有限，相应的传输距离也仅有百纳米。本项目拟构筑特殊悬空石墨烯等离激元结构以突破介电环境对等离激元性能的限制，能够同时排除基底电荷杂质散射、声子散射及介电损耗多个衰减通道，由此提升其品质因子和传输距离（达到微米级）。此外，悬空石墨烯提供了纯净（无基底干扰）的特殊等离激元环境，可对石墨烯等离激元各个衰减机制单独研究，从而获得石墨烯等离激元衰减通道的定量结果。本项目的研究内容为实现高性能的石墨烯等离激元器件提供了新思路，为石墨烯等离激元在纳米光子学领域的广阔应用提供实验和理论支持。

石墨烯为代表的二维材料中的极化激元具有响应频率宽、波长压缩大和易于调控的优势，已被证实可以轻易突破光学衍射极限，将光波长压缩到纳米尺度。因此有望实现高效光电互联，从而进一步提升光电融合系统的性能。但是，前期的国外研究表明，二维材料比表面积巨大，上下两层原子裸露，使得其支持的极化激元光学模式极易受到介电环境的干扰。本项目的研究目标是突破介质环境对等离激元性能的限制，实现极化激元的高效传输和调控功能。.经过三年研究，项目已经超额完成立项研究目标，完成内容包括：石墨烯的光电响应性能表征研究；高质量悬空石墨烯等离激元结构的制备和基本传输、调控规律的研究；悬空石墨烯等离激元体系中不同因素对等离激元损耗规律的研究；石墨烯与α相氧化钼异质结中杂化极化激元的耦合与调控研究；石墨烯等离激元与氧化钼声子极化激元的面内纳米聚焦、拓扑转变和负折射调控等功能。.依托上述研究内容，取得如下成果：（1）发表高水平论文5篇（包括Science, Nature Nanotechnology, Nature Communication, 和Advanced Materials等各1篇）。（2）申请国家发明专利5项，其中授权1项。（3）制备了高质量的悬空石墨烯波导器件；室温下实现3.3微米传导距离；揭示了载流子迁移率、费米能级、入射波长和不同悬空高度对等离激元传导性能的调控规律；建立了介电损耗、热声子损耗、电子散射损耗的理论模型。（4）在石墨烯等离激元长程传输的基础上，通过与声子极化激元的杂化耦合发现了新型的拓扑极化激元模式，实现了纳米尺度的面内纳米聚焦、拓扑转变和负折射调控等功能。（5）2人次获得2021年北京市度自然科学奖二等奖（排名第三、第九）、入选北京市科技新星计划。（6）培养硕士研究生4名，1人次获得2020年度国家奖学金。