亚波长金属结构高效太赫兹光电导发射天线研究

This proposal focuses on a growing interest in developing high-efficiency terahertz photoconductive emitter, which is in highly demand for portable and compact broadband terahertz sensing and imaging systems. By proposing a novel design of large-area interdigitated sub-wavelength metal structure contact electrodes, high quantum efficiency photocarriers generation and ultrafast photocarriers transportation can be achieved simultaneously. By considering the photocarriers generation, photocarriers transportation and terahertz radiation process together, the limitation of previous investigation on photoconductive terahertz emitter, which was investigated either by referring to the photo-generated carrier dynamics or by referring to the ultrafast current pulsed-radiation separately, can be overcome. The tradeoff among the “laser factors”, “material factors” and “antenna factors”, which are all in close relation with the optical-to-terahertz conversion efficiency in a photoconductive terahertz emitter, can be perfectly solved. The use of large-area interdigitated sub-wavelength metal structure contact electrodes offers nanoscale carrier transport path lengths for the photocarriers generated from GaAs photoconductor illuminated by femtosecond laser pulses centered at 800nm, enabling efficient collection of the majority of photocarriers in a sub-picosecond time-scale, effectively enhancing the transient photocurrent and promoting the optical-to-terahertz conversion efficiency of photoconductive terahertz emitters with such plasmonic contact electrodes. It also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the photoconductive antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers.

本项目针对便携式、小型化宽频谱太赫兹波谱成像系统对高效太赫兹光电导发射天线的迫切需求，结合亚波长金属结构及其相关特性研究，创新性地提出引入优化设计的纳米尺度亚波长金属结构电极实现“光生载流子高效产生”和“光生载流子超快输运”和谐统一的设计理念，综合考虑亚波长金属结构光电导天线太赫兹源工作机制中的载流子产生与输运过程和太赫兹波辐射过程，将亚波长金属结构光电导天线太赫兹源涉及的“激光因素”、“材料因素”和“天线因素”有机融合，克服以往实验和理论方法只是孤立地涉及载流子动力学过程或者孤立地涉及电磁波辐射过程的不足，优化设计以GaAs材料为衬底的亚波长结构金属电极间距，缩短光生载流子输运路径，增加电极“收集”光生载流子的比表面积，兼顾亚波长金属结构对800nm激光的透射增强效果，通过将更多的光生载流子在亚皮秒时间尺度输运到天线电极，有效增强瞬态光电流，提升太赫兹光电导发射天线的太赫兹波产生效率。

太赫兹光电导发射天线是一类具有体积小、室温运行、频谱覆盖范围宽等优点的太赫兹波源，已广泛应用于太赫兹时域光谱仪、太赫兹成像系统等，但相对较低的输出功率和产生效率限制了这类太赫兹波源的实际应用。为此，在本项目资助下，项目负责人带领研究小组将亚波长金属结构与太赫兹光电导天线相结合，创新性地引入优化设计的纳米尺度亚波长金属结构电极实现“光生载流子高效产生”和“光生载流子超快输运”，开展了亚波长金属结构太赫兹光电导发射天线的深入研究。具体研究内容包括：1）构建电流瞬冲模型和基于有限元方法的多物理场模型研究了影响太赫兹光电导发射天线效率的相关因素以及亚波长金属结构参数对太赫兹光电导发射天线性能的提升作用，深入剖析了激光强度和激光脉宽等“激光因素”、载流子寿命和载流子迁移率等“材料因素”的影响规律，形成了一套完整的亚波长金属结构太赫兹光电导天线设计流程；2）设计了多种性能优异的亚波长金属结构太赫兹光电导天线，包括基于亚波长金属光栅结构的太赫兹光电导天线、基于掩膜层的亚波长光栅结构太赫兹光电导天线以及基于嵌入金属纳米结构的全介质超表面太赫兹光电导天线，实现了衬底材料对泵浦光的全吸收和表面局域电场增强，提升了太赫兹波的产生功率；3）研发了由高阻硅正方形柱构成的全介质超表面准直和聚焦透镜，分别用于太赫兹光电导发射天线的输出准直以及太赫兹光电导探测天线的输入聚焦，有利于太赫兹波谱成像系统的小型化和轻便化。上述工作实现了太赫兹光电导天线产生效率和辐射功率的提升，较好地完成了既定研究目标。项目研究产生的原创性成果推动了亚波长金属结构与太赫兹光电导天线两个领域的深入结合，在包括《Carbon》、《Nanomaterials》、《Optics Letters》、《Optics Express》、《J. Phys. D: Appl. Phys.》、《Scientific Reports》、《Optics Communications》等国际高水平学术期刊发表论文13篇，申请国家发明专利3项，出版学术专著《宽频带太赫兹波谱技术及其应用》1部（245千字），其中高性能太赫兹光电导发射天线是不可或缺的关键核心器件。