Super Resolution THz Imaging of Nanostructures

最近发明的超分辨率显微镜正在对化学、生物和材料科学领域产生巨大影响，因为这些显微镜提供了以前闻所未闻的物体图像，其尺寸接近分子。然而，由于与所使用的光相关的能量限制，导电和半导体纳米材料的重要化学和电学特征不能用现有的显微镜可视化。在化学测量和成像计划的支持下，以及国家科学基金会化学部门的高分子、超分子和纳米化学项目以及材料研究部门的电子和光子材料项目的部分共同资助下，圣母大学的哈特兰教授和他的团队正在开发一种新的超分辨率显微镜，通过一种新的方法来记录材料的图像，这种新方法利用了电磁光谱太赫兹区域的低能光。太赫兹显微镜能够分辨的最小尺寸的特征是几百纳米，这是以前可能的100到1000倍。哈特兰教授、研究生和本科生正在利用太赫兹显微镜新释放的力量，研究有望实现更高效太阳能电池的材料，以及有朝一日可能用于超灵敏化学检测系统的纳米材料。预计新型太赫兹显微镜将被研究生物制品、超导体和集成电路设备以及识别微量化学物质(如爆炸物残留物)的方法的研究人员采用。努力不仅仅是在大学一级培训学生，主要重点是从埃尔克哈特社区学校招聘的高中生和教师的参与，埃尔克哈特社区学校是当地的一个学区，学生来自高度不同的社会经济背景。埃尔克哈特学校合作的持续影响是通过高中教师因其暑期研究活动而获得研究生学分而实现的。这一计划使教师参与者能够教授印第安纳大学的双重招生课程，从而扩大了埃尔克哈特社区学校提供的教育机会。太赫兹光谱学被广泛用于检测半导体和塑料；然而，目前现有的太赫兹显微镜由于衍射限制，空间分辨率很低。在传统的太赫兹显微镜中，可以分辨的最小特征尺寸由入射光的波长除以2来确定，在太赫兹区约为50微米。哈特兰小组采取的独特方法是基于一种紧密聚焦的可见光探测光束，该光束通过光热效应监测太赫兹泵浦光束的吸收。这些实验中的分辨率由可见的探测光束决定，结果是几百纳米而不是几十微米。光热太赫兹显微镜正被用来研究薄膜和单个纳米结构的光导性和太赫兹光谱。这些实验为光激发载流子在不同结构中的空间分布和运动，以及载流子的衰变路径提供了新的信息。这一信息对开发光伏太阳能电池材料非常重要。太赫兹光谱学实验也在纳米材料的低频共振上进行，这将产生有关这些材料如何与环境相互作用的新信息。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Recently invented super-resolution microscopes are having a large impact on the fields of chemistry, biology, and materials science, due to these microscopes providing previously unheard-of images of objects with dimensions approaching that of molecules. However, important chemical and electrical features of conducting and semiconducting nanomaterials cannot be visualized with existing microscopes, because of energy limitations associated with the light used. With support from the Chemical Measurement and Imaging Program, and partial co-funding from the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry and the Electronic and Photonic Materials Program in the Division of Materials Research of the National Science Foundation, Professor Hartland and his group at the University of Notre Dame are developing a new super-resolution microscope for recording images of materials through a novel approach that takes advantage of low-energy light in the terahertz region of the electromagnetic spectrum. The smallest sized feature the terahertz microscope can resolve is a few hundred nanometers, which is 100 to 1000 times better than previously possible. The newly unleashed power of the terahertz microscope is being used by Professor Hartland and graduate and undergraduate students to study materials that hold promise for more efficient solar cells and nanomaterials that may one day be used in ultra-sensitive chemical detection systems. It is anticipated that the new terahertz microscope will be adopted by researchers who study biologicals, superconductors, and integrated circuit devices, and ways to identify trace amounts of chemicals, such as residues from explosives. Efforts go beyond training of students at the university level, with a major focus being participation of high school students and teachers recruited from the Elkhart Community Schools, a local school district with students who come from highly diverse socioeconomic backgrounds. Sustained impact of the Elkhart School collaboration is being achieved by the high school teachers receiving graduate credits for their summer research activities. This program leads to teacher participants being able to teach Indiana University dual-enrollment courses, thereby broadening the educational opportunities provided by the Elkhart Community Schools.Terahertz spectroscopy is widely used to examine semiconductors and plastics; however, currently existing terahertz microscopes have very low spatial resolution due to the diffraction limit. In a traditional terahertz microscope, the minimum feature size that can be resolved is determined by the wavelength of the impinging light divided by two, which is approximately 50 micrometers in the terahertz region. The unique approach taken by the Hartland group is based on a tightly focused visible probe beam that monitors absorption of a terahertz pump beam through the photothermal effect. The resolution in these experiments is dictated by the visible probe beam, and, as a result, is several hundred nanometers rather than tens of micrometers. The photothermal terahertz microscope is being used to examine photo-conductivity and the terahertz spectroscopy of thin films and individual nanostructures. These experiments are providing new information about the spatial distribution and motion of photo-excited charge carriers in the different structures, and the decay pathways for the charge carriers. This information is important for developing materials for photo-voltaic solar cells. Terahertz spectroscopy experiments are also being performed on the low frequency resonances of nanomaterials, which is generating new information about how these materials interact with their environment.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.