Exploiting excitons in atomic monolayers for dielectric sensing

非技术描述：包括半导体在内的许多晶体都是由弱键层构成的，因此，可以分离成原子厚度的稳定单位。在这些孤立的原子膜上，电子（带负电荷的粒子）和空穴（带正电荷的粒子）被强烈的吸引力束缚在一起，形成激子。激子是类氢原子粒子；它们具有一系列的量子态，并在吸收光谱中显示特征峰。另一方面，激子比氢原子大得多；虽然电子和空穴被限制在原子膜内，但它们的相互作用基本上延伸到外部。这种特性赋予了激子——吸收峰的能量和强度——对周围环境极其敏感。当膜放置在金属附近时，电子-空穴相互作用被显著屏蔽；激子吸收光谱发生了很大的变化。相反，附近的绝缘体对激子谱的影响要小得多。在这个项目中，研究小组利用原子膜中激子的这种独特性质来开发一种新的传感技术，该技术可以应用于广泛的材料，包括那些传统技术无法达到的材料。该团队将该技术应用于在二维空间中探测新型绝缘体和超导体。该项目支持一名研究生和几名本科生的研究和开发。其他活动包括使康奈尔大学的物理高级实验课程现代化，并为拓展活动开发材料，包括专门针对年轻女孩的校园科学、技术、工程和数学（STEM）项目。技术描述：原子薄的过渡金属二硫族半导体已经成为强光-物质相互作用的新平台。单层材料的光学响应是由激子（结合的电子-空穴对）主导的，它们对周围的介电环境非常敏感，因为大多数负责激子结合的电场线都在单层材料之外。该项目利用单层半导体中激子的这种独特特性，开发了一种新的光学传感技术，用于纳米材料的介电功能或电子可压缩性。目标是开发成像和时间分辨测量能力，以及对该技术的全面理解，其适用性和局限性。选择两个实验来关注每个测量能力。第一个实验研究了该技术对石墨烯中量子霍尔效应的敏感性，并探索了在量子霍尔体系中成像手性边缘态的可能性。第二个实验研究了该技术对超导跃迁的敏感性，并探索了光子激发后二维超导体的超快动力学。这些方法涉及到范德华异质结构和器件的制造以及光谱学，包括反射对比、超光谱成像和泵浦探针光谱学。新的传感技术可以应用于广泛的材料，包括那些不能形成良好电接触的传统电容或传输测量。它为相关材料的量子多体动力学、拓扑手性边缘态和二维超导体的研究开辟了前所未有的机会。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nontechnical description: Many crystals including semiconductors are made of weakly bonded layers, and as such, can be separated into stable units of atomic thickness. Electrons (negatively charged particles) and holes (positively charged particles) in these isolated atomic membranes are bound by strong attractions to form excitons. Excitons are hydrogen atom-like particles; they possess a series of quantum states and display characteristic peaks in the absorption spectrum. On the other hand, excitons are much larger than hydrogen atoms; while the electrons and holes are confined in the atomic membrane, their interactions extend substantially outside. This property endows the excitons--both the energy and the intensity of the absorption peaks--with extreme sensitivity to surroundings. When a membrane is placed near a metal, the electron-hole interactions are significantly screened; and the exciton absorption spectrum is substantially altered. Conversely, a nearby insulator affects the exciton spectrum much less. In this project, the research team exploits this unique property of excitons in atomic membranes to develop a new sensing technique that can be applied to a wide range of materials including those that are inaccessible by conventional techniques. The team applies the technique to probe new forms of insulators and superconductors in two dimensions. The project supports the research and development of one graduate student and several undergraduate students. Other activities involve modernizing the physics advanced laboratory course at Cornell University and developing materials for outreach activities, including several science, technology, engineering, and mathematics (STEM) programs on campus that specifically target young girls.Technical description: Atomically thin transition metal dichalcogenide semiconductors have emerged as a new platform for strong light-matter interactions. The optical response of monolayers is dominated by excitons (bound electron-hole pairs), which are extremely sensitive to the surrounding dielectric environment because most of the electric-field lines responsible for exciton binding are outside the monolayer material. The project exploits this unique property of excitons in monolayer semiconductors to develop a new optical sensing technique for dielectric function or electronic compressibility of nanoscale materials. The goals are to develop imaging and time-resolved measurement capabilities, as well as a comprehensive understanding of the technique, its applicability and limitations. Two experiments are selected to focus on each of the measurement capabilities. The first experiment studies the sensitivity of the technique to the quantum Hall effect in graphene and explores the possibility of imaging the chiral edge states in the quantum Hall regime. The second experiment investigates the sensitivity of the technique to superconducting transitions and explores ultrafast dynamics of two-dimensional superconductors following photon-excitations. The methods involve the fabrication of van der Waals heterostructures and devices and optical spectroscopies, including the reflection contrast, hyper-spectral imaging and pump-probe spectroscopy. The new sensing technique can be applied to a wide range of materials including those that do not form good electrical contacts for conventional capacitance or transport measurements. It opens up unprecedented opportunities for studies of quantum many-body dynamics in correlated materials, topological chiral edge states, and two-dimensional superconductors.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.