Exciton Spectroscopy and Engineering in Strongly Correlated Electron Systems

非技术性：在光照射下，固体中的电子被激发到具有更高能量的状态，在其初始状态下留下一个空穴-一个丢失的电子。带相反电荷的电子和空穴经历静电吸引，并且在电绝缘材料中，可以形成被称为“激子”的原子状束缚物体。激子不仅作为研究固态环境中原子物理现象的平台，而且也是光子和光电技术发展的核心。到目前为止，激子主要是在传统的非磁性绝缘体中研究的，其中电子之间的相互作用很弱。但在理论上，激子也可以存在于一类称为Mott-Hubbard绝缘体的材料中，在这种材料中，强烈的相互作用会束缚电子，并导致它们的固有磁矩采用无数的有序排列。这样的环境可以赋予激子高度非常规的性质，可能导致新一代的高度可调和多功能激子器件。尽管有这些机会，莫特-哈伯德激子在实验上仍然难以捉摸。该项目通过部署新型高速频闪方法来探测Mott绝缘化合物中Mott-Hubbard激子的能量，动力学和空间特性来直接解决这个问题。这项研究的成果正在被纳入一项广泛的外联计划，该计划的目标是留住科学界代表性不足和处境不利的群体。这包括为帕萨迪纳学区的高中生制定广泛的教学和实习计划，并在加州理工学院校园为有兴趣从事STEM领域研究生研究的美国大学生举办年度研讨会。技术：传统的刚性能带半导体中的激子被很好地描述为库仑束缚的类氢电子空穴对。然而，对莫特绝缘体中激子的了解要少得多，莫特绝缘体由库仑和自旋交换相互作用束缚的holon-doublon对组成。这些所谓的哈伯德激子（Hubbard excitons，HEs）的性质，以及它们如何受到莫特绝缘体中常见的复杂电子和磁序的影响，仍然是一个悬而未决的问题。该项目的目标是实验观察HEs和探索他们的能量，动力学和空间特性，使用一套超快光谱技术在太赫兹频率范围内操作。研究活动分为两大重点。推力1的重点是通过激子内跃迁光谱直接解析HEs，并系统地表征它们在Mott系统中的动力学行为，这些系统具有不同的电子和磁序。推力2的重点是使用光学高次谐波产生执行断层成像的HE轨道的空间结构。还将通过场致隧道电离实验对HEs进行强场操纵。建立对高性能电子器件的基本理解将为未来利用强相关材料的光电子应用铺平道路。该奖项反映了NSF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。

Non-technical:Upon irradiation with light, an electron in a solid is excited to a state with higher energy, leaving behind a hole – a missing electron – in its initial state. The oppositely charged electron and hole experience electrostatic attraction and, in electrically insulating materials, can form an atom-like bound object dubbed an “exciton”. Excitons not only serve as a platform for studying atomic physics phenomena in a solid-state environment but are also central to the development of photonic and opto-electronic technologies. To date, excitons have largely been studied in conventional non-magnetic insulators where interactions between electrons are weak. But in theory, excitons can also exist in a class of materials called Mott-Hubbard insulators, where strong interactions immobilize electrons and can cause their intrinsic magnetic moments to adopt myriad ordered arrangements. Such an environment can endow excitons with highly unconventional properties, potentially leading to a new generation of highly tunable and multi-functional excitonic devices. Despite these opportunities, Mott-Hubbard excitons remain experimentally elusive. This project directly addresses this problem by deploying novel high-speed stroboscopic methods to probe the energetic, dynamical, and spatial characteristics of Mott-Hubbard excitons in a range of Mott insulating compounds. Products of this research are being incorporated into a broad outreach plan that targets retention of underrepresented and underprivileged groups in science. This includes developing an extensive teaching and internship program for high school students in the Pasadena School District and holding an annual workshop on Caltech campus for U.S. college level students-of-color interested in pursuing graduate research in STEM fields. Technical:Excitons in conventional rigid-band semiconductors are well described as Coulomb bound hydrogen-like electron-hole pairs. However, much less is understood about excitons in Mott insulators, which consist of holon-doublon pairs bound by both Coulombic and spin exchange interactions. The properties of these so-called Hubbard excitons (HEs), and how they are influenced by the complex electronic and magnetic orders often found in Mott insulators, remain open questions. The goals of this project are to experimentally observe HEs and to explore their energetic, dynamical, and spatial properties using a suite of ultrafast optical spectroscopic techniques operating in the terahertz frequency range. Research activities are divided into two major thrusts. Thrust 1 focuses on directly resolving HEs through intra-excitonic transition spectroscopy, and on systematically characterizing their dynamical behavior across Mott systems hosting different electronic and magnetic orders. Thrust 2 focuses on using optical high-harmonic generation to perform tomographic imaging of the spatial structure of HE orbitals. Strong field manipulation of HEs through field-induced tunneling ionization experiments will also be carried out. Establishing this fundamental understanding of HEs will pave the way for future opto-electronics applications that leverage strongly correlated materials.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.