Non-linear THz optical effects as a probe of Berry's phase in topological materials

非技术摘要：我们对材料的大部分了解来自于它们对其最喜欢的(自然)频率的扰动的反应。例如，一根拨动的小提琴弦的音高取决于它的长度、张力和厚度。以类似的方式，原子在材料中的行为取决于自然频率，对许多固体材料来说，该频率落在太赫兹的光谱范围内，直到最近，从技术上讲还很难达到这个范围。该项目利用最近在太赫兹频率方面的戏剧性技术进步来探索称为拓扑材料的新材料。据预测，这些材料具有一些特性，可以用于开发量子信息技术的电子器件。该项目还包括一项广泛的教育和外联倡议。这项工作在培养具有独特技能的学生方面具有特别的教育价值，为他们成为高科技行业的劳动力做好准备。研究团队将在约翰·霍普金斯物理博览会中发挥积极作用-这是一项外联活动，每年在周六的活动中吸引数百人参观霍普金斯大学的实验室，并让他们接触到各种物理演示和活动。该团队还将在物理博览会上进行演示，并与资源不足的当地学校合作。技术摘要：这是一个利用非线性光学响应来研究一些具有重要Berry相位效应的拓扑和其他材料系统的项目。近年来，物质的拓扑态一直是凝聚态物理学的中心兴趣，但我们缺乏对其中许多系统的电动力学性质的独特测量。理论表明，这些化合物的非线性响应可以给出其基本波函数的基本Berry相结构的独特见解。测量将强调扩展的太赫兹范围(此处为0.1-40太赫兹[0.4-165兆赫])，在该范围内，响应通常针对低能量出射自由度，但实验将使用通过近红外向上的全部光子能量。实验将在拓扑材料和具有重要Berry相效应的平凡材料上进行。材料包括拓扑绝缘体、Weyl半金属、狄拉克半金属和2D过渡金属二卤化物。我们将探索这些体系对线性和圆极化辐射的非线性响应，在一些特定的构型中，旨在阐明它们在这些化合物中的Berry相结构(Berry曲率和Berry连接)。存在许多理论预测，但从实验上看，这是一个几乎完全未被探索的领域，除了其内在重要性外，它还有可能为可能被认为是成熟领域的固体的非线性反应提供重要的新见解。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。

Non-Technical Abstract:Most of what we know about materials comes from their response to perturbations at their favorite (natural) frequencies. For instance, the pitch of sound from a plucked violin string depends on its length, the tension in it, and its thickness. In a similar way, the behavior of atoms in materials depends on the natural frequency, which, for many solid materials, fall in the Terahertz spectral range, a very difficult range to access technically until recently. This project takes advantage of recent dramatic technical advances in Terahertz frequencies to probe new materials called topological materials. These materials have been predicted to possess properties that make them useful for developing electronic devices for quantum information technology. This project also includes a broad initiative in education and outreach. The work is of particular educational value in training students with unique skills to prepare them as the work force in high tech industries. The research team will play active roles in the Johns Hopkins Physics Fair- an outreach activity which brings hundreds of people each year through Hopkins' labs during a Saturday event and exposes them to various physics demonstrations and activities. The team will also give demonstration shows at the Physics Fair and work with under-resourced local schools.Technical Abstract:This is a project to investigate a number of topological and other material systems that have important Berry phase effects using nonlinear optical response. Topological states of matter have been of central interest in condensed matter physics in recent years, yet we are lacking unique measures of many of these systems' electrodynamic properties. Theory has indicated that the nonlinear response of these compounds can give unique insight into the essential Berry phase structure of their underlying wavefunctions. Measurements will emphasize the extended THz range (here 0.1 - 40 THz [0.4 - 165 meV]) where generally responses target the low energy emergent degrees of freedom, but experiments will use a full complement of photon energies up through the near infrared. Experiments will be performed on both topological materials and trivial materials with important Berry phase effects. Materials include topological insulators, Weyl semimetals, Dirac semimetals, and 2D transition metal dichalcogenides. We will explore the nonlinear response of these system to both linear and circularized polarized radiation in a number of specific configurations that are designed to elucidate their Berry phase structure (Berry curvature and Berry connection) in these compounds. Many theoretical predictions exist, but experimentally, this is an almost completely unexplored area, which aside from its intrinsic importance has the potential to give major new insight into what might have been considered a mature area -- the nonlinear response of solids.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.