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

非技术摘要：我们对材料的了解大多来自于它们对最喜欢的（自然）频率的扰动的响应。例如，拨弦的小提琴弦发出的音高取决于它的长度、张力和厚度。同样，材料中原子的行为取决于固有频率，对于许多固体材料来说，固有频率落在太赫兹光谱范围内，这是一个直到最近才从技术上实现的非常困难的范围。这个项目利用了最近在太赫兹频率上的巨大技术进步来探测被称为拓扑材料的新材料。据预测，这些材料具有可用于开发量子信息技术电子设备的特性。该项目还包括一项广泛的教育和外联倡议。这项工作对培养具有独特技能的学生具有特殊的教育价值，为他们成为高科技行业的劳动力做好准备。该研究小组将在约翰霍普金斯大学物理博览会上发挥积极作用，这是一项外展活动，每年都会有数百人在周六的活动中参观霍普金斯大学的实验室，并让他们接触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研究具有重要Berry相位效应的拓扑和其他材料系统的项目。近年来，物质的拓扑状态一直是凝聚态物理学的核心研究方向，然而，我们缺乏许多这些系统电动力学特性的独特测量方法。理论表明，这些化合物的非线性响应可以提供独特的见解，基本的Berry相结构的基础波函数。测量将强调扩展太赫兹范围（这里是0.1 - 40太赫兹[0.4 - 165兆电子伏特]），通常响应目标是低能量涌现自由度，但实验将使用通过近红外的光子能量的全部补充。实验将在具有重要贝里相效应的拓扑材料和平凡材料上进行。材料包括拓扑绝缘体、Weyl半金属、Dirac半金属和二维过渡金属二硫族化合物。我们将探索这些系统对线性和圆极化辐射的非线性响应，在一些特定的配置中，这些配置被设计来阐明它们在这些化合物中的Berry相结构（Berry曲率和Berry连接）。许多理论预测已经存在，但在实验上，这是一个几乎完全未开发的领域，除了其内在重要性之外，它还有可能为可能被认为是成熟领域的固体非线性响应提供重要的新见解。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。

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.