Forbidden Light: Origins and Implications of Optical Frequency Magnetism in Hybrid Organic/Inorganic Perovskites

光是由振荡的电场和磁场组成的。然而，在原子长度尺度上，科学家们一致认为材料只对光的电场分量起反应。由光磁偶极过程的磁场分量驱动的光-物质相互作用被假定为要么极弱，要么严格禁止。最近，首席研究员的研究小组发现了由与磁偶极子相互作用引起的意外的明亮光发射，来自一类原子薄的“二维”层状有机/无机钙钛矿材料。这个项目的主要目标是确定这种光频磁性在这类材料中的起源和含义。通过这样做，研究小组正在澄清光频磁性是否比以前认为的更常见，它是否存在于其他材料系统中，以及它是否以其他不寻常的方式表现出来。这些关于光-物质相互作用的基本性质的研究得到了旨在促进向公众进行有效科学传播的新的外联工作的补充。具体来说，首席研究员正在开发文本编辑器，帮助科学家将他们的研究“翻译”成针对各种普通受众的语言。在光学频率和原子长度尺度下，量子力学光-物质相互作用本质上是非磁性的。也就是说，吸收和发光过程在电偶极（艾德）近似下进行处理，并假设通过纯粹由光的电场分量驱动的量子力学矩阵元发生。高阶磁偶极（MD）过程被假定为弱到可以忽略，如在原子和分子中，或严格禁止，如在典型的半导体中。最近，首席研究员的研究团队在各种二维混合有机/无机钙钛矿（2D HOIP）中发现了非常明亮的，表面上禁止的MD光致发光。这种光频磁性的演示挑战了光学（特别是在超导体中）受电场和电偶极子支配的普遍观念。这项建议的主要目标是确定HOIP中光频磁性的起源和影响。该项目致力于回答一个具有基础科学重要性的问题：为什么2D HOIP表现出表面上“禁止”的MD光发射，为什么这种发射如此明亮？研究工作遵循三个主题：1）确定MD光学过程在HOIP中的普遍性，并显示如何通过化学合成来修改MD过程。2)演示MD光学过程的幅度，能量和动力学如何受到外部刺激（如温度，应变和电场）的影响。3)确定HOIP是否表现出MD吸收或渗透性。这些研究活动将阐明对称性破缺效应在HOIP中的作用，解决有关影响光电器件效率和色纯度的常见边带吸收和发射特征的开放性问题，并为迄今为止被认为不可能的新型大块原子级超材料现象指明方向。更一般地说，光频磁性的存在，在2D HOIP挑战流行的ED为中心的方法来描述轻物质的相互作用。阐明这种效应的起源将澄清光频磁性是否比以前更普遍，它是否存在于其他材料系统中，以及如何利用它来实现新型的光学功能。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

Light consists of oscillating electric and magnetic fields. However, at atomic length scales, scientists uniformly assume that materials react only to the electric field component of light. Light-matter interactions driven by the magnetic field component of light—magnetic dipole processes—are assumed to be either extremely weak or strictly forbidden. Recently, the principal investigator’s research team discovered unexpected, bright, light emission, caused by interaction with magnetic dipoles, from a class of atomically thin “two-dimensional” layered organic/inorganic perovskite materials These results challenge the common notion that optical properties of materials are governed by electric fields alone. The principal objective of this project is to determine the origins and implications of this optical frequency magnetism in thus class of materials. In doing so, the research team is clarifying whether optical frequency magnetism is more common than previously thought, whether it exists in other materials systems, and whether it manifests in other unusual ways. These studies of the fundamental nature of light-matter interactions are complemented by new outreach efforts designed to facilitate effective science communication to the general public. Specifically, the principal investigator is developing text editors that help scientists “translate” their research into language targeted at various general audiences. At optical frequencies and atomic length scales quantum-mechanical light-matter interactions are inherently non-magnetic. That is, absorption and luminescence processes are treated in the electric dipole (ED) approximation and assumed to occur via quantum-mechanical matrix elements that are driven purely by the electric field component of light. Higher order magnetic dipole (MD) processes are assumed to be either negligibly weak, as in atoms and molecules, or strictly forbidden, as in typical semiconductors. Recently, the principal investigator’s research team discovered very bright, ostensibly symmetry-forbidden MD photoluminescence in a variety of two-dimensional hybrid organic/inorganic perovskites (2D HOIPs). This demonstration of optical frequency magnetism challenges the common notion that optics—especially in semiconductors—is governed by electric fields and electric dipoles. The principal objective of this proposal is to determine the origins and implications of optical frequency magnetism in HOIPs. The project is dedicated to answering a question of fundamental scientific importance: Why do 2D HOIPs exhibit ostensibly “forbidden” MD light emission and why is this emission so bright? Research efforts follow three themes: 1) Ascertaining the prevalence of MD optical processes in HOIPs, and showing how MD processes can be modified via chemical synthesis. 2) Demonstrating how the magnitude, energy, and dynamics of MD optical processes are influenced by external stimuli such as temperature, strain, and electric fields. 3) Determining whether HOIPs exhibit MD absorption or permeability. These research activities will clarify the role of symmetry breaking effects in HOIPs, resolve open questions regarding commonly seen sideband absorption and emission features that impact the efficiency and color purity of optoelectronic devices, and point the way to new classes of bulk, atomic-scale metamaterial phenomena heretofore thought impossible. More generally, the existence of optical frequency magnetism in 2D HOIPs challenges the prevailing ED-centric approach used to describe light-matter interactions. Elucidating the origins of this effect will clarify whether optical frequency magnetism is more prevalent than previously, whether it exists in other material systems, and how to exploit it to achieve new types of optical functionality.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.