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

光由振荡的电场和磁场组成。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成分起反应。由光的磁场成分--磁偶极过程--驱动的光-物质相互作用被假定为要么极其微弱，要么被严格禁止。最近，首席研究员的研究小组发现，由于与磁偶极子的相互作用，一类原子薄的二维层状有机/无机钙钛矿材料发出了意想不到的、明亮的光发射。这些结果挑战了通常认为材料的光学性质仅由电场决定的概念。该项目的主要目标是确定这种光学频率磁性在这类材料中的起源和含义。在这样做的过程中，研究小组正在澄清光频磁性是否比之前认为的更常见，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-物质相互作用基本性质的研究得到了新的外展努力的补充，旨在促进向普通公众进行有效的科学交流。具体地说，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上，量子力学与光物质的相互作用本质上是非磁性的。也就是说，吸收和发光过程是在电偶极子(ED)近似下处理的，并假定通过纯粹由光的电场分量驱动的量子力学矩阵元素发生。高阶磁偶极子(MD)过程被认为要么像原子和分子一样可以忽略不计地弱，要么像典型的半导体那样被严格禁止。最近，首席研究员的研究小组在各种二维有机/无机复合钙钛矿(2D HOIP)中发现了非常明亮的、表面上禁止对称的MD光致发光。这种光学频率磁性的演示挑战了通常的观念，即光学--尤其是半导体中的光学--是由电场和电偶极子支配的。这项建议的主要目标是确定光学频率磁性的起源和在高密度热泵中的影响。该项目致力于回答一个具有基础科学意义的问题：为什么2D HOIP表现出表面上被禁止的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)确定分子动力学光学过程在HOIP中的流行程度，并展示如何通过化学合成来改变分子动力学过程。2)演示了温度、应变和电场等外界刺激对MD光学过程的大小、能量和动力学的影响。3)确定HOIP是否具有MD吸收或透过性。这些研究活动将阐明对称性破缺效应在HOIP中的作用，解决有关影响光电子器件效率和颜色纯度的常见边带吸收和发射特性的公开问题，并为迄今为止被认为不可能的大块、原子尺度超材料现象指明方向。更广泛地说，2D HOIP中光频磁性的存在对用于描述光-物质相互作用的主流ED中心方法提出了挑战。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类型的光学功能。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.