EFRI NewLAW: Non-reciprocal, topologically protected propagation using atomically thin materials for nanoscale devices

光学上的互惠可以用我们熟悉的观察来描述：“如果我能看到你，你也能看到我。”这种现象源于这样一个事实，即支配光的自然规律及其在微观层面上与物质的相互作用并不偏爱任何特定的时间方向。换句话说，如果我们假设逆转时间的方向，自然规律的预测将保持不变。当然，在宏观层面上，情况并非如此，因为我们从经验中知道，时间之箭是有固定方向的。事实证明，即使在微观尺度上，外部磁场也可以通过选择首选的传播方向来破坏时间的对称性，这可能会导致互易性的破坏。在目前的光学设备中，使用磁铁来实现这种光的单向传播，这对光通信和互联网至关重要。该项目的一个目标是通过使用一种原子薄的新型材料，在不使用磁铁的情况下实现光的非互易性。这种材料由于其独特的晶体结构而具有有效的磁场，并且可以用来代替用于光的单向传播的外加磁场。该项目旨在实现基于这些新材料的小型化光学器件和电路元件，从而在更小的占地面积上实现更快的光开关和缺陷不敏感的传播。这类设备有可能改变光电信业。在这个项目的过程中，研究团队将向中学生、本科生和研究生，包括那些来自历史上的黑人学院和大学的学生，提供尖端技术方面的科学教育和研究经验，以努力提高未来的科学和工程劳动力。该项目的目标是实现芯片上的、非互易的纳米光子器件和电路元件，这些器件和电路元件工作在光学频率和拓扑保护的光子边缘状态。这类设备将具有新的功能，如可重新配置的单向传播和光转向。在与时间无关的线性系统中，能量和信息的非互易传播需要打破时间反转对称性，这可以通过外部磁场来实现。我们将以一种无磁场的方式实现纳米级的非互易传播，并有可能通过外部刺激进行主动控制。我们的方法将使用原子薄的材料，如具有独特电子和光学性质的过渡金属二卤化物来实现这些目标。在这些打破了反转对称性的材料中，动量空间中存在一个被称为贝里曲率的有效磁场。虽然在这些材料中不存在净Berry曲率，但电控提供了空间局部时间反转破裂和非互易传播的可能性。此外，我们将依靠这些材料中强烈的光-物质相互作用和非线性来增加非互易性，并实现拓扑保护的光的边缘状态。为此，将利用光学和等离子体纳米腔来增强光与物质的相互作用。这个研究项目将促进我们对低维材料中像Berry Curvature这样的有效规范场的基本理解，以及如何利用强烈的光-物质相互作用来实现片上、可重构的非互易和轻物质的拓扑态。

Reciprocity in optics can be described by the familiar observation: "If I can see you, you can see me." This phenomenon stems from the fact that laws of nature governing light and its interaction with matter on a microscopic level do not prefer any particular direction of time. In other words, if we were to hypothetically reverse the direction of time, the predictions of the laws of nature would remain unaltered. Of course, at a macroscopic scale, this is not the case as we know from experience that the arrow of time has a fixed direction. It turns out that an external magnetic field can break the symmetry of time even at the microscopic scale by picking a preferred direction of propagation, which can lead to a break down of reciprocity. In current optical devices, magnets are used to cause such a one-way propagation of light, which is crucial for optical communications and the Internet. One goal of this project is to achieve non-reciprocity of light without the use of magnets by using a new class of materials, which are atomically thin. Such materials feature an effective magnetic field due to their unique crystal structure and can be used in lieu of an applied external magnetic field for one-way propagation of light. This project aims to realize miniaturized optical devices and circuit elements based on these novel materials, which will allow for faster optical switches and defect-insensitive propagation on a reduced footprint. Such devices have the potential to transform the optical telecommunication industry. During the course of this project, the research team will provide science education and research experiences in cutting-edge technologies to middle school, undergraduate and graduate students, including those students from Historically Black Colleges and Universities, in an effort to enhance the science and engineering workforce of tomorrow. The goal of this project is to realize on-chip, non-reciprocal nanophotonic devices and circuit elements operating at optical frequencies and topologically protected edge states for photons. Such devices will feature novel functionalities such as reconfigurable one-way propagation and steering of light. Non-reciprocal propagation of energy and information in a time-independent and linear system requires broken time-reversal symmetry, which can be achieved by an external magnetic field. We will achieve non-reciprocal propagation at nanoscale in a magnetic-free way with the possibility of active control by external stimuli. Our approach will employ atomically thin materials such as transition metal dichalcogenides with unique electronic and optical properties to achieve these goals. In these materials, which break inversion symmetry, an effective magnetic field in the momentum-space called the Berry curvature is present. Although no net Berry curvature is present in these materials, electric control offers the possibility of spatially local time-reversal breaking and non-reciprocal propagation. In addition, we will rely on the strong light-matter interactions and non-linearity in these materials to increase non-reciprocity and also to realize topologically protected edge states of light. To this end, optical and plasmonic nano-cavities, which enhance light-matter interactions, will be exploited. This research project will advance our fundamental understanding of effective gauge-fields like Berry curvature in low-dimensional materials and how strong light-matter interactions can be exploited to achieve on-chip, reconfigurable non-reciprocity and topological states of light-matter.