The Many-Particle Physics of Lasing in Two-Dimensional Transition-Metal Dichalcogenides

非技术总结该奖项支持由二维半导体制成的新型激光系统的物理理论研究和教育。从为互联网供电的通信网络到红绿灯，激光和发光二极管在日常生活中无处不在。与发光二极管相比，激光器发出的光有很大的不同。来自激光的光是相干的，这意味着光波以锁定的步伐在太空中传播，就像行进乐队中的音乐家一样，所有人都一致行走。相干是激光的一个重要特性；它的起源通常是一个复杂的物理过程。它是自发发生的，所以没有任何东西扮演指挥游行的带头人的角色。最近的努力是将激光小型化，使其在与光电信息处理相关的更多应用中有用，这涉及到使用二维材料。这些材料实际上只有一两层原子层厚。这些材料与传统激光中使用的材料非常不同，它们的物理特性直到现在才被发现。PI和他的团队将专注于电子和光学工艺。对这些过程的详细了解最终应该有助于通过控制和优化光发射特性，包括相干和激光作用，使基于二维材料的激光器在技术上更加成熟。PI和他的团队还旨在阐明二维材料中的激光与其他自发形成相干性的系统之间的异同，例如超导体中的相干性与光发射无关，而是与电子的集体量子力学状态有关。该奖项还支持PI参与本科生指导以及在大学和高中水平上接触美洲原住民学生的努力。该项目还支持撰写研究生水平的教科书，非线性半导体光学导论，重点是凝聚态物理的先进理论技术。技术总结该奖项支持由二维半导体组成的新型激光系统物理的理论研究和教育。半导体的相干光发射可以被视为非平衡相变到相位对称性破缺的状态的结果，这导致半导体的偏振期望值非零。从传统的激光到激子激光，再到与费米边缘奇点有关的发射，以及激子和激子-偏振子的Bardeen-Cooper-Schrieffer态和玻色-爱因斯坦凝聚体(BEC)，这类系统发射相干光的概念多种多样。这些静止的非平衡态与它们的平衡态的相似程度各不相同：例如，激子BEC通常被认为非常类似于平衡BEC，而传统的激光器没有直接的平衡类比。半导体中这种有趣的非平衡多粒子态是通过电子和空穴之间强烈的库仑相互作用而产生的，例如，通过电泵浦或光学泵浦产生库仑相互作用。单层过渡金属二卤化物(TMDs)等二维材料引起了人们的广泛关注。这些材料是具有非常强的库仑相互作用效应的半导体。库仑效应决定了纳米激光器和高质量微腔等TMD系统的相干发射。该项目的主要目标是回答这些系统中的基本物理问题，研究激子激光、玻色子和费米子极化激子以及类BCS极化激子激光等非传统激光过程的可能性，并从理论上理解与二维TMD材料相关的微观效应。该项目的目标还包括对可通过实验获得的发射/激光特性进行可测试的预测。所采用的理论方法是基于非平衡格林函数的方法，并结合相关含时运动方程的数值解。该奖项还支持PI参与本科生辅导以及在大学和高中阶段向美洲原住民学生提供外展服务的努力。该项目还支持编写研究生水平的教科书，非线性半导体光学导论，重点是非平衡格林函数和费曼图。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。

NONTECHNICAL SUMMARYThis award supports theoretical research and education on the physics of novel laser systems made of two-dimensional semiconductors.Lasers and light-emitting diodes are ubiquitous in everyday life, from communication networks powering the internet to traffic lights. The light emitted from lasers is very different compared to that from light-emitting diodes. The light from a laser is coherent, meaning that the waves of light propagate through space in lock step, similar to musicians in a marching band, all walking in unison. Coherence is an important characteristic of lasers; its origin is often a complex physical process. It occurs spontaneously so, there is nothing that plays the role of the band leader directing the march.Recent efforts to miniaturize lasers, to make them useful in more applications related to electro-optical information processing, involve the use of two-dimensional materials. These materials are effectively only one or two atomic layers thick. These materials are very different from those used in conventional lasers, and their physical properties are only now being uncovered. The PI and his team will focus on electronic and optical processes. A detailed understanding of these processes should ultimately help make lasers based on two dimensional materials technologically more mature by controlling and optimizing light emission characteristics, including coherence and laser action. The PI and his team also aim to elucidate similarities and differences between lasing in two-dimensional materials and other systems where coherence forms spontaneously, for example superconductors where coherence is not related to light emission but to a collective quantum mechanical state of electrons.This award also supports the PI's efforts to engage in undergraduate student mentoring as well as outreach to Native American students at the college and high school level. The project also supports writing a graduate-student-level text book, an introduction to nonlinear semiconductor optics with an emphasis on advanced theoretical techniques of condensed matter physics.TECHNICAL SUMMARYThis award supports theoretical research and education on the physics of novel laser systems made of two-dimensional semiconductors.Coherent light emission from semiconductors can be viewed as a result of a non-equilibrium phase transition to a state with broken phase symmetry which leads to a non-zero expectation value of the semiconductor's polarization. There are various concepts for such systems to emit coherent light, ranging from conventional lasers, to exciton lasers, to emission related to Fermi-edge singularities, and to Bardeen-Cooper-Schrieffer states and Bose-Einstein condensates (BEC) of excitons and exciton-polaritons. The degree to which these stationary non-equilibrium states are analogous to their equilibrium counterparts varies: an exciton BEC, for example, is often assumed to be very similar to an equilibrium BEC, while a conventional laser has no direct equilibrium analogue. This interesting variety of non-equilibrium many-particle states in a semiconductor arise through strong Coulomb interactions between the electrons and holes, which are created, for example, through electrical or optical pumping. Two-dimensional materials such as monolayer transition-metal dichalcogenides (TMDs) have attracted much attention. These materials are semiconductors with very strong Coulomb interaction effects. The Coulomb effects determine the coherent emission from TMD systems, such as nanolasers and high-quality microcavities. The primary objective of this project is to answer fundamental physics questions in these systems, to study the possibility of unconventional lasing processes such as exciton lasing, bosonic and fermionic polariton lasing as well as BCS-like polariton lasing, and to develop theoretical understanding that incorporates microscopic effects that are relevant in two-dimensional TMD materials. The project objectives also include testable predictions for experimentally accessible emission/laser characteristics. The theoretical approach to be used is based on the method of non-equilibrium Green's functions, combined with numerical solutions of the relevant time-dependent equations of motion. This award also supports the PI's efforts to engage in undergraduate student mentoring as well as outreach to Native American students at the college and high school level. The project also supports writing a graduate-student-level text book, an introduction to nonlinear semiconductor optics with an emphasis on non-equilibrium Green's functions and Feynman diagrams.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.