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

该奖项支持二维半导体新型激光系统的理论研究和物理教育。激光和发光二极管在日常生活中无处不在，从为互联网供电的通信网络到交通信号灯。激光器发出的光与发光二极管发出的光大不相同。激光发出的光是相干的，这意味着光波在空间中以同步的方式传播，就像军乐队里的音乐家一样，步调一致。相干性是激光的一个重要特性；它的起源往往是一个复杂的物理过程。它是自发发生的，没有任何东西扮演指挥进行曲的乐队指挥的角色。为了使激光器在更多与光电信息处理相关的应用中发挥作用，最近的努力涉及到二维材料的使用。这些材料实际上只有一到两个原子层的厚度。这些材料与传统激光器中使用的材料大不相同，它们的物理性质直到现在才被发现。PI和他的团队将专注于电子和光学过程。通过控制和优化光发射特性（包括相干性和激光作用），对这些过程的详细了解最终将有助于使基于二维材料的激光器在技术上更加成熟。PI和他的团队还致力于阐明二维材料和其他系统中激光的异同，这些系统中相干性是自发形成的，例如超导体，其中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在本科生指导方面的努力，以及在大学和高中阶段向美国原住民学生伸出援手。该项目还支持编写一本研究生水平的教科书，介绍非线性半导体光学，重点介绍凝聚态物理的先进理论技术。该奖项支持由二维半导体制成的新型激光系统的理论研究和物理教育。半导体的相干光发射可以看作是一个非平衡相变到一个相位对称性被破坏的状态的结果，这导致半导体偏振的非零期望值。这类系统发射相干光的概念有很多，从传统的激光到激子激光，到与费米边缘奇点相关的发射，以及激子和激子极化子的巴丁-库珀-施里弗态和玻色-爱因斯坦凝聚态（BEC）。这些固定的非平衡态与平衡态的相似程度各不相同：例如，激子BEC通常被认为与平衡BEC非常相似，而传统激光没有直接的平衡模拟。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的强库仑相互作用产生的，例如，通过电或光泵浦产生。二维材料如单层过渡金属二硫族化合物（TMDs）引起了人们的广泛关注。这些材料是具有很强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，研究非常规激光过程的可能性，如激子激光、玻色子和费米子极化激子激光以及类bcs极化激子激光，并发展理论认识，包括与二维TMD材料相关的微观效应。该项目的目标还包括对实验可获得的发射/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，结合相关时相关运动方程的数值解。该奖项还支持PI在本科生指导方面的努力，以及在大学和高中阶段向美国原住民学生伸出援手。该项目还支持编写研究生水平的教科书，介绍非线性半导体光学，重点是非平衡格林函数和费曼图。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。

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.