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

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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.