Excitonic complexes and optical gain in two-dimensional molybdenum ditelluride well below the Mott transition

Abstract

†These authors contributed equally to this work. AbstractStrong Coulomb interaction in 2D materials provides unprecedented opportunities for studying many key issues of condensed matter physics, such as co-existence and mutual conversions of excitonic complexes, fundamental optical processes associated with their conversions, and their roles in the celebrated Mott transition. Recent lasing demonstrations in 2D materials raise important questions about the existence and origin of optical gain and possible roles of excitonic complexes. While lasing occurred at extremely low densities dominated by various excitonic complexes, optical gain was observed in the only experiment at densities several orders of magnitude higher, exceeding the Mott density. Here, we report a new gain mechanism involving charged excitons or trions well below the Mott density in 2D molybdenum ditelluride. Our combined experimental and modeling study not only reveals the complex interplays of excitonic complexes well below the Mott transition, but also provides foundation for lasing at extremely low excitation levels, important for future energy efficient photonic devices.Dynamical processes of quasiparticles such as excitons and their various associated complexes (including charged excitons or trions, bi-excitons, and other highly correlated objects (1)) in solids are at the very core of fundamental condensed matter physics. The evolution of physical states from low to high carrier density involves (2-5) the insulating exciton gas, Bose-Einstein condensate (BEC) (2), co-existence of various excitonic complexes, crossover or transitions to conducting electron-hole plasmas (EHP), or electron-hole liquid (EHL) (3) through the Mott transition (MT) (4). There remain many fundamental issues to be understood associated with the evolution of such quantum quasiparticles and the related phase transitions, especially in the intermediate density regime involving highly correlated complexes. On the other hand, mutual conversions of these excitonic complexes and related phase transitions are also profoundly linked to the natures of different optical processes in semiconductors as carrier density increases (5,6). Light-semiconductor interaction thus plays important dual roles: As an information reporter, the interaction reveals the intrinsic dynamics of evolution, co-existence, and mutual conversion of various excitonic complexes in their passage towards degenerate EHP or EHL through MT, providing deep understandings of physical processes when carrier density is successively increased. At the same time, different conversion processes among various excitonic complexes provide novel absorption or emission mechanisms, forming the physical foundations for photonic functionalities including lasers, solar cells, light emitting diodes, and many other devices. While optical processes are well-understood in the two extremes: pure exciton gas and highly degenerate EHP, the intermediate stages involving various excitonic complexes are much less understood. Recent stud...

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Mechanisms of Spontaneous and Amplified Spontaneous Emission in CH3NH3PbI3 Perovskite Thin Films Integrated in an Optical Waveguide

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