Threshold-like Superlinear Accumulation of Excitons in a Gated Monolayer Transition Metal Dichalcogenide

Coexistence and mutual conversion of various excitonic species in semiconductors are the essence of Mott physics and underpin important device applications such as excitonic or polaritonic lasers. The emergence of monolayer semiconductors provides unprecedented opportunities to study these fundamental issues due to the much larger exciton binding energies. In this paper, we study the evolution of the coupled exciton-trion system in electrically gated monolayer MoTe2 devices. Contrary to the conventional linear scaling, we found that exciton density exhibits an abnormal three-stage scaling behavior: a conventional linear scaling at low pumping levels, followed by a superlinear behavior accompanied by a strong saturation of trion emission. In the third stage, the exciton emission returns to the linear scaling with the further increase of pumping. Although such behavior has a rare similarity in other physical systems, surprisingly we discovered a complete analogy of this behavior with the threshold of a conventional laser and proved mathematically that the exciton-trion equations are identical to the laser equations. We further showed that the power-law index increases with the charge density experimentally and can be as high as 40 at a density of similar to 1 x 1012/ cm2 in principle, leading to extremely nonlinear behavior of exciton accumulation. Our results reveal a new behavior of exciton accumulation and provide an alternative mechanism for multiplying exciton population, in analogy to a condensate. The intricate dynamics between excitons and trions will also enrich our understanding of the complex Mott physics in 2D semiconductors and may lead to new devices based on the exciton nonlinearity.