posted on 2023-01-10, 04:06authored byZhen Wang, Hao Sun, Qiyao Zhang, Jianxing Zhang, Jialu Xu, Jiacheng Tang, Cun-Zheng Ning
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 ∼1 × 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.