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

Semiconductors that can provide optical gain at extremely low carrier density levels are critically important for applications such as energy efficient nanolasers. However, all current semiconductor lasers are based on traditional semiconductor materials that require extremely high density levels above the so-called Mott transition to realize optical gain. The new emerging 2D materials provide unprecedented opportunities for studying new excitonic physics and exploring new optical gain mechanisms at much lower density levels due to the strong Coulomb interaction and co-existence and mutual conversion of excitonic complexes. Here, we report a new gain mechanism involving charged excitons or trions in electrically gated 2D molybdenum ditelluride well below the Mott density. Our combined experimental and modelling study not only reveals the complex interplay of excitonic complexes well below the Mott transition but also establishes 2D materials as a new class of gain materials at densities 4–5 orders of magnitude lower than those of conventional semiconductors and provides a foundation for lasing at ultralow injection levels for future energy efficient photonic devices. Additionally, our study could help reconcile recent conflicting results on 2D materials: While 2D material-based lasers have been demonstrated at extremely low densities with spectral features dominated by various excitonic complexes, optical gain was only observed in experiments at densities several orders of magnitude higher, beyond the Mott density. We believe that our results could lead to more systematic studies on the relationship between the mutual conversion of excitonic species and the existence of optical gain well below the Mott transition. A new mechanism for amplifying optical signals in two-dimensional semiconductors could be used to develop nano-sized lasers with little input power. When high-energy photons strike a semiconductor they produce excitons – pairs comprising an excited electron and the positively-charged ‘hole’ that it leaves behind, or even trions - states comprising two electrons and one hole, or one electron and two holes. However, scientists have so far limited understanding of the many exotic exciton states that can exist and mutually convert. Cun-Zheng Ning at Tsinghua University in Beijing and co-workers studied ultra-thin single and dual layers of molybdenum ditelluride, and observed a complex interplay between excitons and trions. The trion states enabled the system to show optical gain – a vital property for lasers - despite being orders of magnitude below the so-called Mott transition density, a minimum condition for optical gain in conventional semiconductors.