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.

可以以极低的载流子密度水平提供光增益的半导体对于诸如节能纳米激光之类的应用至关重要。然而，当前所有的半导体激光器都是基于传统的半导体材料，其要求在所谓的莫特跃迁之上的极高的密度水平以实现光学增益。由于强库仑相互作用以及激子复合物的共存和相互转化，新出现的2D材料为研究新的激子物理学和探索新的光学增益机制提供了前所未有的机会，从而以低得多的密度进行了研究。在这里，我们报告了一种新的增益机制，其中涉及远低于Mott密度的电控2D二碲化钼中的带电激子或tri。我们结合实验和模型研究，不仅揭示了激子复合物的复杂相互作用远低于Mott跃迁，而且将2D材料确立为一种新型的增益材料，其密度比常规半导体低4-5个数量级，并提供了基础用于未来低能耗光子器件的超低注入水平的发射。此外，我们的研究可能有助于调和最近在2D材料上存在矛盾的结果：虽然已证明以2D材料为基础的激光器在极低的密度下具有由各种激子复合体控制的光谱特征，但是光学增益仅在密度高几个数量级的实验中才能观察到，超过了莫特密度。