Van der Waals semiconductors provide a platform for creating two-dimensional crystals layer-by-layer and engineering excitonic states therein with exceptional properties. We discuss here a few interesting opportunities enabled by interlayer excitons in bilayer transitional metal dichalcogenides (TMDs), including high valley polarizations, lasing in 2D cavities, and tunable interlayer excitons in homo-bilayers.
Monolayer TMDs feature spin-valley locking, enabling valleytronic phenomena and applications. However, strong inter-valley scattering due to electron hole exchange interactions leads to rapid valley depolarization in picoseconds, making it difficult to achieve a high degree of valley polarization. The electron-hole exchange interaction becomes suppressed for interlayer excitons in heterobilayers with type II band alignment. We show highly polarized interlayer excitons with a long valley lifetimein in both spin singlet and brightened triplet states in hetero-bilayers.
TMDs have also garnered intense interest as an active medium, for they feature very strong exciton-photon interactions in a monolayer. However, the rapid radiative decay makes it challenging to establish population inversion. Interlayer excitons decay much more slowly, comparable to excitons in quantum wells of conventional semiconductors. When the interlayer exciton are integrated on a cavity, lasing was established at the cavity resonance accompanied by increased temporal and spatial coherence.
Lastly, due to the strong intra-layer localization of the carriers in 2D materials, interlayer excitons co-exist with intra-layer excitons as meta-stable states even in homo-bilayers without artificial interfaces. These interlayer excitons also feature oscillator strengths between intra-layer excitons and inter-layer ones in hetero-structures, offering a potentially highly tunable system for 2D optoelectronics.