The optical properties of thin layers of transition-metal dichalcogenide (TMDC) semiconductors are predominantly determined by excitons, bound states typically formed from holes in the highest valence band and electrons in the lowest conduction band. We recently demonstrated that excitons in exfoliated 2D layers of van der Waals (vdW) crystals, such as WSe2, also exist in a further intriguing configuration, where the electron instead occupies a high-lying conduction band at almost twice the energy of the band-gap. The electron can be promoted between the band-edge and high-lying bands by interacting with an optical field, resulting in excitonic quantum interference manifesting in the spectrum of second-harmonic generation. This effect is rationalized by excitonic Rabi flopping, in analogy to the phenomenon of electromagnetically induced transparency familiar from atomic gases. While such high-energy exciton states are short-lived in TMDC monolayers, similar longer-living, spatially indirect states also arise in vdW homo- and heterobilayers. We will study the optical properties of such species with the focus on long-lived excited states with interlayer electronic hybridization and static dipole moments, controllable by both electric fields and electrostatic doping. In contrast to conventional excitons, the negative-mass contribution to these high-lying excitons is expected to result in a net acceleration of the electrically neutral species in lateral electric fields – a counterintuitive behavior that may become accessible with the long lifetime of interlayer excited states. In comparison to band-edge excitons, the high-lying excitons are significantly more sensitive to interlayer coupling. Since they are separated in energy from typical near-gap defect states, they are prime candidates to investigate the influence on the upper conduction bands of a periodic moiré potential arising in twisted layers of vdW crystals. In this context, fruitful collaborations with theory groups are anticipated, not least because it is challenging to study the moiré effect on these bands by direct optical probing, e.g., in reflectometry. Because of the divergence of the effective exciton mass, new exciton physics is expected to emerge when the moduli of electron and hole mass are tuned to be equal. Excitonic multi-level systems interact strongly with the optical field and show contributions to nonlinear susceptibilities that are controllable, e.g., by electric fields. Recent reports indicate that interlayer excitons with permanent out-of-plane dipoles may result in deterministic single-photon emission if they are trapped in moiré potentials. We anticipate that the presence of high-lying excitonic states will enable the study of nonlinear optical effects in the ultimate limit of individual excitons.
