Quantum interference between band-edge and high-lying excitons comprising negative-mass electrons

Novoselov and Geim pointed out early on that exfoliated monolayers of TMDCs are “essentially gigantic 2D molecules” (PNAS 2005). Indeed, just as in molecules, owing to the extraordinary oscillator strength of mobile excitons¹, we have identified transitions involving electrons in high-lying conduction bands – semiconductor analogs of molecular S₂-S₀ transitions². These electrons in high-lying conduction bands have negative effective mass, giving rise to a unique phonon progression in the PL². Unlike in molecules, excitons can be coherently controlled and driven into quantum interference, signatures of which emerge in the frequency³ and time domain⁴. In monolayer WSe₂, these high-lying excitons are characterized by narrow bands (<2 meV) in a spectral range far above the bandgap². Under electrical gating, UV-emitting trions form, involving electrons of both positive and negative mass. This field dependence offers tunability of excitonic quantum interference. Because of additional confinement in twisted bilayer structures^6-7, the oscillator strength of high-lying species is enhanced⁸, opening further avenues for tunable excitonic quantum optics.

¹Puchert et al., Nat. Nano. 12, 637 (2017). ²Lin et al., Nat. Comm. 12, 5500 (2021). ³Lin et al., Nat. Phys. 15, 242 (2019). ⁴Bauer et al., Nat. Phot. 10.1038/s41566-022-01080-1 (2022). ⁵Lin et al., Nat. Comm. in press (2022). ⁶Merkl et al., Nat. Mater. 18, 691 (2019). ⁷Merkl et al., Nature Comm. 11, 2167 (2020). ⁸Lin et al., Nat. Comm. 12, 1553 (2021).