Resonant internal quantum transitions and femtosecond radiative decay of excitons in monolayer WSe2

Intraband optical spectroscopy—using an ultrashort mid-infrared probe pulse and a visible pump pulse—probes the bright and dark exciton dynamics simultaneously in a transition metal dichalcogenide monolayer. Atomically thin two-dimensional crystals have revolutionized materials science 1 , 2 , 3 . In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range 4 , 5 , 6 , 7 , 8 , 9 . Their electronic and optical properties are dominated by Coulomb-bound electron–hole pairs called excitons 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , whose unusual internal structure 13 , symmetry 15 , 16 , 17 , many-body effects 18 and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1 s A excitons, regardless of momentum—inside and outside the radiative cone—in single-layer WSe 2 . Phase-locked mid-infrared pulses reveal the internal orbital 1 s –2 p resonance, which is highly sensitive to the shape of the excitonic envelope functions and provides accurate transition energies, oscillator strengths, densities and linewidths. Remarkably, the observed decay dynamics indicates an ultrafast radiative annihilation of small-momentum excitons within 150 fs, whereas Auger recombination prevails for optically dark states. The results provide a comprehensive view of excitons and introduce a new degree of freedom for quantum control, optoelectronics and valleytronics of dichalcogenide monolayers 19 , 20 , 21 , 22 , 23 , 24 .