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

Abstract

Atomically thin two-dimensional crystals have revolutionized materials science. In particular, monolayer transition metal dichalcogenides promise novel optoelectronic applications, owing to their direct energy gaps in the optical range. Their electronic and optical properties are dominated by Coulomb-bound electron-hole pairs called excitons, whose unusual internal structure, symmetry, many-body effects and dynamics have been vividly discussed. Here we report the first direct experimental access to all 1s A excitons, regardless of momentum--inside and outside the radiative cone--in single-layer WSe2. Phase-locked mid-infrared pulses reveal the internal orbital 1s-2p 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.

Direct Observation of Ultrafast Exciton Formation in a Monolayer of WSe2

“…20,21 The properties of excitons in 2D TMDCs are a topic of intense research, investigating, for example, rapid exciton−exciton scattering, 22 interlayer excitons, 23 charged excitons and excitonic molecules, 24,25 ultrafast recombination dynamics, 19,26−28 or efficient coupling to light and lattice vibrations. 4,19,29,30 In many experiments, excitons are created indirectly through nonresonant optical excitation or electronic injection, which may prepare unbound charge carriers with energies far above the exciton resonance. 8,18 Subsequently, the electrons and holes are expected to relax toward their respective band minima and form excitons in the vicinity of the fundamental energy gap.…”

“…Recently, this technique has become sensitive enough to probe the intraexcitonic transitions even in single atomically thin layers of TMDCs following direct optical injection of the excitons. 19,37 The exciton formation, however, as well as the dynamics of unbound photoexcited charge carriers in a 2D TMDC has not been studied by direct low-energy probing so far, to the best of our knowledge.…”

Dielectric Engineering of Electronic Correlations in a van der Waals Heterostructure

“…12,47 The pump-induced change ΔE observed at a fixed electro-optic sampling time t EOS = 0 fs (Figure 4a, black solid line) has been established to be proportional to the exciton density. 12,13 Thus, recording ΔE(t EOS = 0 fs) as a function of t PP provides an alternative access to the temporal evolution of the exciton density. The almost perfect agreement of the temporal shape of n X (t PP ) (Figure 4a, black spheres) with ΔE(t EOS = 0 fs, t PP ) (Figure 4a, black solid line) further corroborates the phenomenological Drude−Lorentz model of eq 1.…”

“…The density of excitons (Figure 4a, black spheres) initially decays within the first hundreds of femtoseconds from n X = 3.93 × 10 12 cm −2 to 2.15 × 10 12 cm −2 followed by a much slower decay on a picosecond scale. 12 This dynamics corresponds to the radiative recombination of coherent bright excitons followed by the decay of incoherent dark states. 12,47 The pump-induced change ΔE observed at a fixed electro-optic sampling time t EOS = 0 fs (Figure 4a, black solid line) has been established to be proportional to the exciton density.…”

“…12 This dynamics corresponds to the radiative recombination of coherent bright excitons followed by the decay of incoherent dark states. 12,47 The pump-induced change ΔE observed at a fixed electro-optic sampling time t EOS = 0 fs (Figure 4a, black solid line) has been established to be proportional to the exciton density. 12,13 Thus, recording ΔE(t EOS = 0 fs) as a function of t PP provides an alternative access to the temporal evolution of the exciton density.…”