Abstract
Atomically thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions. The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. They are not visible in the optical spectra, but can strongly influence the coherence lifetime and the linewidth of the emission from bright exciton states. Here, we investigate the microscopic origin of the excitonic coherence lifetime in two representative materials (WS2 and MoSe2) through a study combining microscopic theory with spectroscopic measurements. We show that the excitonic coherence lifetime is determined by phonon-induced intravalley scattering and intervalley scattering into dark excitonic states. In particular, in WS2, we identify exciton relaxation processes involving phonon emission into lower-lying dark states that are operative at all temperatures.
Highlights
Thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions
In addition to the optically accessible bright excitonic states located at the K and K0 points at the corners of the hexagonal Brillouin zone[6,7,8,9,10], there is a variety of optically forbidden states including p excitons exhibiting a non-zero angular momentum, intravalley excitons with a non-zero centre-of-mass momentum beyond the light cone, spin-forbidden intravalley exciton triplets, as well as intervalley excitons where a hole is located at the K point and the electron either at the K0 point or the L point[11,12,13], cf
Since excitons dominate the optical response of transition metal dichalcogenides (TMDs), a microscopic understanding of their properties is crucial for technological applications in future optoelectronic and photonic devices
Summary
Thin transition metal dichalcogenides are direct-gap semiconductors with strong light–matter and Coulomb interactions The latter accounts for tightly bound excitons, which dominate their optical properties. Besides the optically accessible bright excitons, these systems exhibit a variety of dark excitonic states. It was shown that in tungsten-based TMDs, the intervalley dark excitonic state lies energetically below the optically accessible exciton, resulting in a strong quenching of photoluminescence at low temperatures[14,15]. The presence of dark states has a strong impact on the coherence lifetime of optically accessible states, since they present a possible scattering channel that can be accessed via emission or absorption of phonons. A consistent microscopic theory description of exciton–phonon scattering is not available yet
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