Fine structure and lifetime of dark excitons in transition metal dichalcogenide monolayers

Abstract

The intricate interplay between optically dark and bright excitons governs the light-matter interaction in transition metal dichalcogenide monolayers. We have performed a detailed investigation of the "spin-forbidden" dark excitons in WSe2 monolayers by optical spectroscopy in an out-of-plane magnetic field Bz. In agreement with the theoretical predictions deduced from group theory analysis, magneto-photoluminescence experiments reveal a zero field splitting δ=0.6 ± 0.1 meV between two dark exciton states. The low energy state being strictly dipole forbidden (perfectly dark) at Bz=0 while the upper state is partially coupled to light with z polarization («grey» exciton). The first determination of the dark neutral exciton lifetime τ D in a transition metal dichalcogenide monolayer is obtained by time-resolved photoluminescence. We measure τ D~1 10 ± 10 ps for the grey exciton state, i.e. two orders of magnitude longer than the radiative lifetime of the bright neutral exciton at T=12 K.

Ultra‐Long Lifetimes of Single Quantum Emitters in Monolayer WSe2/hBN Heterostructures

“…In ref. [], it has been shown that the lifetime of the dark excitons is two orders of magnitude longer than the lifetime of the bright excitons in WSe 2 . We see a similar trend in our experimental results compared to previously measured lifetimes of defect‐localized states in WSe 2 .…”

Valley phonons and exciton complexes in a monolayer semiconductor

“…The emergent valley-contrasting properties not only impact quasiparticles, but also are expected to give rise to new physics involving zone edge phonons, or valley phonons, which are collective lattice oscillations at the corners of hexagonal Brillouin zone (±K points). These phonons have been predicted to play an important role in spin and valley pseudospin relaxation through phononassisted intervalley scattering 7,8 . Additionally, valley phonons can possess chirality with intrinsic pseudo-angular momentum 9 , which has recently attracted wide attention [10][11][12][13] .…”

“…Here, we identify the signatures of multiple valley phonons in monolayer WSe2. The monolayer WSe2 hosts stable and long-lived dark exciton and trion as ground states [17][18][19][20][21][22] . The resulting accumulated exciton and trion populations are highly desirable for studying their interactions with phonons.…”

Orbital, spin and valley contributions to Zeeman splitting of excitonic resonances in MoSe 2 , WSe 2 and WS 2 Monolayers

“…Notably large g-factors (about-8) have been recently reported for intra valley dark excitons in WSe 2 monolayer encapsulated in hBN [56]. Our simple consideration takes into account recombination of neutral complexes.…”