Multipath Optical Recombination of Intervalley Dark Excitons and Trions in Monolayer WSe2

Abstract

Excitons and trions (or exciton-polarons) in transition metal dichalcogenides (TMDs) are known to decay predominantly through intravalley transitions. Electron-hole recombination across different valleys can also play a significant role in the excitonic dynamics, but intervalley transitions are rarely observed in monolayer TMDs, because they violate the conservation of momentum. Here we reveal the intervalley recombination of dark excitons and trions through more than one path in monolayer WSe2. We observe the intervalley dark excitons, which can recombine by the assistance of defect scattering or chiral-phonon emission. We also reveal that a trion can decay in two distinct pathsthrough intravalley or intervalley electron-hole recombination -into two different final valley states. Although these two paths are energy degenerate, we can distinguish them by lifting the valley degeneracy under a magnetic field. In addition, the intra-and intervalley trion transitions are coupled to zone-center and zone-corner chiral phonons, respectively, to produce distinct phonon replicas. The observed multipath optical decays of dark excitons and trions provide much insight into the internal quantum structure of trions and the complex excitonic interactions with defects and chiral phonons in monolayer valley semiconductors.

Long-lived populations of momentum- and spin-indirect excitons in monolayer WSe2

“…For the D and XK excitons, we observe a much slower population decay rate, consistent with the much smaller oscillator strength compared to X. Specifically, the radiative emission of D and XK requires a spin-flip of electrons 30 and defect/phonon scattering [21][22][23] , respectively. To capture the dynamics of all the excitons populations, especially the one of XK exciton, we perform a universal fitting with the rate equations, Eq.…”

“…We note that the behaviour of Imax should not be confused with the power dependent peak intensity in the continuous-wave (CW) measurement. 22,23 In Supplementary S3, we show the powerdependent PL measurements with a CW laser at 2.33 eV, where the intensity of XK exhibits a linear dependence on the incident power. This discrepancy can be understood as illustrated in the inset of Figure 4b: Although the maximum intensity is growing superlinearly, the lifetime of the exponential decay also decreases (due to more efficient conversion XK → D → X), resulting in a linear dependence of PL intensity.…”

“…As a result, the energy of a photon emitted by the XK exciton is 61 meV below the X exciton emission, even lower than D exciton. [21][22][23][24] In this work, by measuring the time-resolved photoluminescence (Tr-PL) of the high-quality hexagonal boron nitride (hBN) encapsulated 1L-WSe2 at cryogenic temperature, we observe dense populations of D and XK excitons with sub-ns lifetime.…”

Guide to optical spectroscopy of layered semiconductors

“…In WSe 2 and WS 2 monolayers, dark excitons are prominent (Fig. 2b) and lead to exotic, higher-order complexes (such as biexcitons made up of a dark and a bright exciton, and dark trions 128,144 ), which can be identified by monitoring the orientation of the exciton dipole. The role of out-of-plane dipole emission is also investigated for quantum emitters in WSe 2 (reF.…”

Optical study of excitonic complexes in transition metal dichalcogenides monolayers

“…The peak with the highest energy at ∼2.063 eV is identified as the neutral exciton state X A from the K/K transition in the Brillouin zone, whose energy matches its absorption resonance in Figure 5.1. The peaks at ∼2.033 eV and 2.027 eV are identified as the intervalley trion state (the so-called triplet state T − ) and intravalley trion state (singlet state S − ) states with the trion binding energies of ∼ 30 meV and 36 meV, respectively, as reported in previous studies for W-based monolayers [90,[93][94][95][96][97][100][101][102][103]. The energy splitting between the triplet and singlet state ∆ S−T is ∼6 meV, arising from the exchange interaction δ between the excess electron and the electron-hole pair that have the spin configuration (see the inset in Figure 5.4(a)).…”