Up- and Down-Conversion between Intra- and Intervalley Excitons in Waveguide Coupled Monolayer WSe₂

Abstract

The presence of two spin-split valleys in monolayer (1L) transition metal dichalcogenide (TMD) semiconductors supports versatile exciton species classified by their spin and valley quantum numbers. While the spin-0 intravalley exciton, known as the “bright” exciton, is readily observable, other types of excitons, such as the spin-1 intravalley (spin-dark) and spin-0 intervalley (momentum-dark) excitons, are more difficult to access. Here we develop a waveguide coupled 1L tungsten diselenide (WSe2) device to probe these exciton species. In particular, TM coupling to the atomic layer’s out-of-plane dipole moments enabled us to not only efficiently collect but also resonantly populate the spin-1 dark excitons, promising for developing devices with long valley lifetimes. Our work reveals several upconversion processes that bring out an intricate coupling network linking spin-0 and spin-1 intra- and intervalley excitons, demonstrating that intervalley scattering and spin-flip are very common processes in the atomic layer. These experimental results deepen our understanding of tungsten diselenide exciton physics and illustrate that planar photonic devices are capable of harnessing versatile exciton species in TMD semiconductors.

Symmetry and control of spin-scattering processes in two-dimensional transition metal dichalcogenides

“…Additionally, despite the seemingly simplistic single-particle picture presented here, these results also provide an explanation for the recently observed asymmetry between bright and dark excitons concerning direct to indirect exciton scattering in W-based TMDs [36], where indirect excitons are composed of an electron and a hole in opposite valleys.…”

Bright excitonic multiplexing mediated by dark exciton transition in two-dimensional TMDCs at room temperature

“…These transitions strongly couple to light via in-plane ( x – y direction) dipoles, 19 making the A b -, B b -channels the dominant features in conventional photoluminescence (PL) spectrum. Interestingly, the main PL emission channel in TMDs arises from the A b -exciton, 2,20 where to date, limited increase of the B b -exciton emission signal was realized by several factors such as pump intensity, 21 substrate, 22 sample quality, 23 and bright-to-bright exciton mixing 24 (see ESI† note 1). Nonetheless, an all-optical way enabling strong modulation between the two bright excitonic channels can pave the way for exciting tunable multi-wavelength devices.…”

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

“…We found that 2 for the X exciton is > 74%, even higher than the analogous process in carbon nanotubes (CNTs; < 40%) 29 , suggesting that the dark exciton in 1L-WSe2 behaves as a significant reservoir of X exciton emission through efficient inter-state upconversion. 24…”

“…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.…”