Fig. 1: Formation of high-lying excitons (HX) in monolayer WSe2, with an electron in an upper conduction band and a hole in the top-most valence band.
From: Narrow-band high-lying excitons with negative-mass electrons in monolayer WSe2
a Calculated GW band structure of bare monolayer WSe2. Higher (lower) energy spin-split bands are labeled by + (–) superscripts. The color indicates normalized oscillator strength of electrons making a transition from the top valence band (VB+) to the different conduction bands (CB, CB+1, and CB+2). Red double-headed arrows mark the exciton resonances invoked to explain quantum interference in optical second-harmonic generation (SHG)7. b Resonant pumping of band-edge A excitons (red arrow) results in momentum-selective excitation of higher-lying bands at the K-symmetry points. Electrons from the high-lying band recombine with holes from the valence band to emit UV light (blue arrow), measured as upconverted photoluminescence (UPL) spectra. Electrons in the first conduction band (CB) relax towards the K-points by emitting phonons, whereas electrons in the downwards-curved higher-lying conduction band relax away from the K-points (green curves). c PL spectrum of hBN-encapsulated monolayer WSe2 at 5 K under narrow-band continuous-wave (CW) excitation at 1.732 eV. The peak at 3.464 eV arises from SHG (purple), below which upconverted PL (UPL) from the HX (blue) is observed in the form of a set of 10 peaks. The bottom-left inset indicates the Auger-like process responsible for populating exciton states from the higher-lying conduction band, generating UPL. The intensity of the SHG (purple dots) co-polarized with the laser varies as the laser polarization is rotated with respect to the crystal lattice (right inset), whereas the HX luminescence (blue dots) does not. The top-left inset shows the UPL excitation (UPLE) spectrum of the HX (blue), which matches the band-edge A-exciton PL spectrum (red).
