Figure 1: Exciton resonances in linear spectroscopy and upconversion at T=4 K.
From: Enabling valley selective exciton scattering in monolayer WSe2 through upconversion
The sample consists of a WSe2 monolayer encapsulated in hBN (ref. 61). (a) Strongly bound electron–hole pairs, excitons, dominate optical properties of TMDC monolayers such as WSe2. In WSe2 there exist two different exciton series. The B-exciton is about 400 meV above the A-exciton due to spin-obit splitting of the valence band. (b) Excitons have a binding energy EB, defined as the difference between the free particle bandgap and the optical bandgap observed in photoluminescence (PL) emission. EB is of the order of 500 meV, the first excited state n=2 is about 130 meV above the n=1 state, marked as A:2s and A:1s, respectively, throughout this manuscript. (c) We have performed reflectivity with a white light source to identify the A- and B-exciton at T=4 K. In addition we observe an excited states of the A-exciton labelled A:2s. (d) In photoluminescence we observe neutral A-exciton and trion emission (T), in addition we see hot luminescence of the A:2s state. This is the only spectrum obtained using a pulsed laser in this work, to reach the energy of the B:1s, see Methods. (e) We also demonstrate upconversion PL: the laser is tuned to the A:1s-exciton resonance and strong emission from B:1s and A:2s at much higher energy is recorded, in addition to the trion emission (T) at lower energy. (f) optical microscope image of the studied van der Waals heterostructure, with scale-bar of 5 μm. (g) Scheme of the side view of the sample.
