Room-temperature multi-phonon upconversion photoluminescence in monolayer semiconductor WS_2

摘要

Photon upconversion, a step toward laser cooling of solids, is an anti-Stokes process in which an absorption of a photonleads to a reemission of a photon at energy higher than the excitation energy. Here, we demonstrate room temperatureupconversion photoluminescence process in a monolayer semiconductor WS2, with energy gain up to 150 meV. Weattribute this process to transitions involving trions (T) and many phonons and free exciton complexes (X). We show thatthe energy gain significantly depends on the temperature. In order to gain insight into the temperature dependence of themechanism of the upconverted emission, we combine the normal and upconverted photoluminescence of the monolayerWS2 at low, intermediate and high temperatures. At 7 K the energy gain of upconversion emission amounts about 40meV, which is comparable with the energy difference between the X and T emission lines and also nearly resonates withthe energy of one optical phonon (A’1 or E’). This suggests that at low temperature the upconversion process is related tothe coupling between the T and X states mediated by one optical phonon. The higher energy gain of ~60 meV at 70 Ksuggests that more than one phonon is involved in the upconversion process.