Phonon-assisted upconversion photoluminescence in monolayer MoSe2 and WSe2

Abstract

Phonon-assisted upconversion photoluminescence is the basis of laser cooling effect in solids. Compared to conventional cooling methods, laser cooling has many advantages such as compactness, high efficiency, free of vibrational disturbance and refrigerant-free, etc. Exploring more semiconducting materials in which a net cooling effect can be observed is an important area among the laser cooling community. Monolayer two-dimensional transition metal dichalcogenides (TMDs) like MoSe2 and WSe2 are direct band gap semiconductors. The perfect crystal structure and high extraction efficiency (due to thickness at atomic level) guarantee them the possibility to be potential candidates for laser cooling. In this work, with a Ti-sapphire tunable laser (pumped by a 532 nm laser), we studied the upconversion photoluminescence and its wavelength-dependence. In detail, we implemented experiments on laser power-dependent photoluminescence intensity, the linearity fitting results (at low power) show that it is a phonon-assisted upconversion photoluminescence process; furthermore, this mechanism is also verified from the temperature-dependent upconversion photoluminescence intensity. Further experiments on the investigation of which kinds of phonons are involved in the upconversion process and its efficiency, as well as the design of a sample structure that is free of background absorbance, are still required to help to tell whether net laser cooling can be achieved, or how it can be achieved in monolayers TMDs.