Excitonic Photoluminescence Up-conversion Of 2D Layered Tungsten Disulfides

Two-dimensional layered transition metal dichalcogenides（TMDCs）is a new type of semiconducting materials,which has many unique advantages,such as direct-indirect band gap transition,strong exciton effect,super fast charge transfer,spin-valley coupling electronics,etc.These interesting properties can be used for frontier research of many-body physics and applications of modern optoelectronic devices However,there is still no definite conclusion on the interactions of photon-exciton and phonon-exciton.In this work,we study luminescence up and down conversion of tungsten disulfide（WS₂）at tighly bounded excitonic states with Raman spectroscopy.In this research,the physical properties and influencing factors of the exciton luminescence up-conversion effect of layered material WS₂ are studied.The layer number of WS₂films were determined by atomic force microscopy（AFM）,combining the single-photon absorption/reflection spectrum and the change of optical phonon A_1g Raman peak position with the layer number.The continuous laser of 532nm and Micro-Raman technology are used to explore the phonon mode of WS₂ crystal and the spectral information of X⁰ and X^-exciton under different temperatures and excitation power.By analyzing the luminescence at excitonic states of excitons with one-and two-photon with CW and fs lasers,we can study the exciton-phonon interaction with linear and non-linear optics.We report the double resonance enhancement effect in WS₂ crystals.On the basis of conversion fluorescence enhancement on X⁰ spontaneous radiation,the X⁰,X^-exciton,A_1g and E¹_2g optical phonons in WS₂ crystal can meet the double resonance condition by using 633 nm continuous laser excitation.The up-conversion fluorescence intensity of the X⁰exciton is significantly stronger than that of the down-conversion fluorescence.Under the same condition,the result obtained are compared with the experimental result with only up-conversion effect,and the feasibility of optical refrigeration is verified.On this basis,the influence factors of substrate,temperature and excitation power of X⁰ are further studied and analyzed to explore the condition under which the up-conversion energy gain could be increased,thus providing a new idea for optical refrigeration of two-dimensional semiconductor materials.