Optical Properties of Atomically Thin Layered Transition Metal Dichalchogenide

Semiconducting transition metal dichalcogenides (TMDs) called as "beyond graphene" will open new research fields of atomically thin layered materials. The optical properties of monolayer TMDs are dominated by strongly correlated bound states of electron–hole (excitons) and two-electrons–hole (charged exciton) due to the enhanced Coulomb interactions confined in atomically thin layered materials. Here, we review novel optical properties related to carriers, excitons, and charged excitons in monolayer TMDs, including features of optical absorption, photoluminescence (PL) spectra, control of PL by chemical doping, and nonlinear optical response, which have been studied by continuous-wave and time-resolved optical spectroscopy. The understanding of optical phenomena related to optically excited states including carriers, excitons, and charged excitons (trions) is critical for the development of future optoelectronic devices using TMDs.