Control of ultrafast excitonic shift current-induced THz emission efficiency of layered single-crystalline MoS2

Following ultrafast photoexcitation, a semiconductor exhibits competing dynamics among photocarriers, many-body transient states of highly energetic excitons, and electron–hole liquid (EHL). Here, we show that femtosecond optical pulse excitation induces a transient excitonic shift current contributing to stronger terahertz (THz) emission from a single-crystalline bulk MoS2 at low temperatures. The control of dominating excitonic shift current is elucidated from excitation density-dependent experiments at varying temperatures. A strong decrease in the excitonic contribution beyond a critical fluence of 150 μJ/cm2 is observed at a very low temperature of 20 K. This behavior suggests the formation of a new quantum condensate, i.e., the EHL, in the regime when the exciton density is overwhelmingly large that the average spacing between exciton pairs is comparable to the exciton radius. Furthermore, the exciton density-dependent THz emission at varying temperatures is consistent with the Varshni model and the crystal Debye temperature of 260 K.