Photo-induced excitonic structure renormalization and broadband absorption in monolayer tungsten disulphide

Atomically thin transition metal dichalcogenides (TMDCs) have emerged as a new class of two-dimensional (2D) material for novel optoelectronic applications. In particular, 2D TMDCs are viewed as intriguing and appealing materials to construct Q-switching and mode-locked modulators, due to their broadband saturable absorption even of photon energy below their excitonic energies. However, the dynamics and mechanism of saturable absorption inside TMDCs has yet to be investigated. In this paper, the relaxation dynamics of monolayer tungsten disulphide (WS2) was investigated considering different excitonic transitions. WS2 illustrates dramatic changes in optical responses when excited by intense laser pulses, which are characterized by the broadband photo-induced nonresonance absorption and the giant excitonic bands renormalization process. The experimental results show that strong photo-induced restructuring of excitonic bands has picosecond lifetime and full recovery of optical responses takes hundreds of picosecond. Additionally, our observations reveal that heavy renormalization and overlap of excitonic bands are induced by strong many-body Coulomb interactions. Moreover, the broadband absorption feature of WS2 opens up new applications in broadband saturable absorbers and ultrafast photonic devices. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement