Temporally Resolving Synchronous Degenerate and Nondegenerate Two-Photon Absorption in 2D Semiconducting Monolayers

Degenerate and nondegenerate two-photon absorption (TPA) in WS2 and MoSe2 monolayers are synchronously induced and temporally resolved by femtosecond laser pump-probe. Differential transmission signals in the first 500 fs consist of negative and positive components, that originate from direct probe depletion via nondegenerate TPA and carrier accumulation via degenerate TPA, respectively. Temporal cross-correlation of pump and probe pulses allows us to fully decouple the ultrafast nondegenerate and degenerate TPA signals. Subsequently, degenerate and nondegenerate TPA coefficients are calculated as a function of pump irradiance. Under nonresonant pumping, 100 +/- 10 and 250 +/- 25 cm GW(-1) are obtained for degenerate and nondegenerate TPA coefficients of monolayer WS2, respectively, which both present linearly decreasing trends as increasing pump irradiances. However, under resonant pumping of 2p excitonic states in monolayer MoSe2, degenerate TPA coefficients exponentially decrease from 800 to 80 cm GW(-1) as increasing pump irradiances, due to the interplay between band-renormalization and band-filling effects, while nondegenerate TPA coefficient is about 650 +/- 50 cm GW(-1). For comparison, a trilayer MoSe2 is also investigated. These results set a foundation for precisely measuring TPA coefficients and actively controlling nonlinear excitonic dynamics via TPA in 2D semiconducting monolayers.