Two-photon excited ultrafast spin dynamics in monolayer MoS2

Monolayers of transition-metal dichalcogenides (TMDCs) such as MoS2 have drawn great interest because of the direct band gap nature in monolayers to explore 2D valley excitons. So, far there have been various spectroscopic... [ view full abstract ]

Monolayers of transition-metal dichalcogenides (TMDCs) such as MoS2 have drawn great interest because of the direct band gap nature in monolayers to explore 2D valley excitons. So, far there have been various spectroscopic efforts to study exciton dynamics in MoS2 such as reflectance, photoluminescence excitation spectroscopy and pump-probe spectroscopy. However, little is known related to exciton relaxation dynamics under two-photon excitation, where a thorough knowledge of the valley selective selection rules for optically inactive dark excitons in monolayer MoS2 is needed.

In this work, we have studied two-photon excited ultrafast spin dynamics in monolayer MoS2, by performing two-photon excited pump-probe differential reflection and photoluminescence (PL) studies near 1200 nm excitation wavelength. By exploiting the interplay between same and opposite pump-probe circular polarizations the differential reflection signal (Fig. a) associated with the exciton dynamics of optically inactive dark excitons (mj+-2) (Fig. b) created under circularly polarized two-photon excitation are probed using broadband white light supercontinuum via optically active bright excitons (~625 nm and ~695 nm). The pump-probe signal is attributed to transformation of dark exciton into bright exciton via phonon-assisted inter-valley scattering. In our two-photon excited PL studies, a two-photon absorption (TPA) saturation effect (with high unsaturated TPA co-efficient ~ 10^4 cm/GW) is observed at higher excitation intensity (>1 MW/cm2) possible because the pulse duration is shorter than the thermalization time of the generated carriers and with high intense laser pulse. Moreover, the enhanced PL intensity for linearly polarized emission as compared to circularly polarized emission at different polarization angle suggests that the linearly polarized excitation at 1250 nm (Fig. c) creates exciton valley coherence, where phonon assisted inter-valley scattering is prevented due to insufficient excitation energy. Our results provide important experimental information for exciton-valley coupling in monolayer MoS2 - for valleytronics, quantum optics and photonic devices.