2020/02/05 by Tomasz Jakubczyk, Jakubczyk, Tomasz, Miroslav Bartoš +11
Physics and Astronomy · Neuroscience · Materials Science · #Spectroscopy and Quantum Chemical Studies #Photoreceptor and optogenetics research #Phase-change materials and chalcogenides
paper · pdf · doi:10.48550/arxiv.2002.01720
We investigate dynamics of resonantly excited excitons in single-layers of\nMoSe2 and WS2 down to 4.5 K. To this end, we measure the delay dependence\nof the heterodyne four-wave mixing (FWM) amplitude induced by three, short\nlaser pulses. This signal depends not only on the population of optically\nactive excitons, which affects the absorption of the probe, but also on the\npopulation of optically inactive states, by interaction-induced energy shift,\ninfluencing the refractive index experienced by the probe. As such, it offers\ninsight into density dynamics of excitons which do not directly couple to\nphotons. Reproducing the coherent signal detected in amplitude and phase, the\nFWM delay dependence is modeled by a coherent superposition of several\nexponential decay components, with characteristic time constants from 0.1\npicosecond up to 1 nanosecond. With increasing excitation intensity and/or\ntemperature, we observe strong interference effects in the FWM field amplitude,\nresulting in progressively more complex and nonintuitive signal dynamics. We\nattribute this behaviour to increasingly populated exciton dark states, which\nchange the FWM field phase by the relative effect on absorption and refractive\nindex. We observe that exciton recombination occurs on a significantly longer\ntimescale in WS2 with respect to MoSe2, which is attributed to the dark\ncharacter of exciton ground state in the former and the bright in the latter.\n