Two-dimensional semiconductors offer a compelling platform for excitons with robust interaction with light, owing to their confined nature and their numerous manipulable degrees of freedom. In bilayers, interlayer excitons (IX) combine these degrees of freedom with high interactions due to their out-of-plane alignment. However, their oscillator strength is often negligible. Interlayer hybridization provides IX with a significant oscillator strength. Here, we examine the ultrafast dynamics of these hybrid IX in bilayer and trilayer MoSe2. We find that IX are particularly strong in trilayers. These unexplored excitonic species exhibit fundamentally different dynamics from IX in bilayers, with delayed rise times of over 2 ps and significantly longer lifetimes. We attribute this to the origin of this excitonic species and confirm it with theory. Our findings offer insights into high oscillator strength, long-living interlayer excitons in trilayers, superior to their bilayer counterparts.
Hybridization between inter- and intralayer excitons can occur in Transition Metal Dichalcogenide (TMD) bilayers, giving rise to dipolar excitons with high oscillator strength. Such excitons can be exploited to achieve high optical nonlinearities, when TMDs are strongly coupled to light confined in optical microcavities. However, observations of TMD polaritons ultrafast temporal dynamics and their exploitation remain elusive. We performed pump-probe spectroscopy experiments at 8K in a custom-made microscope to study hBN-encapsulated monolayers and bilayers of MoS2 placed in optical microcavities. We probe the ultrafast dynamics of exciton-polaritons in such systems by resonantly exciting the cavities with femtosecond pulses and measuring the transient differential reflectivity. Our experiments revealed an ultrafast sub-picosecond switching from strong to weak coupling regime with a fast reversible recovery, and we demonstrated its high frequency operation (250 GHz) as an optical switch. The rich dynamics of TMD polaritons explored in our work give access to extreme nonlinear phenomena in TMD systems on ultrafast time scales for future optical logic gates.
Strong coupling between light and excitations of a two-dimensional electron gas (2DEG) are important to both pure physics and to the development of future photonic nanotechnologies. Studying the relationship between spin polarisation of a 2DEG in monolayer semiconductor MoSe2, and resultant light-matter interactions modified by a zero-dimensional optical microcavity, finds the robust spin-susceptibility of the 2DEG simultaneously enhances/supresses trion-polariton formation in opposite photon helicities. This leads to optical non-linearities arising from the highly non-linear behaviour of the valley-specific strong light-matter coupling regime and allowing all-optical tuning of the enhanced polaritonic Zeeman splitting from 4 to more than 10 meV.
https://www.nature.com/articles/s41566-022-01025-8
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