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.
The hybrid light-matter character of exciton-polaritons enables strikingly long-range energy transport in organic materials. We use femtosecond transient absorption microscopy to probe this behavior in the initial coherent regime, where photon and exciton wavefunctions are inextricably mixed. We achieve rapid polaritonic transport in highly ordered, pure organic semiconductor films without any external cavity. In a disordered system, confinement within a Fabry-Perot cavity provides enables comparable coherent transport effects. In both cases, the polaritons don’t travel alone: they are accompanied by intracavity dark states, which reduce the transport velocity, extend the lifetime, and provide a new mechanism for external control.
The interaction of organic semiconductors with confined light fields offers one of the easiest means to tune their material properties. In the regime of strong light-matter coupling, the semiconductor exciton and cavity photon mode hybridize to form new 'polariton' states. In organic systems these light-matter hybrids are tuneably separated by as much as 100’s of meV from the parent exciton, enabling radical alteration of the energetic landscape. The effects of strong coupling can be profound, including reports of long-range energy transfer, enhanced carrier mobility and altered chemical reactivity. Theoretical work is now increasingly focused on the potential of polariton to manipulate electronic dynamics in the excited state, but experimental realisation has proved challenging. Here, we demonstrate the ability to manipulate triplet photophysics in singlet exciton fission materials in the strong coupling regime. Within microcavities, we dramatically enhance the emission lifetime and increase delayed fluorescence by >100%, which we explain through a shift in the thermodynamic equilibrium between dark states in the exciton reservoir and the bright polaritons. Indeed, with this approach we can create entirely new radiative pathways, turning completely dark states bright and opening new scope for microcavity-controlled materials.
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