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.
Fourier-plane optical microscopy is a powerful technique for studying the angularly-resolved optical properties of a plethora of materials and devices. The information about the direction of the emission of light by a sample is extracted by imaging the objective back focal plane on a two-dimensional detector, via a suitable optical system. This imaging technique is able to resolve the angular spectrum of the light over a wide angular field of view, but typically it doesn’t provide any spectral information, since it integrates the light intensity over a broad wavelength range. On the other hand, advanced hyperspectral imaging techniques are able to record the spectrum of the transmitted/reflected/emitted light at each pixel of the detector. In this work, we combine an innovative hyperspectral imaging system with Fourier-space microscopy, and we apply the novel device to the characterization of planar organic microcavities. In our system, hyperspectral imaging is performed by Fourier-transform spectroscopy thanks to an innovative common-path birefringent interferometer: it generates two delayed replicas of the light field, whose interference pattern is recorded as a function of their delay. The Fourier Transform of the resulting interferogram yields the intensity spectrum for each element of the microscope angular field-of-view. This system provides an angle-resolved hyperspectral view of the microcavities. The hyperspectral Fourier-space image clearly evidences the cavity modes both in photoluminescence and reflection, whose energy has a parabolic dependence on the emission angle. From the hyperspectral image, we reconstruct a 3D view of the parabolic cavity dispersion across the whole Fourier space.
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