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.
In this novel multimodal wide-field Raman microscope, spectra are obtained by the time-domain Fourier-transform method. The wide-field approach enables faster collection of Raman maps, while the time-domain method disentangles fluorescence and Raman signals. This is obtained by choosing a proper sampling of the interferogram, thanks to the use of an ultrastable common-path birefringent interferometer. Validation of the system is performed on plastic microbeads; multimodality is demonstrated by fluorescence and Raman maps of a few-layers transition metal dichalcogenide sample.
We present a novel wide-field Raman microscope, based on the time-domain Fourier-transform method. This enables parallel acquisition of Raman spectra on all the pixels of the 2D detector; the resulting wide-field approach allows faster collection of Raman maps with respect to standard raster-scanning methods. In addition, the time-domain method disentangles fluorescence and Raman signals. The system is robust and stable, thanks to the use of an ultrastable common-path birefringent interferometer. Validation of the system is performed on plastic microbeads and on a few-layers transition metal dichalcogenide sample.
KEYWORDS: Terahertz spectroscopy, Near infrared, Excitons, Crystals, Waveguide modes, Near field, Dielectrics, Near field optics, Waveguides, Second harmonic generation, Microscopy
Near-field microscopy has emerged as a powerful tool for investigating the optoelectronic properties of van der Waals crystals on deeply subwavelength length scales. Complementary information may be obtained by interrogating the layered materials with electromagnetic radiation oscillating at vastly different frequencies: In the terahertz spectral range, for example, the polarizability of excitons in transition metal dichalcogenide (TMDC) heterostructures can be recorded on subcycle timescales–granting access to ultrafast formation dynamics or the exciton Mott transition with nanometer precision. In contrast, visible or near-infrared light propagates through thin van der Waals slabs in the form of waveguide modes (WMs). By resolving interference patterns in maps of the scattered electric field, the anisotropic dielectric tensor of layered materials is retrieved and signatures of strong light-matter coupling in the dispersion of the WMs are revealed. This approach also allows for boosting the potential of 3R-stacked TMDCs for applications in nonlinear optics by quantifying their birefringence, thus providing essential parameters for future phase-matched waveguide second harmonic generation and compact on-chip optical devices in general.
Optical parametric amplification is a coherent mechanism whereby an optical signal is amplified by a pump via the generation of an idler field and it is the key ingredient of parametric oscillators. Here we demonstrate optical parametric amplification by monolayer transition-metal dichalcogenides, showing that amplification can be attained over a propagation through an atomic layer. The surface-like second-order nonlinear interaction bypasses phase-matching constraints, enabling ultrabroadband collinear amplification, generally unattainable due to material dispersion. Moreover, the amplification process is invariant over signal and pump in-plane polarizations. Our experimental findings pave the way for innumerable applications in nanophotonics and quantum information technology.
We use ultrafast spectroscopy to resolve the interlayer charge scattering processes in heterostructures (HS) of two-dimensional materials. In a WSe2/MoSe2 HS we photogenerate intralayer excitons in MoSe2 and observe hole injection in WSe2 on the sub-picosecond timescale, leading to the formation of interlayer excitons. The temperature dependence of the build-up and decay of interlayer excitons provide insights into the coupling mechanisms. In a graphene/WS2 HS we observe ultrafast charge transfer from graphene to the semiconductor. The data are consistent with hot electron/hole transfer, whereby a tail the hot Fermi-Dirac carrier distribution in graphene tunnels through the Schottky barrier into WS2.
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