Coupling of MoS_{2} Excitons with Lattice Phonons and Cavity Vibrational Phonons in Hybrid Nanobeam Cavities.

We report resonant Raman spectroscopy of neutral excitons X^{0} and intravalley trions X^{-} in hBN-encapsulated MoS_{2} monolayer embedded in a nanobeam cavity. By temperature tuning the detuning between Raman modes of MoS_{2} lattice phonons and X^{0}/X^{-} emission peaks, we probe the mutual coupling of excitons, lattice phonons and cavity vibrational phonons. We observe an enhancement of X^{0}-induced Raman scattering and a suppression for X^{-}-induced, and explain our findings as arising from the tripartite exciton-phonon-phonon coupling. The cavity vibrational phonons provide intermediate replica states of X^{0} for resonance conditions in the scattering of lattice phonons, thus enhancing the Raman intensity. In contrast, the tripartite coupling involving X^{-} is found to be much weaker, an observation explained by the geometry-dependent polarity of the electron and hole deformation potentials. Our results indicate that phononic hybridization between lattice and nanomechanical modes plays a key role in the excitonic photophysics and light-matter interaction in 2D-material nanophotonic systems.

Twist-Dependent Intra- and Interlayer Excitons in Moiré MoSe_{2} Homobilayers.

Optoelectronic properties of van der Waals homostructures can be selectively engineered by the relative twist angle between layers. Here, we study the twist-dependent moiré coupling in MoSe_{2} homobilayers. For small angles, we find a pronounced redshift of the K-K and Γ-K excitons accompanied by a transition from K-K to Γ-K emission. Both effects can be traced back to the underlying moiré pattern in the MoSe_{2} homobilayers, as confirmed by our low-energy continuum model for different moiré excitons. We identify two distinct intralayer moiré excitons for R stacking, while H stacking yields two degenerate intralayer excitons due to inversion symmetry. In both cases, bright interlayer excitons are found at higher energies. The performed calculations are in excellent agreement with experiment and allow us to characterize the observed exciton resonances, providing insight about the layer composition and relevant stacking configuration of different moiré exciton species.

Tuning the Optical Properties of a MoSe2 Monolayer Using Nanoscale Plasmonic Antennas.

Nanoplasmonic systems combined with optically active two-dimensional materials provide intriguing opportunities to explore and control light-matter interactions at extreme subwavelength length scales approaching the exciton Bohr radius. Here, we present room- and cryogenic-temperature investigations of a MoSe2 monolayer on individual gold dipole nanoantennas. By controlling nanoantenna size, the dipolar resonance is tuned relative to the exciton achieving a total tuning of ∼130 meV. Differential reflectance measurements performed on >100 structures reveal an apparent avoided crossing between exciton and dipolar mode and an exciton-plasmon coupling constant of g = 55 meV, representing g/(ℏωX) ≥ 3% of the transition energy. This places our hybrid system in the intermediate-coupling regime where spectra exhibit a characteristic Fano-like shape. We demonstrate active control by varying the polarization of the excitation light to programmably suppress coupling to the dipole mode. We further study the emerging optical signatures of the monolayer localized at dipole nanoantennas at 10 K.

Room-Temperature Synthesis of 2D Janus Crystals and their Heterostructures.

Janus crystals represent an exciting class of 2D materials with different atomic species on their upper and lower facets. Theories have predicted that this symmetry breaking induces an electric field and leads to a wealth of novel properties, such as large Rashba spin-orbit coupling and formation of strongly correlated electronic states. Monolayer MoSSe Janus crystals have been synthesized by two methods, via controlled sulfurization of monolayer MoSe2 and via plasma stripping followed thermal annealing of MoS2 . However, the high processing temperatures prevent growth of other Janus materials and their heterostructures. Here, a room-temperature technique for the synthesis of a variety of Janus monolayers with high structural and optical quality is reported. This process involves low-energy reactive radical precursors, which enables selective removal and replacement of the uppermost chalcogen layer, thus transforming classical transition metal dichalcogenides into a Janus structure. The resulting materials show clear mixed character for their excitonic transitions, and more importantly, the presented room-temperature method enables the demonstration of first vertical and lateral heterojunctions of 2D Janus TMDs. The results present significant and pioneering advances in the synthesis of new classes of 2D materials, and pave the way for the creation of heterostructures from 2D Janus layers.