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In moiré crystals formed by stacking van der Waals materials, surprisingly diverse correlated electronic phases and optical properties can be realized by a subtle change in the twist angle. Here, we discover that phonon spectra are also renormalized in MoS2 twisted bilayers, adding an insight to moiré physics. Over a range of small twist angles, the phonon spectra evolve rapidly owing to ultra-strong coupling between different phonon modes and atomic reconstructions of the moiré pattern. We develop a low-energy continuum model for phonons that overcomes the outstanding challenge of calculating the properties of large moiré supercells and successfully captures the essential experimental observations. Remarkably, simple optical spectroscopy experiments can provide information on strain and lattice distortions in moiré crystals with nanometre-size supercells. The model promotes a comprehensive and unified understanding of the structural, optical and electronic properties of moiré superlattices. 

Atomically thin, two-dimensional, transition-metal dichalcogenide (TMD) monolayers have recently emerged as a versatile platform for optoelectronics. Their appeal stems from a tunable direct bandgap in the visible and near-infrared regions, the ability to enable strong coupling to light, and the unique opportunity to address the valley degree of freedom over atomically thin layers. Additionally, monolayer TMDs can host defect-bound localized excitons that behave as single-photon emitters, opening exciting avenues for highly integrated 2D quantum photonic circuitry. By introducing plasmonic nanostructures and metasurfaces, one may effectively enhance light harvesting, direct valley-polarized emission, and route valley index. This review article focuses on these critical aspects to develop integrated photonic and valleytronic applications by exploiting exciton–plasmon coupling over a new hybrid material platform.

 

Interlayer exciton diffusion in transition metal dichalcogenide heterostructures is controlled by the moiré potential.

 

Optical dielectric constants are critical to modeling the electronic and optical properties of materials. Silver, as a noble metal with low loss, has been extensively investigated. The recently developed epitaxial growths of single crystalline Ag on dielectric substrates have prompted efforts to characterize their intrinsic optical dielectric function. In this paper, we report spectral ellipsometry measurements and analysis of a thick, epitaxially-grown, single-crystalline Ag film. We focus on the range of 0.18 – 1.0 eV or 1.24 – 7 µm, an energy and wavelength range that has not been examined previously using epitaxial films. We compare the extracted dielectric constants and the predicted optical performances with previous measurements. The loss is appreciably lower than the widely quoted Palik’s optical constants (i.e., up to a factor of 2) in the infrared frequency range. The improved knowledge of fundamental optical properties of the high-quality epitaxial Ag film will have a broad impact on simulations and practical applications based on Ag in the long wavelength range.