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Register a new userStrong light-matter coupling in MoS$_2$
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Polariton-based devices require materials where light-matter coupling under ambient conditions exceeds losses, but our current selection of such materials is limited. Here we measured the dispersion of polaritons formed by the $A$ and $B$ excitons in thin MoS$_2$ slabs by imaging their optical near fields. We combined fully tunable laser excitation in the visible with a scattering near-field optical microscope to excite polaritons and image their optical near fields. We obtained the properties of bulk MoS$_2$ from fits to the slab dispersion. The in-plane excitons are in the strong regime of light-matter coupling with a coupling strength ($40-100,$meV) that exceeds their losses by at least a factor of two. The coupling becomes comparable to the exciton binding energy, which is known as very strong coupling. MoS$_2$ and other transition metal dichalcogenides are excellent materials for future polariton devices.
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To translate electrical into optical signals one uses the modulation of either the refractive index or the absorbance of a material by an electric field. Contemporary electroabsorption modulators (EAMs) employ the quantum confined Stark effect (QCSE), the field-induced red-shift and broadening of the strong excitonic absorption resonances characteristic of low-dimensional semiconductor structures. Here we show an unprecedentedly strong transverse electroabsorption (EA) signal in a monolayer of the two-dimensional semiconductor MoS2. The EA spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to the conventional QCSE, the signal increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dBnm-1 bear the scope for extremely compact, ultrafast, energy-efficient EAMs for integrated photonics, including on-chip optical communication.
Transition metal dichalcogenides like MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$ have attracted enormous interest during recent years. They are van-der-Waals crystals with highly anisotropic properties, which allows exfoliation of individual layers. Their remarkable physical properties make them promising for applications in optoelectronic, spintronic, and valleytronic devices. Phonons are fundamental to many of the underlying physical processes, like carrier and spin relaxation or exciton dynamics. However, experimental data of the complete phonon dispersion relations in these materials is missing. Here we present the phonon dispersion of bulk MoS$_2$ in the high-symmetry directions of the Brillouin zone, determined by inelastic X-ray scattering. Our results underline the two-dimensional nature of MoS$_2$. Supported by first-principles calculations, we determine the phonon displacement patterns, symmetry properties, and scattering intensities. The results will be the basis for future experimental and theoretical work regarding electron-phonon interactions, intervalley scattering, as well as phonons in related 2D materials.
Two-dimensional (2D) crystals have renewed opportunities in design and assembly of artificial lattices without the constraints of epitaxy. However, the lack of thickness control in exfoliated van der Waals (vdW) layers prevents realization of repeat units with high fidelity. Recent availability of uniform, wafer-scale samples permits engineering of both electronic and optical dispersions in stacks of disparate 2D layers with multiple repeating units. We present optical dispersion engineering in a superlattice structure comprised of alternating layers of 2D excitonic chalcogenides and dielectric insulators. By carefully designing the unit cell parameters, we demonstrate > 90 % narrowband absorption in < 4 nm active layer excitonic absorber medium at room temperature, concurrently with enhanced photoluminescence in cm2 samples. These superlattices show evidence of strong light-matter coupling and exciton-polariton formation with geometry-tunable coupling constants. Our results demonstrate proof of concept structures with engineered optical properties and pave the way for a broad class of scalable, designer optical metamaterials from atomically-thin layers.
Inversion-symmetric crystals are optically isotropic and thus naively not expected to show dichroism effects in optical absorption and photoemission processes. Here, we find a strong linear dichroism effect (up to 42.4%) in the conduction band of inversion-symmetric bilayer MoS$_2$, when measuring energy- and momentum-resolved snapshots of excited electrons by time- and angle-resolved photoemission spectroscopy. We model the polarization-dependent photoemission intensity in the transiently-populated conduction band using the semiconductor Bloch equations and show that the observed dichroism emerges from intralayer single-particle effects within the isotropic part of the dispersion. This leads to optical excitations with an anisotropic momentum-dependence in an otherwise inversion symmetric material.
We present an ultra-high vacuum scanning tunneling microscopy (STM) study of structural defects in molybdenum disulfide thin films grown on silicon substrates by chemical vapor deposition. A distinctive type of grain boundary periodically arranged inside an isolated triangular domain, along with other inter-domain grain boundaries of various types, is observed. These periodic defects, about 50 nm apart and a few nanometers in width, remain hidden in optical or low-resolution microscopy studies. We report a complex growth mechanism that produces 2D nucleation and spiral growth features that can explain the topography in our films.
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