Van der Waals heterostructures are synthetic quantum materials composed of stacks of atomically thin two-dimensional (2D) layers. Because the electrons in the atomically thin 2D layers are exposed to layer-layer coupling, the properties of van der Waals heterostructures are defined not only by the constituent monolayers, but also by the interactions between the layers. Many fascinating electrical, optical, and magnetic properties have recently been reported in different types of van der Waals heterostructures. In this review we focus on unique excited-state dynamics in transition metal dichalcogenide (TMDC) heterostructures. TMDC monolayers are the most widely studied 2D semiconductors, featuring prominent exciton states and accessibility to the valley degree of freedom. Many TMDC heterostructures are characterized by a staggered band alignment. This band alignment has profound effects on the evolution of the excited states in heterostructures, including ultrafast charge transfer between the layers, the formation of interlayer excitons, and the existence of longlived spin and valley polarization in resident carriers. Here we review recent experimental and theoretical efforts to elucidate electron dynamics in TMDC heterostructures, extending from time scales of femtoseconds to microseconds, and comment on the relevance of these effects for potential applications in optoelectronic and valleytronic/spintronic devices. Main text Advances in the isolation and manipulation of atomically-thin sheets of two-dimensional (2D) crystals, starting with the investigations of graphene a decade ago, have ushered in a new era of basic scientific research and technological innovation. 2D layers with diverse properties can now be prepared separately and then stacked together to form new types of quantum materials, known as van der Waals (vdW) heterostructures. The ability to combine materials with monolayer precision enables the design and creation of functional 2D materials that do not exist in nature. Today we have at our disposal a wide variety of atomically thin 2D layers, ranging from semiconducting MoS 2 and insulating hexagonal boron-nitride (h-BN) to magnetic CrI 3 and superconducting NbSe 2 , that can be stacked one upon the other. Since the electrons in atomically thin layers are exposed, different quantum states found in the individual layers can interact and couple to one another in ways that are not possible in other systems. VdW heterostructures constitute a vast family of new quantum materials, since they are defined not only by the combination of constituent monolayer materials, but also by the stacking sequence and relative crystallographic alignment of the layers. Further control of physical properties in 2D vdW heterostructures can be achieved through the application of electrostatic gating and fields, as well as substrate and strain engineering. Many fascinating physical phenomena have been reported in different vdW heterostructures, as exemplified by transport measurements revealing Hofstadter butterfly states,...

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Interlayer excitons, electron-hole pairs bound across two monolayer van der Waals semiconductors, offer promising electrical tunability and localizability. Because such excitons display weak electron-hole overlap, most studies have examined only the lowest-energy excitons through photoluminescence. We directly measured the dielectric response of interlayer excitons, which we accessed using their static electric dipole moment. We thereby determined an intrinsic radiative lifetime of 0.40 nanoseconds for the lowest direct-gap interlayer exciton in a tungsten diselenide/molybdenum diselenide heterostructure. We found that differences in electric field and twist angle induced trends in exciton transition strengths and energies, which could be related to wave function overlap, moiré confinement, and atomic reconstruction. Through comparison with photoluminescence spectra, this study identifies a momentum-indirect emission mechanism. Characterization of the absorption is key for applications relying on light-matter interactions.

We report light emission around 1200 nm from a vertical heterostructure consisting of M0S2 and WSe2 monolayers. The emission, arising from the fundamental interlayer exciton, can be tuned by nearly 100 nm by electrical gating.

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We report direct observations of Rabi oscillations and self-induced transparency in a quantum dot optical amplifier operating at room temperature. The experiments make use of pulses whose durations are shorter than the coherence time which are characterized using Cross-Frequency-Resolved Optical Gating. A numerical model which solves the Maxwell and Schrödinger equations and accounts for the inhomogeneously broadened nature of the quantum dot gain medium confirms the experimental results. The model is also used to explain the relationship between the observability of Rabi oscillations, the pulse duration and the homogeneous and inhomogeneous spectral widths of the semiconductor.

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