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In the emerging field of valleytronics, it is aimed to coherently manipulate the valley pseudospin as an information‐bearing degree of freedom. The 2D transition‐metal dichalcogenides (TMDCs) provide a unique possibility to generate an excitonic valley pseudospin, opening the way to valley information. Although significant development of valley pseudospin in layered materials has been achieved recently, looking for new TMDCs featuring robust valley phenomenon at room temperature is still desirable for practical applications. Herein, the valley pseudospin of the spiral WS2 with different layer thicknesses at room temperature is investigated by both circular and linear polarization‐resolved photoluminescence spectroscopy. In the experimental results, it is demonstrated that the spiral WS2 emerges robust valley polarization and valley coherence, the degree of circular polarization, and linear polarization gradually increase with the lift of the layer thicknesses, reaching up to 0.91 for valley polarization and 0.94 for valley coherence, respectively. The robust layer‐dependent valley pseudospin may originate from the intrinsic broken inversion symmetry, due to the spiral structure of the multilayer WS2. The robust and near‐unity valley polarization and valley coherence at room temperature in the spiral WS2 may provide a new platform for optical manipulation of the valley pseudospin for further valleytronics applications. [ABSTRACT FROM AUTHOR]

In the emerging field of valleytronics, it is aimed to coherently manipulate the valley pseudospin as an information‐bearing degree of freedom. The 2D transition‐metal dichalcogenides (TMDCs) provide a unique possibility to generate an excitonic valley pseudospin, opening the way to valley information. Although significant development of valley pseudospin in layered materials has been achieved recently, looking for new TMDCs featuring robust valley phenomenon at room temperature is still desirable for practical applications. Herein, the valley pseudospin of the spiral WS2 with different layer thicknesses at room temperature is investigated by both circular and linear polarization‐resolved photoluminescence spectroscopy. In the experimental results, it is demonstrated that the spiral WS2 emerges robust valley polarization and valley coherence, the degree of circular polarization, and linear polarization gradually increase with the lift of the layer thicknesses, reaching up to 0.91 for valley polarization and 0.94 for valley coherence, respectively. The robust layer‐dependent valley pseudospin may originate from the intrinsic broken inversion symmetry, due to the spiral structure of the multilayer WS2. The robust and near‐unity valley polarization and valley coherence at room temperature in the spiral WS2 may provide a new platform for optical manipulation of the valley pseudospin for further valleytronics applications.

As an indispensable component of optoelectronic system, an on‐chip light source with well‐defined polarization is desirable in optical communication, signal processing and display applications. The emerging atomically thin transition metal dichalcogenides (TMDCs), due to their high quantum yield and robust valley coherence, provide an unprecedented platform to realize the high‐efficient linearly polarized light emission. Although the valley‐related optical selection rules suggest that valley coherence should be possible, extreme conditions such as cryogenic temperatures are required, which is a long‐term challenge for their practical applications. In this paper, the strongly enhanced linearly polarized emission is realized by integrating WS2 monolayers with a delicate designed plasmonic nanocavity. It is demonstrated that strong plasmon–exciton coupling gives rise to the plasmon–exciton polariton. The polariton valley coherence results in a linear polarization up to 0.32 at room temperature and contributes to 52% of the total linear polarization. Enhancement of linear polarization through polariton valley coherence can be understood as the consequence of the extra relaxation channel introduced by its plasmonic counterpart. The potential of 2D TMDC‐plasmon hybrid structure appears to be high and of significant technological interest, as well as inspires new perspectives on quantum manipulations in 2D solid‐state systems. [ABSTRACT FROM AUTHOR]

Moiré superlattice (mSL)-induced sub-bands in twisted van der Waals homo- and heterostructures govern their optical and electrical properties, rendering additional degrees of freedom such as twist angle. Here, we demonstrate the moiré superlattice effects on the intralayer excitons and trions in a twisted bilayer of MoS 2 of H-type stacking at marginal twist angles. We identify the emissions from localized and delocalized sub-bands of intralayer moiré excitons and show their electrical modulation by the corresponding trion formation. The electrical control of the oscillator strength of the moiré excitons also results in the strong tunability of resonant Raman scattering. We find that the gate-induced doping significantly modulates the electronic moiré potential; however, leaves the excitonic moiré confinement unaltered. This effect, coupled with variable moiré trap filling by tuning the optical excitation density, allows us to delineate the different phases of localized and delocalized moiré trions. We demonstrate that the moiré excitons exhibit strong valley coherence that changes in a striking nonmonotonic W-shape with gating due to motional narrowing. These observations from the simultaneous electrostatic control of quasiparticle-dependent moiré potential will lead to exciting effects of tunable many-body phenomena in moiré superlattices.

The emerging field of valleytronics aims to exploit the valley pseudospin of electrons residing near Bloch band extrema as an information carrier. Recent experiments demonstrating optical generation and manipulation of exciton valley coherence (the superposition of electron-hole pairs at opposite valleys) in monolayer transition metal dichalcogenides (TMDs) provide a critical step towards control of this quantum degree of freedom. The charged exciton (trion) in TMDs is an intriguing alternative to the neutral exciton for control of valley pseudospin because of its long spontaneous recombination lifetime, its robust valley polarization, and its coupling to residual electronic spin. Trion valley coherence has however been unexplored due to experimental challenges in accessing it spectroscopically. In this work, we employ ultrafast two-dimensional coherent spectroscopy to resonantly generate and detect trion valley coherence in monolayer MoSe 2 demonstrating that it persists for a few-hundred femtoseconds. We conclude that the underlying mechanisms limiting trion valley coherence are fundamentally different from those applicable to exciton valley coherence.