Monolayer group VI transition metal dichalcogenides (TMDs) have recently emerged as promising candidates for photonic and opto-valleytronic applications. The optoelectronic properties of these atomically-thin semiconducting crystals are strongly governed by the tightly bound electron-hole pairs such as excitons and trions (charged excitons). The anomalous spin and valley configurations at the conduction band edges in monolayer WS₂ give rise to even more fascinating valley many-body complexes. Here we find that the indirect Q valley in the first Brillouin zone of monolayer WS₂plays a critical role in the formation of a new excitonic state, which has not been well studied. By employing a high-quality h-BN encapsulated WS₂ field-effect transistor, we are able to switch the electron concentration within K-Q valleys at conduction band edges. Consequently, a distinct emission feature could be excited at the high electron doping region. Such feature has a competing population with the K valley trion, and experiences nonlinear power-law response and lifetime dynamics under doping. Our findings open up a new avenue for the study of valley many-body physics and quantum optics in semiconducting 2D materials, as well as provide a promising way of valley manipulation for next-generation entangled photonic devices.

The quest for realizing novel fundamental physical effects and practical applications in ambient conditions has led to tremendous interest in microcavity exciton polaritons working in the strong coupling regime at room temperature. In the past few decades, a wide range of novel semiconductor systems supporting robust exciton polaritons have emerged, which has led to the realization of various fascinating phenomena and practical applications. This paper aims to review recent theoretical and experimental developments of exciton polaritons operating at room temperature, and includes a comprehensive theoretical background, descriptions of intriguing phenomena observed in various physical systems, as well as accounts of optoelectronic applications. Specifically, an in-depth review of physical systems achieving room temperature exciton polaritons will be presented, including the early development of ZnO and GaN microcavities and other emerging systems such as organics, halide perovskite semiconductors, carbon nanotubes, and transition metal dichalcogenides. Finally, a perspective of outlooking future developments will be elaborated.

Optical parametric oscillators (OPOs) have been widely applied in spectroscopy, squeezed light, and correlated photons, as well as quantum information. Conventional OPOs usually suffer from a high power threshold limited by weak high-order nonlinearity in traditional pure photonic systems. Alternatively, polaritonic systems based on hybridized exciton–photon quasi-particles exhibit enhanced optical nonlinearity by dressing photons with excitons, ensuring highly nonlinear operations with low power consumption. We report an on-chip perovskite polariton parametric oscillator with a low threshold. Under the resonant excitation at a range of angles, the signal at the ground state is obtained, emerging from the polariton–polariton interactions at room temperature. Our results advocate a practical way toward integrated nonlinear polaritonic devices with low thresholds.