Chang-Hee Cho

Exciton‐polaritons are composite quasiparticles hybridized between excitons and photons, which are very promising to develop quantum information devices such as entangled photon pair sources and polariton qubit devices by utilizing the fascinating properties of strong nonlinearity, Bose‐Einstein condensation, and superfluidity. Organic–inorganic hybrid lead halide perovskites have attracted much interest in cavity quantum electrodynamics due to their excellent excitonic properties, including strong exciton binding energy and high oscillation strength. Here, tunable Rabi oscillation of exciton‐polaritons in the lead halide perovskite microcavity is demonstrated, which experiences a phase transition between orthorhombic, tetragonal, and cubic phases by varying the temperature. Over the phase transition, the Rabi frequency is probed by tracing the dispersion relation of the exciton‐polaritons using Fourier plane spectroscopy. Due to the emergence of ferroelectricity in the tetragonal phase of the perovskites, the Rabi splitting can be tuned by ≈20%, while the corresponding exciton oscillator strength is varied by ≈44%. These results provide insight into novel functionalities of polariton devices by utilizing ferroic semiconductors, which can facilitate the development of tunable quantum devices.

Hexagonal boron nitride (h-BN) is a key ingredient for various two-dimensional (2D) van der Waals heterostructure devices, but the exact role of h-BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, we report that h-BN encapsulation greatly removes the defect-related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS2 crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS2 crystals and are fixated by h-BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS2 and h-BN. Optical spectroscopic studies show that h-BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS2 crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS2. Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h-BN encapsulated WS2 crystals.

Hexagonal boron nitride (h‐BN) is a key ingredient for various 2D van der Waals heterostructure devices, but the exact role of h‐BN encapsulation in relation to the internal defects of 2D semiconductors remains unclear. Here, it is reported that h‐BN encapsulation greatly removes the defect‐related gap states by stabilizing the chemisorbed oxygen molecules onto the defects of monolayer WS₂ crystals. Electron energy loss spectroscopy (EELS) combined with theoretical analysis clearly confirms that the oxygen molecules are chemisorbed onto the defects of WS₂ crystals and are fixated by h‐BN encapsulation, with excluding a possibility of oxygen molecules trapped in bubbles or wrinkles formed at the interface between WS₂ and h‐BN. Optical spectroscopic studies show that h‐BN encapsulation prevents the desorption of oxygen molecules over various excitation and ambient conditions, resulting in a greatly lowered and stabilized free electron density in monolayer WS₂ crystals. This suppresses the exciton annihilation processes by two orders of magnitude compared to that of bare WS₂. Furthermore, the valley polarization becomes robust against the various excitation and ambient conditions in the h‐BN encapsulated WS₂ crystals.

Controlling coherent light–matter interactions in semiconductor microcavities is at the heart of the next‐generation solid‐state polaritonic devices. Organic–inorganic hybrid perovskites are potential materials for room‐temperature polaritonics owing to their high exciton oscillator strengths and large exciton binding energies. Herein, we report on strong exciton‐photon coupling in the micro‐platelet and micro‐ribbon shaped methylammonium lead bromide single crystals. Owing to high crystallinity and large refractive index, the as‐grown perovskite microcrystals serve as self‐hybridized optical microcavities along different orientations due to their distinct physical dimensionalities. In this regard, the perovskite micro‐platelet forms a simple Fabry–Perot microcavity in out‐of‐plane orientation, while the micro‐ribbon functions as a Fabry–Perot type waveguide microcavity within the plane of the perovskite sample. Consequently, excitons in these microcavities strongly interact with their corresponding uncoupled cavity modes, yielding multimode exciton‐polaritons with Rabi splitting energies ∼205 and 235 meV for micro‐platelet and micro‐ribbon geometry, respectively. Furthermore, micro‐ribbon geometry displays Young’s double‐slit‐like interference patterns, which together with the numerical simulation readily reveals the parity and the mode order of the uncoupled cavity modes. Thus, our results not only shed light on strong exciton‐photon coupling in various morphologies of methylammonium lead bromide microcrystals but also open an avenue for advanced polaritonic devices.

A metal-free donor–acceptor combined conjugated porous polyimide (PeTt–POP) photocatalyst exhibited visible-light driven photocatalytic CO₂ reduction to CH₄ in the gas–solid mode.