Room-temperature strong coupling in a single-photon emitter-metasurface system.

Solid state single-photon sources with high brightness and long coherence time are promising qubit candidates for modern quantum technology. To prevent decoherence processes and preserve the integrity of the qubits, decoupling the emitters from their surrounding environment is essential. To this end, interfacing single photon emitters (SPEs) with high-finesse cavities is required, especially in the strong coupling regime, when the interaction between emitters can be mediated by cavity fields. However, achieving strong coupling at elevated temperatures is challenging due to competing incoherent processes. Here, we address this long-standing problem by using a quantum system, which comprises a class of SPEs in hexagonal boron nitride and a dielectric cavity based on bound states in the continuum (BIC). We experimentally demonstrate, at room temperature, strong coupling of the system with a large Rabi splitting of ~4 meV thanks to the combination of the narrow linewidth and large oscillator strength of the emitters and the efficient photon trapping of the BIC cavity. Our findings unveil opportunities to advance the fundamental understanding of quantum dynamical system in strong coupling regime and to realise scalable quantum devices capable of operating at room temperature.

Ultrafast exciton fluid flow in an atomically thin MoS2 semiconductor.

Excitons (coupled electron-hole pairs) in semiconductors can form collective states that sometimes exhibit spectacular nonlinear properties. Here, we show experimental evidence of a collective state of short-lived excitons in a direct-bandgap, atomically thin MoS2 semiconductor whose propagation resembles that of a classical liquid as suggested by the nearly uniform photoluminescence through the MoS2 monolayer regardless of crystallographic defects and geometric constraints. The exciton fluid flows over ultralong distances (at least 60 µm) at a speed of ~1.8 × 107 m s-1 (~6% the speed of light). The collective phase emerges above a critical laser power, in the absence of free charges and below a critical temperature (usually Tc ≈ 150 K) approaching room temperature in hexagonal-boron-nitride-encapsulated devices. Our theoretical simulations suggest that momentum is conserved and local equilibrium is achieved among excitons; both these features are compatible with a fluid dynamics description of the exciton transport.

Observation of Strong Valley Magnetic Response in Monolayer Transition Metal Dichalcogenide Alloys of Mo0.5W0.5Se2 and Mo0.5W0.5Se2/WS2 Heterostructures.

Monolayer transition metal dichalcogenide (TMD) alloys have emerged as a unique material system for promising applications in electronics, optoelectronics, and spintronics due to their tunable electronic structures, effective masses of carriers, and valley polarization with various alloy compositions. Although spin-orbit engineering has been extensively studied in monolayer TMD alloys, the valley Zeeman effect in these alloys still remains largely unexplored. Here we demonstrate the enhanced valley magnetic response in Mo0.5W0.5Se2 alloy monolayers and Mo0.5W0.5Se2/WS2 heterostructures probed by magneto-photoluminescence spectroscopy. The large g factors of negatively charged excitons (trions) of Mo0.5W0.5Se2 have been extracted for both pure Mo0.5W0.5Se2 monolayers and Mo0.5W0.5Se2/WS2 heterostructures, which are attributed to the significant impact of doping-induced strong many-body Coulomb interactions on trion emissions under an out-of-plane magnetic field. Moreover, compared with the monolayer Mo0.5W0.5Se2, the slightly reduced valley Zeeman splitting in Mo0.5W0.5Se2/WS2 is a consequence of the weakened exchange interaction arising from p-doping in Mo0.5W0.5Se2 via interlayer charge transfer between Mo0.5W0.5Se2 and WS2. Such interlayer charge transfer further evidences the formation of type-II band alignment, in agreement with the density functional theory calculations. Our findings give insights into the spin-valley and interlayer coupling effects in monolayer TMD alloys and their heterostructures, which are essential to develop valleytronic applications based on the emerging family of TMD alloys.

Halide Perovskite Semiconductor Lasers: Materials, Cavity Design, and Low Threshold.

Solution-processable semiconductor lasers have been a long-standing challenge for next-generation displays, light sources, and communication technologies. Metal halide perovskites, which combine the advantages of inorganic and organic semiconductors, have recently emerged not only as excellent candidates for solution-processable lasers but also as potential complementary gain materials for filling the "green gap" and supplement industrial nanolasers based on classic II-VI/III-V semiconductors. Numerous perovskite lasers have been developed successfully with superior performance in terms of cost-effectiveness, low threshold, high coherence, and multicolor tunability. This mini review surveys the development, current status, and perspectives of perovskite lasers, categorized into thin film lasers, nanocrystals lasers, microlasers, and device concepts including polariton and bound-in-continuum lasers with a focus on material fundamentals, cavity design, and low-threshold devices in addition to critical issues such as mass fabrication and applications.

Bright Exciton Fine-Structure in Two-Dimensional Lead Halide Perovskites.

The fast-growing field of atomically thin semiconductors urges a new understanding of two-dimensional excitons, which entirely determine their optical responses. Here, taking layered lead halide perovskites as an example of unconventional two-dimensional semiconductors, by means of versatile optical spectroscopy measurements, we resolve fine-structure splitting of bright excitons of up to ∼2 meV, which is among the largest values in two-dimensional semiconducting systems. The large fine-structure splitting is attributed to the strong electron-hole exchange interaction in layered perovskites, which is proven by the optical emission in high magnetic fields of up to 30 T. Furthermore, we determine the g-factors for these bright excitons as ∼+1.8. Our findings suggest layered lead halide perovskites are an ideal platform for studying exciton spin-physics in atomically thin semiconductors that will pave the way toward exciton manipulation for novel device applications.

Direct and indirect exciton transitions in two-dimensional lead halide perovskite semiconductors.

Atomically thin layers of two-dimensional lead halide perovskite semiconductors exhibit prominent light emission due to the inherently strong quantum and dielectric confinement. Electronic band structures and coupled electron-hole pairs (excitons), which govern the optical properties, are not well understood in these emergent two-dimensional materials. Here, we have performed both the steady-state and time-resolved photoluminescence spectroscopies with varying temperature to study the optical responses of a high-quality (PEA)2PbI4 single crystal. We observe a multitude of exciton transitions with different responses to temperature that suggests their different origins. Furthermore, our results suggest that the photoluminescence of layered perovskites is dominated by direct exciton transitions at low temperatures, while by an indirect exciton at high temperatures that can be explained by our proposed exciton band structure incorporating the interplay of Coulomb and Rashba effects. Our study sheds light on the intrinsic optical properties of two-dimensional perovskites that may be beneficial for the novel applications of perovskite-based devices.

Linearly Polarized Luminescence of Atomically Thin MoS2 Semiconductor Nanocrystals.

Atomically thin layers of transition-metal dichalcogenides semiconductors, such as MoS2, exhibit strong and circularly polarized light emission due to inherent crystal symmetries, pronounced spin-orbit coupling, and out-of-plane dielectric and spatial confinement. While the layer-by-layer confinement is well-understood, the understanding of the impact of in-plane quantization in their optical spectrum is far behind. Here, we report the optical properties of atomically thin MoS2 colloidal semiconductor nanocrystals. In addition to the spatial-confinement effect leading to their blue wavelength emission, the high quality of our MoS2 nanocrystals is revealed by narrow photoluminescence, which allows us to resolve multiple optically active transitions, originating from quantum-confined excitons (coupled electron-hole pairs). Surprisingly, in stark contrast to monolayer MoS2, the luminescence of the lowest-energy levels is linearly polarized and persists up to room temperature, meaning that it could be exploited in a variety of light-emitting applications.

Manipulating efficient light emission in two-dimensional perovskite crystals by pressure-induced anisotropic deformation.

The hybrid nature and soft lattice of organolead halide perovskites render their structural changes and optical properties susceptible to external driving forces such as temperature and pressure, remarkably different from conventional semiconductors. Here, we investigate the pressure-induced optical response of a typical two-dimensional perovskite crystal, phenylethylamine lead iodide. At a moderate pressure within 3.5 GPa, its photoluminescence red-shifts continuously, exhibiting an ultrabroad energy tunability range up to 320 meV in the visible spectrum, with quantum yield remaining nearly constant. First-principles calculations suggest that an out-of-plane quasi-uniaxial compression occurs under a hydrostatic pressure, while the energy is absorbed by the reversible and elastic tilting of the benzene rings within the long-chain ligands. This anisotropic structural deformation effectively modulates the quantum confinement effect by 250 meV via barrier height lowering. The broad tunability within a relatively low pressure range will expand optoelectronic applications to a new paradigm with pressure as a tuning knob.

A 3D Haloplumbate Framework Constructed From Unprecedented Lindqvist-like Highly Coordinated [Pb6 Br25 ]13- Nanoclusters with Temperature-Dependent Emission.

Searching novel haloplumbate building units to construct three-dimensional (3D) frameworks is very important and highly desirable because such materials would possess new physical properties and potential applications. Here, by employing tetrakis(N-imidazolemethylene)methane(TIMM) as a structure-directing agent, the first 3D haloplumbate framework constructed from unprecedented Lindqvist-like highly coordinated [Pb6 Br25 ]13- nanoclusters has been successfully prepared under hydrothermal condition, where all Pb2+ centres in [Pb6 Br25 ]13- nanoclusters adopt seven-/eight-coordinated configurations. The as-obtained material is a wide-gap semiconductor (≈3.1 eV) and can be stable up to 320 °C. More importantly, this material has been demonstrated to show temperature-dependent emission. Our results could provide a new strategy to explore novel metal-halide open-framework materials.