mixing or exchange of cations or anions, covering the complete visible and near-IR wavelength region; (iii) they show lasing at carrier densities as low as 10 16 cm −3 , which is two orders of magnitude lower than those in conventional semiconductor NWs. [9] However, all lead halide perovskite NW lasers demonstrated thus far are limited to pulsed operation. While continuous wave (CW) lasing is vital to achieving electrically injected NW lasers and to applications such as optical communication and spectroscopy, [14,15] achieving CW lasing in lead halide perovskite NWs was thought to be exceptionally challenging due to thermal damage and to screening of the exciton resonance. [16,17] The latter leads to a reduction in oscillator strength and gain at high excitation densities necessary for population inversion. [16,17] However, lasing in a NW cavity may not necessarily require population inversion. Coupling between the exciton resonance and light is enhanced in a NW due to the reduced mode volume of the photons and the cavity enhanced oscillator strength through the relation Ω ∝ / f V , where Ω is the vacuum Rabi splitting; f is the oscillator strength; and V is the mode volume. [18] In this regime, coherent light emission can originate from the steady-state leakage of an exciton-polariton condensate below the threshold for population inversion. [19] An exciton-polariton, or polariton for short, is a bosonic quasiparticle formed by the superposition of strongly coupled exciton and photon states, which generates an upper and lower polariton branch (UPB and LPB, respectively) from the avoided crossing of the two dispersions. [20,21] The LPB is exciton like at high momentum (k) and photon like at lower k (see Figure S1 in the Supporting Information). Consequentially, polaritons relax along the LPB by acoustic phonon emission and accumulate near the avoided crossing, i.e., the polariton bottleneck, due to the reduced lifetimes and density-of-states in the photon-like region at lower energies. [21] Polaritons undergo Bose-stimulated scattering, which surpasses spontaneousscattering at a critical density to produce the coherent condensate state [22] and the light leaking out of the cavity from such a coherent state has been called polariton lasing. [20,21] Condensation in the bottleneck region was observed in CdTe microcavities as a ring of emission in angle-resolved fluorescence measurements. [23,24] The NW cavity differs from the planar microcavity. While both are Fabry-Perot cavities defined by end facets in a NW [7,8,13] or distributed Bragg reflectors (DBRs) in Lead halide perovskite nanowires (NWs) have been demonstrated in pulsed lasing with high quantum yields, low thresholds, and broad tunability. However, continuous-wave (CW) lasing, necessary for many optoelectronic applications, has not been achieved to date. This is thought to be due to many-body screening, which reduces the excitonic resonance enhancement of the oscillator strength at high excitation densities necessary for population inversion. Here CW lasi...

Lead halide perovskites are emerging as an excellent material platform for optoelectronic processes. There have been extensive discussions on lasing, polariton formation, and nonlinear processes in this material system, but the underlying mechanism remains unknown. Here we probe lasing from CsPbBr3 perovskite nanowires with picosecond (ps) time resolution and show that lasing originates from stimulated emission of an electron-hole plasma. We observe an anomalous blue-shifting of the lasing gain profile with time up to 25 ps, and assign this as a signature for lasing involving plasmon emission. The time domain view provides an ultra-sensitive probe of many-body physics which was obscured in previous time-integrated measurements of lasing from lead halide perovskite nanowires.

Singlet fission, the generation of two triplet excited states from the absorption of a single photon, may potentially increase solar energy conversion efficiency. A major roadblock in realizing this potential is the limited number of molecules available with high singlet fission yields and sufficient chemical stability. Here, we demonstrate a strategy for developing singlet fission materials in which we start with a stable molecular platform and use strain to tune the singlet and triplet energies. Using perylene diimide as a model system, we tune the singlet fission energetics from endoergic to exoergic or iso-energetic by straining the molecular backbone. The result is an increase in the singlet fission rate by 2 orders of magnitude. This demonstration opens a door to greatly expanding the molecular toolbox for singlet fission.

CONSPECTUS: Lead halide perovskites (LHPs) are attractive material systems for light emission, thanks to the ease and diverse routes of synthesis, the broad tunability in color, the high emission quantum efficiencies, and the strong light−matter coupling which may potentially lead to excitonpolariton condensation. This account contrasts the laser-like coherent light emission from highly lossy Fabry−Perot cavities, formed naturally from LHP nanowires (NWs) and nanoplates (NPs), with highly reflective cavities made of LHP gain media, sandwiched between two distributed Bragg reflector (DBR) mirrors. The mechanism responsible for the operation of conventional semiconductor lasers involves stimulated emission of electron and hole pairs bound by the Coulomb potential, i.e., excitons or, at excitation density above the so-called Mott threshold, an electron−hole plasma (EHP). We discuss how lasing from LHP NWs or NPs likely originates from stimulated emission of an EHP, not excitons or exciton-polaritons. A character central to this kind of lasing is the dynamically changing photonic properties in the naturally formed cavity. In contrast to the more static conditions of a DBR cavity, lasing modes and gain profiles are extremely sensitive to material properties and excitation conditions in an NW/NP cavity. While such unstable photonic cavities pose engineering challenges in the application of NW/NP lasers, they provide excellent probes of many-body physics in the LHP material. For sufficiently strong light−matter coupling expected for LHPs in DBR cavities, an exciton-polariton, i.e., the superposition state between the exciton and the cavity photon, can form. An exciting prospect of strong light−matter coupling is the potential formation of an exciton polariton condensate, which possesses many interesting quantum and nonlinear effects, such as superfluidity, long-range coherence, and laserlike light emission. However, it is difficult to distinguish coherent light from an exciton-polariton condensate and that from conventional stimulated laser emission. Several reports have established the condition of strong coupling for LHPs in DBR cavities. We stress, however, that these studies have not included necessary experiments to unambiguously establish the formation of exciton-polariton condensation, and several experiments and routes of analysis are needed to make a more convincing case for exciton-polariton condensation in LHP based systems. The potential of exciton-polariton condensation expands the horizon of LHP materials from conventional optoelectronics to quantum devices.

Spin-orbit coupling (SOC) is responsible for a range of spintronic and topological processes in condensed matter. Here, we show photonic analogs of SOCs in exciton-polaritons and their condensates in microcavities composed of birefringent lead halide perovskite single crystals. The presence of crystalline anisotropy coupled with splitting in the optical cavity of the transverse electric and transverse magnetic modes gives rise to a non-Abelian gauge field, which can be described by the Rashba-Dresselhaus Hamiltonian near the degenerate points of the two polarization modes. With increasing density, the exciton-polaritons with pseudospin textures undergo phase transitions to competing condensates with orthogonal polarizations. Unlike their pure photonic counterparts, these exciton-polaritons and condensates inherit nonlinearity from their excitonic components and may serve as quantum simulators of many-body SOC processes.