ABSTRACT
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.
ABSTRACT
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 exciton-polariton 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.
ABSTRACT
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.
ABSTRACT
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.
