Continuous‐Wave Lasing in Cesium Lead Bromide Perovskite Nanowires

Abstract

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...

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