![]() To circumvent such undesired situations in the coherent optical studies of single perovskite NCs, it is imperative to realize resonant excitation of the emission state. In the aforementioned pioneering works 7, 8, a single perovskite NC was first excited into the absorption state with the coherent information being extracted next from the emission state, which would cause not only linewidth broadening to reduce the exciton coherence 9, 10 but also timing jitter in the photon generation and emission events 11. These coherent optical properties, which are rarely achievable in traditional colloidal NCs despite several decades of active pursuits, promise great potential of single perovskite NCs in quantum-information applications. By means of photon-correlation Fourier spectroscopy, the PL linewidth was measured to be ~17.0 μeV for the emission-state excitons of single CsPbBr 3 NCs 7, while the exciton dephasing time of the absorption state could be longer than ~10 ps in single CsPbI 3 NCs based on the quantum interference measurement 8. 1), with the subsequent observations of single-photon emission 2, 3, suppressed photoluminescence (PL) blinking and spectral diffusion 4, and stable exciton fine structures 5, 6. Semiconductor perovskite nanocrystals (NCs) have attracted great research attention in the single-particle optical studies since their first successful synthesis in 2015 (ref. The ability to collect resonance fluorescence from single CsPbI 3 nanocrystals, with the subsequent revelation of exciton-acoustic phonon coupling, has marked a critical step towards their steady advancement into superior quantum-light sources. Meanwhile, size-quantized acoustic phonons can be resolved from a single CsPbI 3 nanocrystal, whose coupling with the exciton is proposed to arise from the piezoelectric potential. This allows us to estimate an ultranarrow photoluminescence excitation linewidth of ~11.32 µeV for the emission state of a single CsPbI 3 nanocrystal, corresponding to an exciton dephasing time of ~116.29 ps. Here we have realized resonant and near-resonant excitations of single perovskite CsPbI 3 nanocrystals, with the scattered laser light contributing to only ~10% of the total collected signals. Single perovskite nanocrystals have attracted great research attention very recently due to their potential quantum-information applications, which critically depend on the development of powerful optical techniques to resolve delicate exciton photophysics.
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