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Black phosphorus is a layered semiconducting material, demonstrating strong layer-dependent optical and electronic properties. Probing the photophysical properties on ultrafast timescales is of central importance in understanding many-body interactions and non-equilibrium quasiparticle dynamics. Here, we applied temporally, spectrally and spatially resolved pump-probe microscopy to study the transient optical responses of mechanically exfoliated few-layer black phosphorous, with layer number ranging from 2 to 9. We have observed layer-dependent resonant transient absorption spectra with both photobleaching and redshifted photoinduced absorption features, which could be attributed to bandgap renormalization of higher subband transitions. Surprisingly, coherent phonon oscillations with unprecedented intensities were observed when the probe photons were in resonance with the optical transitions, which correspond to the low-frequency layer-breathing mode. Our results reveal strong Coulomb interactions and electron-phonon couplings in photoexcited black phosphorous, providing important insights into the ultrafast optical, nanomechanical and optoelectronic properties of this novel two-dimensional material.
PMID: 29684276 [PubMed - as supplied by publisher]