Multiple-phonon assisted anti-Stokes photoluminescence in CsPbBr₃ nanocrystals

Cesium lead bromide nanocrystals, in contrast to most other materials, exhibit near-unity photoluminescence quantum yields (PLQY). This means that absorption of photons with energies greater than the band gap separating occupied valence bands from unoccupied conduction bands almost certainly leads to photon emission. Even more surprising is that when excited below the band gap, they absorb the photons and show anti-Stokes photoluminescence (ASPL), emitting higher energy, band-gap photons. Simultaneous existence of near-unity PLQY and ASPL can be used to optically cool these materials. In this talk, I will report near-unity ASPL efficiencies in CsPbBr₃ nanocrystals and attribute it to resonant multiple-phonon absorption by polarons. The theory explains paradoxically large efficiencies for intrinsically disfavored, multiple-phonon-assisted ASPL in nanocrystals.