Lead-halide perovskite nanocrystals are promising candidates for semiconductor laser cooling due to their near-unity photoluminescence quantum yields and efficient photon/phonon up-conversion process. This unexpected, efficient sub-gap energy up-conversion implies an unexpectedly strong electron-phonon interaction in perovskite nanocrystals. However, the underlying mechanism remains mostly unclear. Detailed experiments, along with theory, have now been conducted to elucidate the efficient up-conversion in CsPbBr3 NCs, utilizing a combination of techniques: photothermal absorption spectroscopy, up-conversion detuning spectroscopy, and ultrafast transient differential absorption spectroscopy.
Cesium lead bromide nanocrystals, in contrast to most other materials, exhibit near-unity photoluminescence quantum yields (PLQY). 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.
Achieving condensed phase optical refrigeration requires near-unity emission quantum yields (QYs). Colloidal CsPbBr3 nanocrystals (NCs) are promising candidates in this respect given near unity QY-values, achieved by post-synthetic surface treatment with quaternary ammonium bromide ligands. The origin of these QY enhancements, however, is not understood. Systematic nuclear magnetic resonance studies of the organic ligand passivation of near unity QY CsPbBr3 NC surfaces are therefore conducted to better reveal their surface-ligand interactions.
CsPbBr3 perovskite nanocrystals have been identified as a potential medium to realize condensed phase optical refrigeration. This is due to its near unity emission quantum yields and efficient anti-Stokes photoluminescence (ASPL). Despite much work on CsPbBr3’s optical response, the origin of its efficient ASPL remains unclear. We conduct detailed optical spectroscopy measurements in conjunction with theory to establish mechanistic insights into CsPbBr3’s up-conversion process. Experimental techniques utilized include: temperature-dependent and detuning energy-dependent ASPL measurements, temperature-tunable photothermal heterodyne absorption spectroscopy, and ultrafast transient differential absorption (TDA) spectroscopy.
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