Zero-Point Motion of Polar Optical Phonons Revealed by Up-Converted Photoluminescence from a Single Perovskite Nanocrystal at Cryogenic Temperatures

Quantum mechanics suggests a persistent lattice vibration even at zero temperature; however, this elusive zero-point motion is difficult to measure for further analyses and exploitations. Here, we have performed near-resonant excitations on single perovskite CsPbI_{3} nanocrystals (NCs) at cryogenic temperatures, showing that pronounced up-converted photoluminescence (UC-PL) still takes place even at ∼4 K despite the vanishing phonon-assisted absorption. This surprising result can be explained in terms of a dynamic electric field caused by the zero-point motion of polar optical phonons, which gives rise to an absorption tail as in the Franz-Keldysh electroabsorption process. Furthermore, laser cooling is demonstrated in the ∼4 K UC-PL process of a single CsPbI_{3} NC, which opens a novel research avenue for the realization of even lower temperatures beyond the limit set forth by the helium-based cryostat.