Unusually Strong Biexciton Repulsion Detected in Quantum Confined CsPbBr Nanocrystals with Two and Three Pulse Femtosecond Spectroscopy.

Transient absorption measurements were conducted on pristine and monoexciton saturated CsPbBr nanocrystals varying in size within the regime of a strong quantum confinement. Once the difference spectra were translated to absolute transient changes in absorption cross section, a single exciton is shown to completely bleach the band edge absorption peak and induce a new absorption roughly two times weaker ∼100 meV to the blue. Difference spectra obtained during Auger recombination of biexciton demonstrate that the addition of a second exciton, rather than double the effect of a first, bleaches the blue-induced absorption band without producing a net stimulated emission at the band edge. Accompanied by high time resolution transient absorption spectra pumping at the lowest exciton band, these results identify the blue-induced absorption as the second transition to 1S1S which is shifted in energy due to unusually strong and promptly rising biexciton repulsion. Possible mechanisms giving rise to this repulsion and prospects for applying it to enhance optical gain applications of these particles are discussed.