Ultrafast Supercontinuum Spectroscopy of Carrier Multiplication and Biexcitonic Effects in Excited States of PbS Quantum Dots

We examine the population dynamics of multiple excitons in PbS quantum dots using spectrally resolved ultrafast supercontinuum transient absorption (SC-TA) measurements. We simultaneously probe the first three excitonic transitions. The transient spectra show the presence of bleaching of absorption for the 1S(h)-1S(e) transition, as well as transients associated with the 1P(h)-1P(e) transition. We examine signatures of carrier multiplication (multiple excitons arising from a single absorbed photon) from analysis of the bleaching features in the limit of low absorbed photon numbers (< N-abs > similar to 10(-2)) for pump photon energies from two to four times that of the band gap. The efficiency of multiple-exciton generation is discussed both in terms of the ratio between early- to long-time transient absorption signals and of a broadband global fit to the data. Analysis of the population dynamics shows that bleaching associated with biexciton population is red shifted with respect to the single exciton feature, which is in accordance with a positive binding energy for the biexciton.