Multi-Exciton and Phonon Dynamics Impact Intraband Transitions in CdSe/ZnS Colloidal Quantum Dots

Intraband transitions in colloidal quantum dots (QDs) are of interest for mid-infrared optoelectronics, given their large oscillator strength and size-tunable spectral coverage. However, due to an absence of suitable ultrafast optical methods to study the broad transitions, little is known about how short-lived excitations, such as biexcitons and nonequilibrium phonons, impact these transitions. In this study, we employ femtosecond visible pump–mid-IR continuum probe spectroscopy for the first time to study the intraband transitions in three sizes of CdSe/ZnS QDs across the entire mid-infrared spectrum with ultrafast time resolution. While the multiexciton intraband transitions track the expected Auger recombination, we also show a size-dependent blue-shift of the intraband absorption of up to 350 cm–1 (43 meV), a remarkable observation for a type I QD architecture. In addition, a clear imprint of nonequilibrium phonons is observed on the intraband transitions. Our results reveal not only the novel photophysics of intraband excitons in CdSe/ZnS, but also the potential of ultrafast continuum infrared spectroscopy to study such processes in nanomaterials.