Two-Photon Absorption and Multi-Exciton Generation in Lead Salt Quantum Dots

Understanding the nonlinear optical processes in semiconductor nanostructures leads to possible applications in areas including laser amplifiers, optical switches, and solar cells. Here we present a study of the frequency degenerate twophoton absorption (2PA) spectrum of a series of PbS and PbSe quantum dots (QDs). The influence of the quantum confinement is analyzed using a four-band model which considers the mixing of valence and conduction bands. In contrast to our observations of CdSe QDs, the present results point to an increase of the 2PA cross-section (normalized by the QD volume) as the quantum dot size is made smaller. This is explained by the symmetry between the valence and conduction bands which allows the density of states to remain high even for small QDs. A study of the ultrafast carrier dynamics of the PbS quantum dots is also presented. Through nondegenerate femtosecond pump-probe experiments we show evidence of multi-exciton generation with quantum yield (number of excitons generated per absorbed photon) up to 170% for excitation with hω > 3 Eg (where Eg is the bandgap energy).