Saturation of Exciton Absorption in CdSe/CdS Nanoplatelets under Transient Excitation Conditions

Nonlinear absorption of light by colloidal CdSe nanoplatelets with a CdS shell is investigated under the conditions of transient single-photon generation of excitons. By changing the shell thickness, it is possible to implement either resonant excitation of light-hole and heavy-hole excitons or nonresonant excitation. The saturation of absorption by colloidal solutions of nanoplatelets is observed and attributed to the effect of exciton phase space filling. The difference between the saturation intensities for CdSe nanoplatelets with different CdS shell thicknesses is explained by the details of exciton generation and relaxation processes. An increase in absorption observed at the excitation intensity significantly exceeding the saturation intensity is explained by the processes of exciton–exciton interaction and energy exchange between heavy- and light-hole excitons.