Relaxation in the Exciton Fine Structure of Semiconductor Nanocrystals

A spectroscopic study is reported of relaxation in the exciton fine structure of CdSe nanorods measured using ultrafast cross-polarized heterodyned third-order transient grating (CPH-3TG) spectroscopy. The CPH-3TG spectroscopy probes the dynamics of population transfer between states accompanying exciton spin flip (radiationless transitions in the exciton fine structure associated with changes in the sign of the total angular momentum). Analysis of the data enabled elucidation of pathways for exciton fine structure relaxation (EFSR) involving all of the exciton fine structure states. The mechanism and origin of the EFSR dynamics in CdSe nanorods are discussed in the exciton state picture with an analogy to molecular radiationless transition processes such as internal conversion and intersystem crossing. Key conclusions are that the fast transitions are between states with total angular momentum of +/- 1 and -/+ 2 and that the rate of this transition follows an inverse diameter to the fourth power size dependence, which originates from the Dresselhaus spin-orbit coupling effect.