Characterizing Electron–Hole Plasma Dynamics at Different Points in Individual ZnO Rods

We have used two-photon emission microscopy to characterize the charge carrier dynamics at different locations within a single ZnO rod. Photoexcitation by a focused laser produces carriers (electrons and holes) in a localized region. Emission is detected using both time-integrated and time-resolved methods. Results show that the electron–hole plasma (EHP) state plays a larger role at the end of the rod compared to other points within the structure, where electron–hole recombination proceeds through an excitonic state. The origin of this spatial dependence is attributed to the physical confinement at the end of the structure that prevents an expansion of the photoexcited electron–hole cloud through processes such as carrier diffusion. Whispering gallery modes are identified as contributing to a periodic emission pattern along the length of the structure.