Plasmonic Control of Near-Interface Exciton Dynamics in Defect-Rich ZnO Thin Films

Zinc oxide (ZnO) is an attractive material for many electro-optical applications, but the control of impurities remains an issue in device fabrication. For this paper, the dynamics of defect states produced by annealing ZnO thin films at temperatures of 400–800 °C were probed by band-edge pump-probe spectroscopy in differential reflection and transmission. The distinction between the differential reflection and transmission spectra allowed for the analysis of ultrafast near-interface dynamics, which cannot be separated from the bulk thin-film dynamics by traditional ultrafast spectroscopies. In particular, simultaneous differential reflection and transmission spectroscopy provided clear evidence that the band-edge recombination dynamics in samples annealed at 400 °C were absent near the ZnO/substrate interface. However, the Purcell enhancement observed in Ag/ZnO heterostructures resulted in the dramatic emergence of the band-edge recombination signal nearly two orders of magnitude greater in intensity than the defect differential reflectivity. This indicates that the spatial range of the Purcell effect is at least twice as large as inferred from previous photoluminescence studies.