Recombination dynamics and pronounced re-absorption effect in ZnO single crystals under two-photon excitation

Abstract

Time-resolved photoluminescence measurements were conducted on ZnO single crystals using two-photon (2P) excitation at low temperatures ranging from 25 to 260 K. The decay constants of various emission peaks are determined and compared between one-photon and two-photon excitations using a recently modified localized-state ensemble model. The analysis is focused on the redshift of the phonon-assisted free exciton transition energy with increasing temperature. A significantly longer radiative recombination lifetime of approximately 840 ps is observed under 2P excitation. The extended lifetime of excitons during 2P excitation is achieved through the interplay of multiple mechanisms. Photon-recycling, enabled by high absorption coefficients and total internal reflection, facilitates re-absorption and new carrier generation. Phonon scattering lowers photon energy, allowing their escape from the crystal, while exciton-polariton interactions delay photon travel, collectively contributing to the extension of exciton lifetimes. The re-absorption effect and elongated carrier lifetime observed from our work could be beneficial for applications of ZnO materials in 2P imaging, particularly of biological samples.