Coherent dynamics of excitons and biexcitons in intermixed ZnO/Zn1-xMgxO quantum wells by ion implantation

We have performed one and two colour, spectrally resolved four-wave mixing (FWM) experiments on a series of ZnO/Zn_1-xMg_xO quantum wells with different oxygen ion implantation doses. The results show that at room temperature and with resonant excitation, excitonic coherences are maintained beyond the duration of the laser pulse. A transient signal observed at negative delays in the two-colour experiments is attributed to biexcitonic coherence. The use of the two colours allows the purely biexcitonic contribution to be resolved from any other many-body or excitonic contributions at negative delays; therefore the transient signal observed indicates the creation of biexcitons in the ZnO quantum well at room temperature. Coherence times as long as 480 fs at room temperature are found for both excitons and biexcitons and are shown to be independent of implantation dose.