Time-resolved photon echoes from donor-bound excitons in ZnO epitaxial layers

The coherent optical response from 140 nm and 65 nm thick ZnO epitaxial layers is studied using four-wave-mixing spectroscopy with picosecond temporal resolution. Resonant excitation of neutral donor-bound excitons results in two-pulse and three-pulse photon echoes. For the donor-bound A exciton (${\mathrm{D}}⁰{\mathrm{X}}_{\text{A}}$) at temperature of 1.8 K we evaluate optical coherence times ${T}₂=33--50$ ps corresponding to homogeneous line widths of $13--19\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}\mathrm{eV}$, about two orders of magnitude smaller as compared with the inhomogeneous broadening of the optical transitions. The coherent dynamics is determined mainly by the population decay with time ${T}₁=30--40$ ps, while pure dephasing is negligible. Temperature increase leads to a significant shortening of ${T}₂$ due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (${\mathrm{D}}⁰{\mathrm{X}}_{\text{B}}$) is significantly faster (${T}₂=3.6\phantom{\rule{0.16em}{0ex}}\mathrm{ps}$) and governed by pure dephasing processes.