Time-resolved photon echoes from donor-bound excitons in ZnO epitaxial layers
AbstractThe 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 (D0XA) at temperature of 1.8 K we evaluate optical coherence times T2=33–50 ps corresponding to homogeneous line widths of 13–19μeV, 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 T1=30–40 ps, while pure dephasing is negligible. Temperature increase leads to a significant shortening of T2 due to interaction with acoustic phonons. In contrast, the loss of coherence of the donor-bound B exciton (D0XB) is significantly faster (T2=3.6ps) and governed by pure dephasing processes.