Time-of-flight study of bound exciton polariton dispersive propagation in ZnO

We report direct observations of polariton dispersive propagation at the donor-bound exciton (D 0 X) resonance in single-crystal ZnO. Time-resolved photoluminescence spectra are measured from both surfaces of a ZnO single crystal that is excited in a thin layer on one surface. When the excitation and the detector are at opposite sides of the crystal, the time-resolved luminescence spectrum in the streak camera display has a curved shape in the t versus plane, indicating dispersion in transport time across the crystal thickness. The group velocity at the donor-bound exciton resonance (3.36 eV) is measured to be 6 × 10 6 m s −1 . The EHP (electron–hole plasma) relaxation time is about 36 ps, in agreement with published results. The polariton reflection at the crystal boundary is observed in the time domain, confirming the dispersion hypothesis. The low-temperature transmission spectrum shows that optical emission from the donor-bound exciton polariton is attenuated by 95% on traversing 0.5 mm crystal thickness due to reflections and polariton decay. From these data the D 0 X polariton lifetime is estimated as 37 ps. The temperature-dependent change of streak camera curvatures corresponds to thermal de-trapping of the donor-bound excitons.