Femtosecond-time-resolved imaging of the dielectric function of ZnO in the visible to near-IR spectral range

We demonstrate micrometer-resolved imaging of the transient dielectric function of a c-ZnO thin film with femtosecond resolution in the visible to near-IR spectral range measured by pump-probe ellipsometry at five different probe photon-energies. The spatial profile of the real part of the dielectric function broadens drastically with increasing time delay, which we associate with the combined effect of carrier cooling and fast carrier transport with an effective diffusion coefficient of ( 1.1 ± 0.1 ) × 10 4 cm2/s. A ring structure is detected in the image after a few picoseconds, which can be explained by a random-walk model including ballistic transport due to the thermal gradient induced by the hot-phonon effect.We demonstrate micrometer-resolved imaging of the transient dielectric function of a c-ZnO thin film with femtosecond resolution in the visible to near-IR spectral range measured by pump-probe ellipsometry at five different probe photon-energies. The spatial profile of the real part of the dielectric function broadens drastically with increasing time delay, which we associate with the combined effect of carrier cooling and fast carrier transport with an effective diffusion coefficient of ( 1.1 ± 0.1 ) × 10 4 cm2/s. A ring structure is detected in the image after a few picoseconds, which can be explained by a random-walk model including ballistic transport due to the thermal gradient induced by the hot-phonon effect.