The full transient dielectric-function (DF) tensor of ZnO after UV-laser excitation in the spectral range 1.4–3.6 eV is obtained by measuring an $m$-plane-oriented ZnO thin film with femtosecond (fs)-time-resolved spectroscopic ellipsometry. From the merits of the method, we can distinguish between changes in the real and the imaginary part of the DF as well as changes in birefringence and dichroism, respectively. We find pump-induced switching from positive to negative birefringence in almost the entire measured spectral range for about 1 ps. Simultaneously, weak dichroism in the spectral range below 3.0 eV hints at contributions of inter-valence-band transitions. Line-shape analysis of the DF above the band gap based on discrete exciton, exciton-continuum, and exciton-phonon-complex contributions shows a maximal dynamic increase in the transient exciton energy by 80 meV. The absorption coefficient below the band gap reveals an exponential line shape attributed to Urbach-rule absorption mediated by exciton–longitudinal-optic-phonon interaction. The transient DF is supported by first-principles calculations for ${10}^{20}{\mathrm{cm}}^{-3}$ excited electron-hole pairs in ideal bulk ZnO.

• Received 2 October 2020
• Revised 19 January 2021
• Accepted 16 February 2021

DOI:https://doi.org/10.1103/PhysRevResearch.3.013246

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Published by the American Physical Society