Transient birefringence and dichroism in ZnO studied with fs-time-resolved spectroscopic ellipsometry

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 1020cm−3 excited electron-hole pairs in ideal bulk ZnO.
Funding: project "Advanced research using high intensity laser produced photons and particles" (ADONIS) from the European Regional Development Fund [CZ.02.1.01/0.0/0.0/16 019/0000789]; project "Structural dynamics of biomolecular systems"(ELIBIO) from the European Regional Development Fund (EFRE) [CZ.02.1.01/0.0/0.0/15 003/0000447]; Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)German Research Foundation (DFG) [SFB 762, 31047526, FOR 1616 (SCHM2710/2)]; project I-COSIMA [SAB 100315366]; Ministry of Education, Youth and Sports from the National Programme of Sustainability II; National Science FoundationNational Science Foundation (NSF) [DMR-1555153, CBET-1437230, OCI-0725070, ACI-1238993]; state of Illinois; Universitat Leipzig within core research area Complex Matter; Leipzig University