Abstract
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.3 MoreReceived 2 October 2020Revised 19 January 2021Accepted 16 February 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.013246Published 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 SocietyPhysics Subject Headings (PhySH)Research AreasCarrier dynamicsExcitonsOptical transient phenomenaPhysical SystemsOxidesSemiconductorsThin filmsTechniquesEllipsometryUltrafast pump-probe spectroscopyCondensed Matter, Materials & Applied PhysicsNonlinear Dynamics
Highlights
Transient optical properties [1,2] represented by the dielectric function (DF) following ultrafast laser excitation can be obtained using time-resolved spectroscopic ellipsometry [3,4]
Even if the molecular-beam epitaxy (MBE)-grown ZnO thin film is subject to an anisotropic strain relaxation governed by interfacial dislocations [56], we suggest the exciton–longitudinal optic (LO)-phonon interaction, which is paramount for the optical properties of polar semiconductors, to be responsible for the exponential line shape of the absorption coefficient: The charge carrier relaxation generates hot phonons that perturb the lattice potentials, which act like disorder on the excitons
We expect similar carrier and phonon dynamics: Initially, the carrier distribution in momentum space follows the spectral distribution of the pump laser; that is, it features a peak and is nonthermal
Summary
Transient optical properties [1,2] represented by the dielectric function (DF) following ultrafast laser excitation can be obtained using time-resolved spectroscopic ellipsometry (tSE) [3,4]. In contrast to conventional pump-probe experiments that do not investigate the change in light polarization upon reflection (transmission), spectroscopic ellipsometry measures simultaneously relative amplitude and phase information of the reflected (transmitted) electromagnetic fields [5]. This allows a clear distinction between changes in extinction coefficient and refractive index. Transient off-diagonal DF tensor elements can be obtained from magneto-optical Kerr-effect measurements [16] All these effects of transient optical anisotropy can be studied individually, but they are subsumed in the underlying material properties, which means in general the transient DF. Earlier time-resolved spectroscopy studies on ZnO in the reflection [3,21,22,23,24] or transmission [21,25,26,27] configuration mostly neglected its anisotropic optical properties
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