Abstract
Many linked processes occur concurrently in strongly excited semiconductors, such as interband and intraband absorption, scattering of electrons and holes by the heated lattice, Pauli blocking, bandgap renormalization and the formation of Mahan excitons. In this work, we disentangle their dynamics and contributions to the optical response of a ZnO thin film. Using broadband pump-probe ellipsometry, we can directly and unambiguously obtain the real and imaginary part of the transient dielectric function which we compare with first-principles simulations. We find interband and excitonic absorption partially blocked and screened by the photo-excited electron occupation of the conduction band and hole occupation of the valence band (absorption bleaching). Exciton absorption turns spectrally narrower upon pumping and sustains the Mott transition, indicating Mahan excitons. Simultaneously, intra-valence-band transitions occur at sub-picosecond time scales after holes scatter to the edge of the Brillouin zone. Our results pave new ways for the understanding of non-equilibrium charge-carrier dynamics in materials by reliably distinguishing between changes in absorption coefficient and refractive index, thereby separating competing processes. This information will help to overcome the limitations of materials for high-power optical devices that owe their properties from dynamics in the ultrafast regime.
Highlights
Many-body systems under non-equilibrium conditions, for instance caused by photo-excitation, still challenge the limits of our understanding at microscopic length and ultrashort time scales [1,2,3]
Due to the flatness of the valence bands, excited holes have enough excess energy to scatter towards the edge of the Brillouin zone and promote IVB transitions [figure 1(a), short black arrows] which are observed as low-energy absorption, similar to observations for strongly doped p-type semiconductors [75, 76]
It is a unique tool to study the dynamics of electronic systems in solids that beats conventional transient spectroscopy in several areas
Summary
Many-body systems under non-equilibrium conditions, for instance caused by photo-excitation, still challenge the limits of our understanding at microscopic length and ultrashort time scales [1,2,3]. In addition to advancing the fundamental understanding of exotic quantum states, e.g., involving large densities of free charge carriers [6, 7], understanding such many-body systems supports technological breakthroughs and the development of novel applications including high-speed optical switching [8, 9] and computing [10, 11], fast transparent electronics [12, 13], light harvesting [14, 15], or even new means of propulsion for spacecrafts [16] The implementation of such next-generation devices requires development of techniques that probe transient states of matter and precisely control ultrafast dynamics of excited electronic systems in solids. This is especially significant for applications of transparent semiconductors like wide-gap oxides [18]
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