Abstract
We gain hitherto missing access to the spatio-temporal evolution of lattice distortions caused by carrier self-trapping in the class of oxide materials - and beyond. The joint experimental/theoretical tool introduced combines femtosecond mid-infrared probe spectroscopy with potential landscape modeling and is based on the original approach that the vibration mode of a biatomic molecule is capable to probe strongly localized, short-lived lattice distortions in its neighborhood. Optically generated, small, strong-coupling polarons in lithium niobate, mediated by OH− ions present as ubiquitous impurities, serve as a prominent example. Polaron trapping is found to result in an experimentally determined redshift of the OH− stretching mode amounting to Δνvib = −3 cm−1, that is successfully modeled by a static Morse potential modified by Coulomb potential changes due to the displacements of the surrounding ions and the trapped charge carrier. The evolution of the trapping process can also be highlighted by monitoring the dynamics of the vibrational shift making the method an important tool for studying various systems and applications.
Highlights
Sign and magnitude of the shift can only be explained by taking into account, in addition to the polaronic charge, the distortion of the lattice in the imminent neighborhood - a very striking result that supports the microscopic approach for the bulk photovoltaic effect based on small, strong-coupling polarons in LN16 and will foster its advances
The time resolution can be increased to the sub-ps time regime by using up-conversion MIR-spectroscopy; this enables a larger signal-to-noise ratio accompanied with a better time resolution
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Summary
The step is to verify the relation of this shift to the presence of small, strong-coupling polarons For this purpose, the temporal relaxation dynamics in the mid-infrared spectral range is highlighted in a semi-logarithmic plot in Fig. 3 (red data points) and compared with the temporal decay dynamics of the light-induced absorption detected in the near-infrared and visible spectral range (blue data points) close to the maximum of the GP and Probed effect Molecular vibration Electronic transition. The values (τ,β) obtained from fitting the experimental data are listed in Table 1 and are comparable within the error margins These findings are remarkable since the absorption in the NIR and VIS region is related to changes in the electronic structure, whereas the absorption in the MIR is characteristic for the molecular vibration of OH−. Appropriate calculations may be used straightforwardly for the determination of further small polaron parameters, like the electron phonon coupling strength
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