Photoinduzierte Effekte in funktionalen Oxiden untersucht mittels zeitaufgelöster optischer Spektroskopie
Zusammenfassung der Projektergebnisse
Within the project “Photo-induced effects in functional oxides studied by time-resolved optical spectroscopy” the photostriction effect in ferroelectrics and multiferroics was investigated by optical spectroscopy and pump-probe measurements. Photostriction is the deformation of materials induced by the illumination with light and is generally explained in terms of a combination of photovoltaic and inverse piezoelectric effects. In particular, the multiferroic BiFeO3 shows photostriction with an ultrafast response time in the ps range, which is orders of magnitude faster than that of classical ferroelectrics. Most recent studies propose the excitation of charge-neutral excitons, which are either self-trapped, leading to an instantaneous stress due to the inverse piezoelectric effect in BiFeO3 , or diffuse to the surface or interface, where they dissociate causing the screening of the depolarization field, which leads to a structural piezoelectric response. We carried out various spectroscopic studies, in order to observe the suggested excitons induced during the illumination with light in BiFeO3 and other ferroelectric materials. Our experiments revealed that the features in the optical response, which were induced by laser illumination in BiFeO3 , were extremely weak and difficult to observe. Only in the nearinfrared/visible frequency range, i.e., close to the d-d crystal field excitations and the onset of the fundamental absorption edge, traces of the weak light-induced spectral features could be observed. The absorption features were studied as a function of temperature, pressure, and time. For the illumination, laser light with various wavelengths, power, and polarization was used (besides LEDs). Based on temperature-dependent optical measurements on BiFeO3 single crystals under illumination we could relate the three absorption features to (crystal field) excitons, with a possible coupling of the involved electronic levels with other degrees of freedom such as phonons or magnons. Corresponding pressure-dependent studies suggest that the absorption features are linked to the ferroelectric R3c phase, since they disappear for pressures above the first structural phase transition at 3-4 GPa. No evidence for the excitonic features could be found in BiFeO3 films and doped BFO films with a chemical composition close to the morphotropic phase boundary, where the competition of two structural phases leads to enhanced piezoelectric properties. In the ferroelectric PbTiO3 one rather broad absorption features was observed under illumination with energy just below the absorption edge. The photo-induced changes in the optical response of BiFeO3 single crystals were also studied by FTIR spectroscopy in the step-scan mode with a time resolution in the µs range. No clear feature was observed in the step-scan spectra, most probably because the photo-induced features appear and decay on a shorter time scale. The time-dependent decay of the lowest-energy absorption feature A at 1.2 eV associated with the lower-energy crystal field excitons was studied by pump-probe measurements in collaboration with the group of Markus Betz, Technische Univeristät Dortmund. Based on the relative change of the transmission as a function of delay time we could obtain two relaxation times on the ps time scale. The short relaxation time τ1 ∼6 ps could be related to the fast decay of high-energy electrons releasing their energy through the emission of phonons within a few ps. We attribute the relaxation time τ2 ∼53 ps to the decay of the lower-energy crystal field excitons. This short relaxation time suggests that the crystal field excitons play an important role regarding the ultrafast photostriction effect in BiFeO3.
Projektbezogene Publikationen (Auswahl)
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(2019) Temperature-dependent photo-response in multiferroic BiFeO 3 revealed by transmission measurements. Journal of Applied Physics 125 (11) 114104
Meggle, F.; Viret, M.; Kreisel, J.; Kuntscher, C. A.
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Optical spectroscopy study on the photoresponse in multiferroic BiFeO3 , Appl. Phys. Lett. 109, 182903 (2016)
F. Burkert, J. Kreisel, and C. A. Kuntscher
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Chemical pressure effect in Sm and La substituted ferroelectric BiFeO3 thin films: Insights from infrared spectroscopy, J. Appl. Phys. 121, 144103 (2017)
F. Burkert, M. Janowski, X. Zhang, I. Takeuchi, and C. A. Kuntscher
