SrTiO(3)/DyScO(3) epitaxial multilayers with variable number and thickness (10-100 nm) of bilayers deposited on DyScO(3) substrates were investigated by means of time-domain terahertz spectroscopy at room temperature. A tensile strain develops in the SrTiO(3) films and shifts the eigenfrequency of the ferroelectric soft mode down by ∼25-45 cm(-1) with respect to the value found for single crystals. In all films the soft mode strongly hardens upon the electrical bias and a linear coupling to a silent excitation of relaxation type at 10 cm(-1) is observed. We show that the change in the THz and sub-THz response of the layers with an increasing field is determined solely by the soft mode eigenfrequency and we propose a phenomenological model describing the origin of the tunability and the peculiar properties of the ferroelectric soft mode in the terahertz spectral range.

Institute of Physics Academy of Sciences of the Czech Republic Na Slovance 2 182 21 Prague 8 Czech Republic

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