Surface acoustic waves as a sensitive probe for photoresponsive polarization memory in SrTiO 3

Transient electric polarization in single crystalline SrTiO3 (STO) substrates was studied by measuring the transmission of surface acoustic waves (SAWs). We applied a large dc electric field (8 × 106 Vm−1) to interdigital transducers (IDTs) on STO substrates in order to induce local piezoelectricity, which is required to generate and transmit SAWs. The resulting electric polarization and the retention thereof were analyzed as a function of time after the applied electric field was removed, by measuring transient SAW signals. The retained polarization turned out to provide strong electromechanical coupling, comparable to that resulting from the dc-field-induced piezoelectricity. SAW signals were observed for more than 30 h (in absence of an external dc electric field), which is evidence for a long-lasting retention of electric polarization. Remarkably, this polarization was found to be rapidly suppressed as the sample was exposed to visible light. By measuring the transient SAW transmission under illumination with light at different wavelengths, we identified photoconductivity and/or acceleration of oxygen vacancy migration by photon-induced splitting of bound vacancy pairs as the main mechanisms behind this photoresponsive memory effect.