Dielectric property and tunability of multilayer BST-BTO thin film in the terahertz range

The dielectric property and tunability of Ba_0.6Sr_0.4TiO₃ (BST) and BaTiO₃ (BTO) multilayer thin film were investigated by using terahertz time-domain spectroscopy (THz-TDS) at room temperature. The film samples used in the study had a same total thickness but different numbers of periodic layer N (a single BTO layer plus a single BST layer) and different layer thickness ratios x (x for total BTO layer thickness to total film thickness; 1-x for total BST layer thickness to total film thickness). We found that the permittivity of the samples declined with the increase of N, which can also be modulated by external optical pumping. The dielectric loss of the sample with N = 1 was much higher than that of other samples, which could be attributed to the interfacial polarization between BST and BTO thin films. In short, the dielectric property of layered BST-BTO composite structure can be appreciably changed by selecting the number of periodical layers and the corresponding layer thickness. These results indicate that the multilayer thin film structure is possible for the applications of tunable photonic devices in the terahertz range.