Second harmonic generation in AB-type LaTiO₃/SrTiO₃superlattices

Conventional generation of second harmonic generation (SHG) is by using non-central symmetric materials, or a combination of them. However this is limited by the material itself, which is usually of bulk size. In this work, we employ the surface and interfaces to generate and control the SHG from a series of central symmetric oxide LaTiO₃/SrTiO₃ superlattices. We demonstrate that the nonlinear optical susceptibility χ⁽²⁾ and the SHG pattern can be tuned by tailoring the number of superlattice interfaces. Significantly, unlike the conventional ABC-type nanostructures, our AB-type nanostructures can also produce SHG signals. The χ⁽²⁾ value (24.3 pm/V) is even one to two orders of magnitude greater than the conventional ones. Hence, we demonstrate that SHG is a local microscopic process—it does not rely on macroscopic broken inversion symmetry. Moreover, we show that temperature dependence or structural phase transition can further enhance the SHG intensity. Our results introduce a scheme for SHG, extending the generation and control of SHG in nano-photonics to nearly all the accessible centrosymmetric materials.