Imaging of the two-dimensional polarization landscape at the LaAlO3/SrTiO3 interface

LaAlO3/SrTiO3 interfaces are a challenging topic in the field of perovskite oxides. A conductive two-dimensional electron liquid (2DEL) appears at the interface for a LaAlO3 thickness of n≥4 unit cells. However, many questions regarding the origin and characteristics of the 2 DEL have to be addressed yet. In particular, this includes the spatial homogeneity of the interface reconstruction. Second Harmonic Generation (SHG) is an ideal tool for studying interfaces, since it is sensitive to the interfacial symmetry breaking along the stacking direction. Furthermore, the spatial degrees of freedom of optical techniques can be exploited. Here we use SHG with amplified femtosecond laser pulses for imaging the LaAlO3/SrTiO3 interface with a spatial resolution of ∼ 2 μm. For n=1 a pronounced interfacial inhomogeneity on a lengthscale of < 1 μm is observed. The lateral dimension of these inhomogeneities increases for n=2 pointing to a step-like reconstruction of the interface. At n=3, the interface becomes homogeneous which may reflect the completion of a fully reconstructed interface, though yet without the emergence of conductivity.