  The electronic symmetry of the SrTiO3/LaAlO3 interface was investigated by
optical second harmonic generation, using superlattices with varying
periodicity to study the evolution of the electronic reconstruction while
avoiding substrate contributions. The superlattices show large perpendicular
optical nonlinearity, which abruptly increases when the sublattice thickness
goes above 3 unit cells, revealing substantial effects of the polar-nonpolar
interface. The nonlinear 'active' area is primarily in SrTiO3, develops with
increasing thickness, and extends up to 8 unit cells from the interface.
