Epitaxial CaMnO 3 films grown with 2.3% tensile strain on (001)-oriented LaAlO 3 substrates are found to be incipiently ferroelectric below 25 K. Optical second harmonic generation (SHG) was used for the detection of the incipient polarization. The SHG analysis reveals that CaMnO 3 crystallites with in-plane orientation of the orthorhombic b axis contribute to an electric polarization oriented along the orthorhombic a (respectively c) axis in agreement with the predictions from density functional calculations.

We report the results of second-harmonic optical spectroscopy of LaAlO 3 films grown on SrTiO 3 (001) substrates having either TiO 2 or SrO atomic terminations. The atomic termination is found to have a large effect on the observed spectra, with a strong dependence on the ω-2ω light polarizations. These results are analyzed through a model based on symmetry-controlled selection rules and atomic orbital overlaps. The analysis shows that, as suspected from transport measurements, the SrO termination prevents the formation of a mobile interfacial electron gas. Yet, by explicitly probing and identifying the electronic states at the O 2− -Ti 3+ band gap we now obtain unique and direct evidence that this is caused by the absence of any detectable charge injection in the interfacial region rather than by carrier localization.

Phase-sensitive, spatially resolved optical second-harmonic-generation experiments were performed on LaAlO 3 /SrTiO 3 heterostructures. Lateral inhomogeneities on a length scale of ≈ 30 μm are found when a one-unit-cell-thick epitaxial monolayer of LaAlO 3 is grown on TiO 2 -terminated SrTiO 3 single crystals. The inhomogeneity is absent in samples with LaAlO 3 layers of more than one unit cell. The results are discussed in the framework of electronic, oxidic, and chemical inhomogeneities.

Optical second harmonic generation (SHG) spectra of LaAlO 3 /SrTiO 3 , LaGaO 3 /SrTiO 3 , and NdGaO 3 /SrTiO 3 interfaces with SrTiO 3 substrates have been measured up to 4.2 eV as a function of film thickness and temperature. This spectral range is characterized by two-photon transitions from the valence to conduction bands of SrTiO 3 which are classified with a model based on symmetry, selection rules, and atomic orbital overlaps. This model is further confirmed in spectral measurements as a function of film thickness, material overlayer, and temperature. SHG enhancement at low temperature indicates an increase of the interfacial polarity with decreasing temperature. This confirms the relation between SHG and the spatial translation of Ti ions which are more prone to move at lower temperatures because of the quantum-paraelectric nature of SrTiO 3. We finally show that SHG spectroscopy captures structural details of the interfaces. In particular, we find evidence for proximity effects such as LaAlO 3-induced distortions of the TiO 6 octahedra or bond buckling. We also observe a correlation between SHG and the in-plane lattice mismatch between the SrTiO 3 substrate and the LaAlO 3 , LaGaO 3 , and NdGaO 3 overlayers.

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