Angular-dependent second harmonic generation (SHG) spectroscopy is used study to the lattice and electronic symmetry of quantum material thin film buried interfaces. Transition metal oxide thin film heterostructures, having potential superconducting, magnetic, and topological properties are investigated where the SHG signal is measured as a function of sample azimuthal angle under different incident and output polarization configurations. In one case, La2/3Sr1/3MnO3 (LSMO) thin films are grown on SrTiO3 (STO) substrates. In another case, STO thin films are grown on NdGaO3 (NGO) substrates. Each of the different crystalline materials is centrosymmetric so that the SHG signal originates from the surface and buried interface. The results are compared to the STO, LSMO, and NGO surfaces alone to determine the symmetry characteristics of the buried interfaces. Additionally, the angular-dependent SHG signal of GaAs is measured as an established reference sample. Complementary analysis using high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM) is used to obtain atomically-resolved images of the interfaces. The LSMO/STO interface shows a 4-fold SHG symmetry indicating an octahedral tilt that lowers the overall symmetry. The STO/NGO interface shows a 2-fold SHG symmetry accompanied by a two-photon fluorescence (TPF) anisotropy that is altered in comparison to NGO. Additional results are presented on the temperature dependence of these results. In general, angular-dependent SHG spectroscopy is demonstrated to be a powerful nonlinear optical technique for determining the lattice and electronic symmetry of buried interfaces for characterizing transition metal oxide thin film heterostructures for developing potential applications in quantum materials.

Angular dependent second harmonic generation spectroscopy studies of quantum material thin film heterostructures

ACS Fall 2021

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