Probing electrocolored Fe-doped SrTiO3 bulks using optical second harmonic generation

We have investigated the spatial distribution of non-centrosymmetric defects in the electrocolored bulks of oxidized and reduced Fe-doped SrTiO3 (Fe:STO) single crystals by optical second harmonic generation (SHG) spectroscopy in the transmission geometry. Transmitted SHG intensities were examined across the anodic, mixed, and cathodic bulk regions of both Fe:STO crystals. The obtained nonlinear susceptibility ratios show that the structural distortions in the anodic and cathodic bulks of the electrocolored crystals are dominated by Fe:TiO bond bending around Fe4+ defect centers and Fe:TiO bond stretching around Fe3+ defect centers, respectively. Moreover, we identified a stronger local strain field at the color front of the oxidized crystal over the reduced one. This is attributed to more efficient strain relaxation during oxygen vacancy demixing in the reduced crystal.

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