Optical second harmonic generation fingerprint of order-disorder transition in electron-doped SrTiO₃

The coexistence of itinerant carriers and polar crystal distortions in a quantum paraelectric metal poses challenges to the ferroelectric theory and raises new questions about nonequilibrium polarization control. Here, we present our study of electron-doped SrTiO₃ single crystals using static and time-resolved rotational anisotropy second harmonic generation (RA-SHG) . We identify a ferroelectric transition near 43 K with order–disorder characteristics from the polar nanoregions. . Notably, optical excitation allows full control of the polarization of the polar nanoregions in the vicinity of the transition temperature. These findings shed light on the understanding of quantum ferroelectric phase transitions and photoinduced symmetry control in an incipient ferroelectric.