Photovoltaic effect by soft phonon excitation in ferroelectric BaTiO₃

The far-infrared/terahertz bands lack fast and sensitive detectors operated at room temperature despite various future applications. This limitation is mainly because the low-energy photovoltaic effect for the small-band-gap material inevitably suffers from thermal noise due to the intrinsically high conductivity. In this study, we demonstrate the shift current induced by soft-phonon excitations by using terahertz light without creation of electron-hole pairs in ferroelectric BaTiO₃. The photocurrents caused by the soft-phonon excitation is as large as that for electronic excitation across the band gap, which can be well accounted for by the shift current model with the quantum-mechanical geometric phase considering the electron-phonon coupling, as supported by the first-principles calculation. Our findings suggest the novel principle for the high-performance terahertz photodetectors.