  Photodetection is an indispensable function of optoelectronic devices in
modern communication and sensing systems. Contrary to the near-infrared/visible
regions, the fast and sensitive photodetectors operated at room temperature for
the far-infrared/terahertz regions are not well developed despite a possibly
vast range of applications. The bulk photovoltaic effect (BPVE) in single-phase
noncentrosymmetric materials based on the shift current mechanism enables
less-dissipative energy conversion endowed with instantaneous responsivity
owing to the quantum-mechanical geometric phase of electronic states.
Nevertheless, the small-band-gap material for the low-energy BPVE inevitably
suffers from the thermal noise due to the intrinsically high conductivity.
Here, we demonstrate the shift current induced by soft-phonon excitations
without creation of electron-hole pairs in the archetypal ferroelectric BaTiO3
by using the terahertz light, whose energy scale is three orders of magnitude
smaller than the electronic band gap. At and above room temperature, we observe
appreciable photocurrents caused by the soft-phonon excitation as large as that
for electronic excitation and their strong phonon-mode dependence. The observed
phonon-driven BPVE can be well accounted for by the shift current model
considering the electron-phonon coupling in the displacement-type
ferroelectrics as supported by the first-principles calculation. Our findings
establish the efficient quantum BPVE arising from low-energy elementary
excitations, suggesting the novel principle for the high-performance terahertz
photodetectors.
