  Terahertz (THz) radiation has been observed from multiferroic BiFeO$_3$ thin
films via ultrafast modulation of spontaneous polarization upon carrier
excitation with illumination of femtosecond laser pulses. The radiated THz
pulses from BiFeO$_3$ thin films were clarified to directly reflect the
spontaneous polarization state, giving rise to a memory effect in a unique
style and enabling THz radiation even at zero-bias electric field. On the basis
of our findings, we demonstrate potential approaches to ferroelectric
nonvolatile random access memory with nondestructive readability and
ferroelectric domain imaging microscopy using THz radiation as a sensitive
probe.
