  Terahertz lights are usually generated through the optical rectification
process within a femtosecond laser pulse in non-centrosymmetric materials.
Here, we report a new generation mechanism of terahertz lights based upon a
photoinduced phase transition (PIPT), in which an electronic structure is
rapidly changed by a photoirradiation. When a ferroelectric organic molecular
compound, tetrathiafulvalene-p-chloranil, is excited by a femtosecond laser
pulse, the ionic-to-neutral transition is driven and simultaneously a strong
terahertz radiation is produced. By analyzing the terahertz electric-field
waveforms and their dependence on the polarization direction of the incident
laser pulse, we demonstrate that the terahertz radiation originates from the
ultrafast decrease of the spontaneous polarization in the photoinduced
ionic-to-neutral transition. The efficiency of the observed terahertz radiation
via the PIPT mechanism is found to be much higher than that via the optical
rectification in the same material and in a typical terahertz emitter, ZnTe.
