Modulation of terahertz emission in time-domain waveform via a photoinduced phase transition in a charge ordered organic ferroelectric

The time-domain waveform of the terahertz (THz) electric field emitted from the charge-ordered organic ferroelectrics α - (ET)₂I₃ [ET:bis(ethylenedithio)-tetrathiafulvalene)] can be significantly modulated depending on incident fluence of femtosecond pulses. Weak irradiation triggers the emission, showing ∼1 THz oscillation in the time-domain waveform with more than 5 oscillation cycles and a corresponding narrow spectral shape. In contrast, strong irradiation almost completely suppresses the trailing portion of the waveform after the first cycle, resulting in over 3 times broader bandwidth. As revealed by time-resolved experiments, this modulation arises from the cooperative nature of photoinduced melting of the charge order accompanying quenching of infrared-activity of the intermolecular vibrations which strongly couple with the emission.