Efficient terahertz-wave generation and its ultrafast optical modulation in charge ordered organic ferroelectrics

Efficient terahertz (THz) wave generation in strongly correlated organic compounds α-(ET)₂I₃ and α′-(ET)₂IBr₂ (ET:bis(ethylenedithio)-tetrathiafulvalene) was demonstrated. The spontaneous polarization induced by charge ordering or electronic ferroelectricity was revealed to trigger the THz-wave generation via optical rectification; the estimated 2nd-order nonlinear optical susceptibility for α-(ET)₂I₃ is over 70 times larger than that for prototypical THz-source ZnTe. Ultrafast (<1 ps) and sensitive (∼40%) photoresponse of the THz wave was observed for α-(ET)₂I₃, which is attributable to photoinduced quenching of the polarization accompanied by insulator(ferroelectric)-to-metal transition. Modulation of the THz wave was observed for α′-(ET)₂IBr₂ upon the poling procedure, indicating the alignment of polar domains.