  The terahertz (THz) response in 10-100 cm^-1 was investigated in an organic
dimer-Mott (DM) insulator kappa-(ET)_2Cu_2(CN)_3 that exhibits a relaxor-like
dielectric anomaly. 30 cm^-1 band in the optical conductivity was attributable
to collective excitation of the intra-dimer electric dipoles which are formed
by an electron correlation. We succeeded in observing photoinduced enhancement
of this 30 cm^-1 band, reflecting the growth of the electric dipole cluster in
the DM phase. Such optical responses in kappa-(ET)_2Cu_2(CN)_3 reflect
instability near the boundary between the DM-ferroelectric charge ordered
phases.
