  We have demonstrated transient charge localization effects with a driving
high-frequency field of 7-fs, 1.5-cycle near infrared light in correlated
organic conductors. In a layered organic conductor alpha-(BEDT-TTF)2I3
(BEDT-TTF: bis[ethylenedithio]-tetrathiafulvalene), a transient short-range
charge order (CO) state is induced in a metallic phase. In contrast to such
drastic change in the electronic state from the metal to the transient CO in
alpha-(BEDT-TTF)2I3, dynamics of a field-induced reduction of a transfer
integral are captured as a red shift of the plasma-like reflectivity edge in a
quasi-one-dimensional organic conductor (TMTTF)2AsF6 (TMTTF:
tetramethyltetrathiafulvalene). These studies on the field-induced charge
localization have been motivated by the theory of dynamical localization on the
basis of tight-binding models with no electron correlation under a strong
continuous field. However, the results of pump-probe transient reflectivity
measurements using nearly single-cycle 7-fs, 11 MV/cm pulses and the
theoretical studies which are presented in this review indicate that the pulsed
field contributes to the similar phenomenon with the help of a characteristic
lattice structure and Coulomb repulsion.
