Yohei Kawakami

Optical freezing of charge motion in an organic conductor

Dynamical localization, that is, reduction of the intersite electronic transfer integral t by an alternating electric field, E(ω), is a promising strategy for controlling strongly correlated systems with a competing energy balance between t and the Coulomb repulsion energy. Here we describe a charge localization induced by the 9.3 MV cm(-1) instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor α-(bis[ethylenedithio]-tetrathiafulvalene)(2)I(3). A large reflectivity change of >25% and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the ~10% reduction of t driven by the strong, high-frequency (ω ≧ t/ħ) electric field.

Polarization selectivity of charge localization induced by a 7-fs nearly single-cycle light field in an organic metal

Polarization selectivity of light-field-induced charge localization was investigated in an organic metal alpha-(BEDT-TTF)2I3 with a triangular lattice. Dependences of transient reflectivity spectra on polarizations of the 7-fs pump and probe lights indicate that a short-range charge order (CO) is efficiently induced from the metallic phase for the pump polarization perpendicular to the 1010-type CO axis. Numerical solution of a time-dependent Schrodinger equation clarified that the 1010-CO is induced by field-induced re-distribution of charges cooperating with competing inter-site Coulomb repulsions in the triangular lattice.

Ultrafast response of plasma-like reflectivity edge in (TMTTF)2AsF6 driven by 7-fs, 1.5-cycle strong-light field

The strong light-field effect of (TMTTF)2AsF6 was investigated utilizing 1.5-cycle, 7-fs infrared pulses. The ultarfast (20 fs) and large (40%) response of the plasma-like reflectivity edge (0.7 eV) was analyzed by the changes in omega_p=sqrt(ne2/(epsilon_0*epsilon(infty)*m)} (n: number of charges in the 1/4 filled-band, m: mass of charge, epsilon(infty): dielectric constants for high-frequency and vacuum, e: elementary charge). The 3% reduction in omega_p is attributed to the 6% increase in m. Furthermore, 20 fs oscillation of omega_p in the time domain indicates that the plasma-like edge is affected by the charge gap (0.2 eV) nature. Theoretical calculations suggest that the Coulomb repulsion plays an important role in the increase in m.

Nonlinear charge oscillation driven by a single-cycle light field in an organic superconductor

Application of an intense light field to solids produces enormous and ultrafast nonlinear phenomena such as high-harmonic generation1,2 and attosecond charge dynamics3,4. These are distinct from conventional photonics. However, the main targets for investigation have been limited to insulators and semiconductors, although theoretical approaches have also been developed for correlated metals and superconductors5. Here, in a layered organic superconductor, a nonlinear charge oscillation driven by a nearly single-cycle strong electric field of >10 MV cm−1 is observed as a stimulated emission. The charge oscillation is different from a linear response and ascribed to a polar charge oscillation with a period of ∼6 fs. This nonlinear polar charge oscillation is enhanced by critical fluctuations near a superconducting transition temperature and a critical end-point of first-order Mott transitions. Its observation on an ultrafast timescale of ∼10 fs clarifies that Coulomb repulsion plays an essential role in the superconductivity of organic superconductors.A nonlinear charge oscillation driven by a 6 fs light field of 11 MV cm–1 is observed in a layered organic superconductor. The initial response time of the oscillation on the timescale of 10 fs clarifies that Coulomb repulsion is essential for the superconductivity.

Strong light-field effects driven by nearly single-cycle 7 fs light-field in correlated organic conductors

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.

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.

Photo-induced macroscopic oscillation between insulator and metal in layered organic Mott insulator

Photo-induced insulator to metal transition in two-dimensional organic Mott insulator ĸ-(d-ET)2Cu[N(CN)2]Br was investigated by mid-IR pump-probe spectroscopy. Photo-induced macroscopic GHz oscillation between the Mott insulator and the metal, reflecting the competitive phase diagram, was observed.