We demonstrate the ultrafast photoinduced Mott transition from a charge transfer insulator to a metal in a halogen-bridged Ni-chain compound by pump-probe reflection spectroscopy. Upon the irradiation of a 130-femtosecond laser pulse, the spectral weight of the gap transition is transferred to the inner-gap region. When the photoexcitation density exceeds 0.1/Ni site, the Drude-like high-reflection band appears in the infrared region, signaling the formation of a metallic state. The photogeneration of the metallic state and the subsequent recovery to the original gapped state occur within a few picoseconds.

We report experimental evidence for the generation of ferroelectric polarization in an organic conductor -[bis(ethylenedithio)tetrathiafulvalene] 2 I 3 obtained by optical second-harmonic generation. The spontaneous polarization emerges along with a metal-to-insulator transition that is driven by the Wigner crystallization of electrons. The strong optical nonlinearity and its ultrafast photoresponse demonstrated by this study exemplify the nature of the ferroelectric polarization that originates from the electron ordering.

Photoinduced transitions from ionic ͑I͒ to neutral ͑N͒ and neutral ͑N͒ to ionic ͑I͒ phases in an organic charge transfer (CT) complex, tetrathiafulvalene-p-chloranil (TTF-CA), were investigated by femtosecond pumpprobe reflection spectroscopy. Transient reflectivity changes of the intramolecular transition band of TTF sensitive to the degree of CT between a donor molecule of TTF and an acceptor molecule of CA are measured as a function of excitation energy, excitation density, and temperature. By adopting the multilayer model for the analysis of the obtained transient reflectivity spectra, we have derived the time characteristics of amounts and spatial distributions of photoinduced N ͑I͒ states in the I ͑N͒ phase. The results reveal that the I to N ͑IN͒ transition induced by the resonant excitation of the CT band at 4 K is composed of three processes; (1) formation of a confined one-dimensional (1D) N domain, that is, a sequence of D 0 A 0 pairs, just after the photoexcitation, (2) multiplication of the 1D N domains to the semimacroscopic N states up to 20 ps within the absorption depth of the excitation light, and (3) proceeding of the IN transition along the direction normal to the sample surface. At 77 K near the NI transition temperature ͑T c =81 K͒, the size of the 1D N domain initially produced is enlarged and its multiplication process is strongly enhanced. When the excitation energy is increased, the initial photoproduct is changed from the confined 1D N domain to the positively and negatively charged N states. The spatial size of the latter is considerably larger than that of the former, indicating that the introduction of charge carriers makes the neighboring I state strongly unstable. The dynamics of the photoinduced N to I ͑NI͒ transition has also been investigated. The 1D I domains are initially produced by lights, however, they decay within 20 ps even if the density of the I domains is increased. The results demonstrate that there is a clear difference of the dynamics between the photoinduced IN and NI transitions. In these photoinduced transitions, three kinds of coherent oscillations with the period of ϳ0.6, ϳ50, and ϳ85 ps have been detected on the photoinduced reflectivity changes, which are reasonably assigned to the dynamical dimeric displacements of molecules associated with the spin-Peierls instability, the shock wave driven by the sudden volume change due to the photoinduced transitions, and the oscillation of the NI domain boundary. On the basis of the results, dynamical aspects of the photoinduced IN and NI transitions have been discussed in detail.

Photoinduced melting of charge-order (CO) in [bis(ethylenedithiolo)]-tetrathiafulvalene (BEDT-TTF) salts was investigated by femotosecond spectroscopy. Comparative studies on two polytypes exhibiting large [theta-(BEDT-TTF)2RbZn(SCN)_{4}] and small [alpha-(BEDT-TTF)2I3] molecular rearrangements through the CO transition were performed. Ultrafast melting of CO for both compounds demonstrates the major contribution of the electronic instability which is due to Coulomb interaction. The roles of the molecular rearrangements on the formation of the CO and the metallic domain are discussed on the basis of low-frequency lattice dynamics.

Ultrafast optical switching from an ionic ( I) to a neutral ( N) state in TTF-CA was observed in femtosecond reflection spectroscopy. Charge transfer excitation in the I phase produces N donor ( D0) acceptor ( A0) strings within 2 ps. These initial N states decay with a lifetime of 300 ps at 4 K, but rather multiply leading to macroscopic I-N conversion within 20 ps at 77 K just below the N-I transition temperature T(NI). Near T(NI), we also found the evidence for the coherent motion of the macroscopic N-I domain boundary with a period of 85 ps.

Ultrafast dynamics of the light-matter interaction in a charge-ordered molecular insulator α-(BEDT-TTF)2I3 were studied by pump-probe spectroscopy using few-optical-cycle infrared pulses (pulse width 12 fs). Coherent oscillation of the correlated electrons and subsequent Fano destructive interference with intramolecular vibration were observed in time domain; the results indicated a crucial role for electron-electron interplay in the light-matter interaction leading to the photoinduced insulator-to-metal transition. The qualitative features of this correlated electron motion were reproduced by calculations based on exact many-electron-phonon wave functions.

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Dynamical localization, that is, reduction of the intersite electronic transfer integral t by an alternating electric field, E(o), 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 MVcm À 1 instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor a-(bis[ethylenedithio]-tetrathiafulvalene) 2 I 3 . A large reflectivity change of 425% 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 B10% reduction of t driven by the strong, high-frequency (oZt/:) electric field.