Croconic acid crystals show proton displacive-type ferroelectricity with a large spontaneous polarization reaching 20  μC/cm^{2}, which originates from the strong coupling of proton and π-electron degrees of freedom. Such a coupling makes us expect a large polarization change by photoirradiations. Optical-pump second-harmonic-generation-probe experiments reveal that a photoexcited croconic-acid crystal loses the ferroelectricity substantially with a maximum quantum efficiency of more than 30 molecules per one absorbed photon. Based on density functional calculations, we theoretically discuss possible pathways toward the formation of a one-dimensional domain with polarization inversion and its recovery process to the ground state by referring to the dynamics of experimentally obtained polarization changes.

The ultrafast dynamics of correlated electron systems after photoexcitation are now attracting considerable attention. This is based upon recent developments in femtosecond laser technology, which enabled us to detect ultrafast electronic responses to a light pulse in solids [1][2][3][4] . Applications of femtosecond pump-probe spectroscopy to correlated electron systems enable us to observe exotic photoinduced phase transitions 5-18 as represented by a photoinduced Mott-insulator-to-metal transition 6,12,[16][17][18] and also to derive detailed information about the interplays between the charge, spin, and lattice degrees of freedom from the transient responses of each degree to a light pulse [5][6][7][8][9][10][11][13][14][15] . Such information can hardly be obtained from the steady-state transport and magnetic measurements. The growing interest in the ultrafast dynamics of correlated electron systems synchronizes to the development of a new field called 'non-equilibrium quantum physics in solids'. In fact, new theoretical approaches have recently been explored to analyse the charge, spin, and lattice dynamics of non-equilibrium states after photoirradiation, as exemplified by the dynamical mean field theory 19 and the time-dependent density-matrix renormalization group method 20,21 .In the non-equilibrium quantum physics of correlated electron systems, the charge dynamics of photoexcited Mott insulators is the most fundamental subject to be studied from both the experimental 6,12,16-18 and theoretical viewpoints [22][23][24][25][26][27][28] . Recent studies have focussed on Mott insulator states realized not only in solids such as transition metal compounds 6,16-18 and organic molecular materials 12 but also in ultra-cold atoms on an optical lattice 29,30,31 . In fact, in the ultra-cold atoms, non-equilibrium dynamics can be investigated by tuning the intersite interaction using a Feshbach resonance 29 . Among various Mott insulators, a one-dimensional (1D) Mott insulator with large on-site Coulomb repulsion energy U is particularly important since the charge and spin degrees 4 of freedom are decoupled 32,33 , In the system, we can obtain clear information about the effects of Coulomb interactions on the charge dynamics. When the electronic structure and low-energy excitations in a 1D Mott insulator are theoretically analysed, the Hubbard model, which includes U and the transfer energy t as the important parameters, is generally used. In the photoexcited states, on the other hand, electron wavefunctions are more delocalized, and the effects of long-range Coulomb interactions will become important in the charge dynamics. However, it is difficult to evaluate these effects experimentally.Based upon these backgrounds, in the present study, we investigated the role of longrange Coulomb interactions in photoexcited states by focusing on the excitons and biexcitons in 1D Mott insulators. The long-range Coulomb interactions can stabilize not only the bound state of a doublon and a holon (that is, an exciton 34-36 ) b...

Rapid polarization control by an electric field in ferroelectrics is important to realize high-frequency modulation of light, which has potential applications in optical communications. To achieve this, a key strategy is to use an electronic part of ferroelectric polarization. A hydrogen-bonded molecular ferroelectric, croconic acid, is a good candidate, since π-electron polarization within each molecule is theoretically predicted to play a significant role in the ferroelectric-state formation, as well as the proton displacements. Here, we show that a sub-picosecond polarization modulation is possible in croconic acid using a terahertz pulse. The terahertz-pulse-pump second-harmonic-generation-probe and optical-reflectivity-probe spectroscopy reveal that the amplitude of polarization modulation reaches 10% via the electric-field-induced modifications of π-electron wavefunctions. Moreover, the measurement of electric-field-induced changes in the infrared molecular vibrational spectrum elucidates that the contribution of proton displacements to the polarization modulation is negligibly small. These results demonstrate the electronic nature of polarization in hydrogen-bonded molecular ferroelectrics. The ultrafast polarization control via π-electron systems observed in croconic acid is expected to be possible in many other hydrogen-bonded molecular ferroelectrics and utilized for future high-speed optical-modulation devices.

Ferroelectrics sometimes show large electro-optical and non-linear optical effects, available for polarization rotation and frequency conversion of light, respectively. If the amplitude of ferroelectric polarization is modulated in the picosecond time domain, terahertz repetition of optical switching via electro-optical and non-linear optical effects would be achieved. Here we show that polarization amplitude can be rapidly modulated by a terahertz electric field in an organic ferroelectric, tetrathiafulvalene-p-chloranil (TTF-CA). In this compound, alternately stacked donor (TTF) and acceptor (CA) molecules are dimerized via the spin-Peierls mechanism, and charge transfer within each dimer results in a new type of ferroelectricity called electronic-type ferroelectricity. Using a terahertz field, the intradimer charge transfer is strongly modulated, producing a subpicosecond change in the macroscopic polarization, which is demonstrated by transient reflectivity and second-harmonic generation measurements. Subsequently, coherent oscillation of the dimeric molecular displacements occur, which is explained by the modulation of the spin moment of each molecule.

This work demonstrates terahertz (THz) line imaging that acquires broadband spectral information by combining echelon-based single-shot THz spectroscopy with high-sensitivity phase-offset electrooptic detection. An approximately 40 dB signal-to-noise ratio is obtained for a THz spectrum from a single line of the camera, with a detection bandwidth up to 2 THz at the peak electric-field strength of 1.2 kV/cm. The spatial resolution of the image is confirmed to be diffraction limited for each spectral component of the THz wave. We use the system to image sugar tablets by quickly scanning the sample, which illustrates the capacity of the proposed spectral line imaging system for high-throughput applications.