Nonlinear Optical Properties in an Epitaxial YbFe2O4 Film Probed by Second Harmonic and Terahertz Generation.

An epitaxial film of YbFe2O4, a candidate for oxide electronic ferroelectrics, was fabricated on yttrium-stabilized zirconia (YSZ) substrate by magnetron sputtering technique. For the film, second harmonic generation (SHG), and a terahertz radiation signal were observed at room temperature, confirming a polar structure of the film. The azimuth angle dependence of SHG shows four leaves-like profiles and is almost identical to that in a bulk single crystal. Based on tensor analyses of the SHG profiles, we could reveal the polarization structure and the relationship between the film structure of YbFe2O4 and the crystal axes of the YSZ substrate. The observed terahertz pulse showed anisotropic polarization dependence consistent with the SHG measurement, and the intensity of the emitted terahertz pulse reached about 9.2% of that emitted from ZnTe, a typical nonlinear crystal, implying that YbFe2O4 can be applied as a terahertz wave generator in which the direction of the electric field can be easily switched.

Petahertz charge dynamics in a correlated organic superconductor.

We report an observation of stimulated emission induced by a nearly single-cycle 6 fs near infrared electric field of 10 MV cm-1 in an organic superconductor (κ-(h-ET)2Cu[N(CN)2]Br). The stimulated emission is attributed to a non-linear synchronized coherent charge oscillation. We also report that, in the same organic superconductor, a light-induced current before the scattering time shows up as carrier-envelope phase (CEP)-sensitive second harmonic generation (SHG). This unconventional SHG in the centrosymmetric compound is regarded as a light induced spatial symmetry breaking. These ultrafast optical non-linearities induced by petahertz charge oscillations show anomalous enhancements around the superconducting transition temperature (11.6 K). These results indicate that the microscopic mechanism of superconducting fluctuations is closely related to the Coulomb repulsive interaction in this compound.

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/h) electric field.