T. Ogasawara

Ultrafast optical nonlinearity in the quasi-one-dimensional mott insulator Sr2CuO3

We report strong instantaneous photoinduced absorption in the quasi-one-dimensional Mott insulator Sr2CuO3 in the IR spectral region. The observed photoinduced absorption is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculation based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of Sr2CuO3 for all-optical switching.

Polarization dependent quantum beats of homogeneously broadened excitons

The polarization dependence of quantum beats from the A-exciton and the B-exciton in a GaN sample of exceptional quality is studied with four-wave mixing experiments. When changing the incident polarizations from collinear to crossed linear, a pi-phase shift of the beats is observed while the decay rate remains unchanged. This confirms previous theoretical predictions.

Photo-destruction of charge/orbital order in layered perovskite manganite: La0.5Sr1.5MnO4

Photo-induced effects in layered perovskite manganite La0.5Sr1.5MnO4 are investigated with time- and polarization-resolved spectroscopy technique. The destruction of charge/orbital ordering, which is observed as the change of the reflectivity and the birefringence, occurs immediately (<1 ps) after the illumination of intense light pulse. The reflectivity change probing the local charge/orbital order reaches more than 25%, reflecting the drastic change in the electronic structure. Much slower than the nanosecond decay in the birefringence signal indicates the size growth of the photo-fragmented charge/orbital-ordered domain.

Ultrafast photo-induced absorption in one-dimensional perovskite copper oxides

Summary form only given. The family of 3d transition metal oxide is known as strongly correlated electron systems, whose low energy properties are dominated by strong on-site Coulomb repulsion and Paulis exclusion principle. Typically, these systems are insulating with finite charge gap at a half filling of conduction band (Mott insulator). In contrast with the conventional band insulators, such as semiconductors, the electronic structure of the transition metal oxides is determined by the strong interaction of electrons themselves. Because of the strong correlation of spins and charges of such system, drastic change in the electronic and magnetic properties easily occurs by chemical doping, for instance the appearance of high-T/sub c/ superconductivity, and colossal magnetoresistance, etc. We report on the observation of efficient change of absorption in one-dimensional perovskite copper oxide Sr/sub 2/CuO/sub 3/ by femtosecond laser pulse.

Quantum beats of homogeneously broadened excitons in GaN

It has been observed that the phase of quantum beats shifts by /spl pi/ if the incident polarization is changed from collinear to crossed linear. Schmitt-Rink et al. (1992) explained this using a single particle model based on the band structure. However, in their experimental results they also observed a decay rate and intensity dependence on the incident polarization, which their model cannot explain. On the other hand, Saiki et al. (1994) proposed an excitonic model and found that the decay rate and intensity changes depending on if the system is homogeneously or inhomogeneously broadened when changing the incident polarization. In this paper, we present the polarization dependence of quantum beats from homogeneously broadened excitons in a high-quality GaN epitaxial layer. The observed /spl pi/-phase shift of quantum beats can be explained with the model for GaAs quantum wells proposed by Schmitt-Rink et al. However, it can neither explain the signal intensity dependence nor the differences between the two cases of homogeneously and inhomogeneously broadened transitions. We analyze our data on the basis of the model by Saiki et al. for the case of homogeneously broadened transitions and achieve a good qualitative agreement. Not only is the polarization dependence of the decay rate accurately modeled but also the intensity as a function of laser pulse polarization.