Using a mixed type-I/type-II GaAs/AlAs multiple-quantum-well sample, we have demonstrated an optically controllable and tunable terahertz (THz) filter. Long-lived electron–hole pairs in the quantum wells allow for efficient THz attenuation over a large THz spot size (2 mm) for extremely low optical cw power. This sample can also be used as an optically tunable THz phase shifter. The optically induced change of the GaAs quantum wells from a dielectric to a conducting material leads to the observed attenuation and the shifting of the THz wave forms.

The temperature dependent exciton dynamics of J-aggregates formed by a perylene bisimide dye is investigated down to liquid nitrogen temperature (77 K) by femtosecond pump-probe spectroscopy. The analysis of the transient absorption data using a diffusion model for the excitons does not only reveal an overall decrease of the exciton mobility, but also a change in the dimensionality of the exciton transport at low temperatures. This change in dimensionality is further investigated by kinetic Monte Carlo simulations, identifying weakly interlinked one-dimensional aggregate chains as the most likely structure at low temperatures. This causes the exciton transport to be highly anisotropic.

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370 / CLE0'99 / THURSDAY MORNING cations of the phenomena; mapping of supercurrent distribution and magnetic-flux trap memory.(a) Two-dimensional mapping of supercurrent distributionSince the amplitude of the THz radiation excited by the fs laser pulses is proportional to the supercurrent density at the laser spot, the two-dimensional supercurrent can be obtained from the THz radiation intensity by scanning the excitation laser beam. Figure 1 shows the distribution of the THz amplitude (root of the intensity) near the bridge of the bow-tie antenna type device under a bias current of 100 mA. It is seen that the supercurrent flows near the edge of the bridge. Until now, the supercurrent distribution is obtained indirectly from the magnetic field measured by a magneto-optical film or Hall sensor. The present method provides a new direct noncontact method for measuring the supercurrent distribution.(b) Magnetic-flux trap memory We found that the THz radiation is emitted into free space from the YBCO films subjected to an external magnetic field and magneticflux trapped state without a bias current. Here, we propose and demonstrate a new type of superconducting optical flux-trap memory using the THz radiation from magnetic-flux trapped states. Figure 2(a) shows the structure of the memory cell and Fig. 2(b) shows the change of the waveform with the bias current and the laser spot position. The cell has a hole in the center of the bridge to trap the magnetic flux. The direction of the magnetic flux trapped in the hole can be changed by the combination of the bias current direction and the laser spot position. The polarity ofthe THz radiation (read-out signal) reflects the direction of the magnetic flux. By integrating this memory cell two-dimensionally, it is possible to make a new type of superconducting optical flux-trap memory.