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Physical Society Colloquium
Brian J. Keay
Department of Physics and Astronomy New Free Electron Laser (FEL) Centers have recently been built all over the world. The interest in FELs is that they offer a powerful source of continuously tunable coherent radiation in spectral regions where no other laser sources exist. This talk will focus on two different experiments, one performed at the UC Santa Barbara Center for Free Electron Laser Studies, and the other performed at Vanderbilt's W.M Keck Free Electron Laser Center. In the Santa Barbara experiments, photon-assisted tunneling was observed in semiconductor nanostructures for the first time. In addition, absolute negative conductance was observed in semiconductor superlattices, a process in which the electrical current flows against the applied dc bias in the presence of terahertz radiation. In the Vanderbilt experiment, the time-resolved infrared transmittance and reflectance of a melt induced by a 10 nsec optical-laser pulse was observed in an undoped crystalline GaAs wafer. Picosecond pulsed, 2.86 GHz repetition rate, infrared radiation from the Free-Electron Laser was used study the formation and propagation of the melt in real-time. The back- reflectance (probed from the side opposite to the incident optical radiation) displays interference oscillations as the melt propagates in the sample. The measurements are in agreement with model calculations which describe the melt with the Drude free-carrier model.
Wednesday, December 1st 1999, 15:30 |