Ultrafast Processes and Spectroscopy with Free Electron Lasers

Abstract : The goal of this project is twofold: 1) to demonstrate that the SCA Free Electron Laser (FEL) is a useful tool for time-resolved spectroscopy; and 2) to use the FEL with other ultrashort pulse sources to uncover the electronic processes in disordered semiconductors. Potentially, the SCA FEL is a near-ideal source for ultrashort time-spectroscopy. It delivers widely tunable, psec pulses with high peak power and at high repetition rate. However, pulse-to-pulse variations in duration, wavelength and phase within one micropulse may be a limitation for high quality spectroscopy. In addition, pulses below one picosecond and tunability around the laser wavelength are desirable. Essential experiments will demonstrate how useful the SCA FEL is in ultrashort time resolved spectroscopy, including pulse compression and generation of white light continuum. The second goal is to uncover the electronic processes in disorder materials. Understanding these processes is the key to understanding the physics in materials such as amorphous semiconductors or liquid semiconductors. In turn, understanding the physics of these materials will lead to novel devices or improved device operation and will impact on our models for other materials. Picosecond and femtosecond time-resolved measurements are carried out on a Si:H and its alloys with Ge and C. The electronic properties of carriers in the vicinity of the mobility edge are emphasized.