Physics of Free-Electron-Laser Applications in the Visible and Infrared.

It is now eighteen years since John Madey published a paper pointing out that a high-brightness relativistic electron beam traversing a spatially periodic magnetic field could stimulate the emission of photons over a broad range of wavelengths, indeed, from the far infrared to the ultraviolet. In a way, the free-electron laser was the ultimate homage paid by the laser, viewed as an optical device, to its antecedents in radar and electron-beam science and technology dating back into the 1940s. In the intervening years, successful infrared and visible free-electron-laser FEL experiments, for example, at Stanford, Orsay, Santa Barbara, and Los Alamos, have shown significant promise for applications based on the unique optical characteristics of the FEL. A variety of accelerators can provide the high-brightness electron beams necessary for the FEL room-temperature pulsed linear accelerators, superconducting accelerators, storage rings, and Van de Graaff generators have all been successfully used so far for this purpose. The existence of this variegated collection of pumps for the stimulated emission generated in the FEL implies a correspondingly broad range of temporal pulse shapes, interpulse spacings, pulse-repetition frequencies, output powers, and spectral ranges for users. With the increasing maturity of the free-electron laser comes a new phase of scientific opportunity for those who are primarily laser users rather than laser physicists. During the past two years, FEL users facilities at Stanford University and the University of California at Santa Barbara began to provide significant quantities of time to photon users, particularly in surface and materials science and bio-medical studies.