A Single-beam, Ponderomotive-optical Trap for Energetic Free Electrons

60 LLE Review, Volume 70 Traditionally, there have been many advantages to using laser beams with Gaussian spatial profiles in the study of high-field atomic physics. High peak intensities are achieved due to their focusability, they are easily generated in most laser systems, and their focal region can be described analytically. However, free electrons interacting with such a field are rapidly accelerated out of the high-intensity region via the ponderomotive force (which is proportional to the gradient of the laser intensity).1,2 As a result, the electrons’ interaction time with the intense portions of the laser focus can be much shorter than the pulse duration. In order to observe harmonic generation from oscillating free electrons, one must first control the expulsion of electrons from the focal region.3 This confinement can be most easily achieved by creating an intensity minimum at the focus, thereby using the ponderomotive force to push the electrons toward the central minimum. If this intensity minimum is non-zero, then the electrons can interact with intense fields while remaining trapped. Such ponderomotive trapping has been proposed in the past,4,5 and specific laser-based schemes to trap electrons in the radial direction have been described.6,7 To our knowledge, such focal spots have not been generated with a high-power laser. In addition, these proposed traps would not confine electrons in the axial (laser propagation) direction. In this article we will present a novel scheme to trap electrons in three dimensions with a single laser beam.