A Single-Beam, Ponderomotive-Optical Trap for Energetic Free Electrons
In 1957, it was shown that charged particles interacting with an oscillating electromagnetic field will experience a force proportional to the gradient of the field intensity in the direction of decreasing intensity.1 This so-called ponderomotive force results in the rapid expulsion of free electrons from a high-intensity Gaussian laser focus.2 If, however, the gradient of the intensity is made to point inwards, this force could be used to confine rather than repel. This was known for the case of RF-fields in 1958,3 and was extended to laser fields in 1966.4 Since that time, laser peak intensities have seen a dramatic rise.5 This has led to the observation of a vast array of non-linear light-matter interactions, but the accompanying increase in the magnitude of the ponderomotive force has made the observation of some effects difficult. In this paper, we re-visit the idea of ponderomotive trapping, but with a specific goal in mind: the confinement of electrons in a high-field region, with the expectation of observing second-harmonic generation from their relativistic oscillations.6 We present a novel scheme to generate a single-beam, three-dimensional trap, along with simulated electron trajectories that demonstrate confinement in a high-field region. Also, we present images of the trapping beam generated with a high-power laser. It is important to note that there exist other schemes of laser-based trapping 7,8 These more complicated methods would trap electrons in only two dimensions, while our single-beam trap offers three dimensions of confinement. To our knowledge, this is the only single-beam, three-dimensional trap for energetic electrons to be proposed, and the only trap of any kind to be generated with a high-power laser.
KeywordsFocal Region Ponderomotive Force Phase Plate Trapping Region Outer Annulus
Unable to display preview. Download preview PDF.
- 3.V. Gaponov and M.A. Miller, J. Exptl. Theoret. Phys. (U.S.S.R.) 34: 242 (1958).Google Scholar