Observation of electron trapping in an intense laser beam

Abstract

Since the discovery of the ponderomotive force over 40 years ago, it has been known that charged particles interacting with an oscillating electromagnetic field will seek regions of low intensity [1]. It was immediately proposed that with the appropriate field distribution, particles could be trapped with this force [2]. The case of electron confinement with a specially shaped laser beam has been discussed since then [3 – 5]. Recently we reported on the optical generation of a three-dimensional, ponderomotive-optical trap with a high-peak-power laser [6]. In this Letter we present the first evidence of electron trapping in a high-intensity laser field, with confinement of electrons with energies up to 10 keV. To our knowledge, this work represents the first controlled manipulation of electrons in a high-intensity laser field by the modulation of the spatial intensity distribution of the beam. This opens up a new direction of study in highintensity laser-electron interactions. Here, we present the effects of trapping on linear Thomson scattering. A trapping beam could also be used to enhance the recently observed nonlinear Thomson scattering [7]. While some further experiments may use the particular geometry described in this Letter, more generally, we have shown that near-field phase control of a high-power laser beam can lead to tailored focal regions which may be optimized for a myriad of experiments. Electrons interact with a laser field via the Lorentz force. For field distributions with a slowly varying temporal and spatial envelope, the motion of the electrons can be decomposed into a high-frequency quiver and a slower, “dark-seeking” drift [8]. The quiver motion is a direct result of the rapidly oscillating electromagnetic field, while the drift is a consequence of the ponderomotive force (the cycle-averaged Lorentz force). The ponderomotive force takes the form Fpond 2=Upond, where Upond e 2 Il 2 2pmc 3 (I is the intensity, l is