Electron acceleration with laser light



In this project we are trying to control the motion of non-relativistic electrons with short laser pulses. In free space electromagnetic waves cannot accelerate or deflect particles efficiently. Because the fields travel faster than any massive particle, it just experiences a quiver motion and therefore no net momentum can be transferred. Accelerating physicists refer to this as Lawson-Woodward theorem [1]. There are a few ways to work around it, though. For example in conventional particle accelerators, superconducting RF-cavities produce standing waves which can be used for acceleration. We want to use evanescent optical modes close to a photonic structure in order to control non-relativistic electrons.

The concept is based on the periodic reversal of the electromagnetic fields close to a photonic grating structure. The basic idea was proposed by T. Plettner, R. L. Byer et al. [2], who we collaborate with. A linearly polarized laser pulse impinges on a transmission grating. The grating grooves periodically reverse the electromagnetic field. If the grating period is matched to the velocity of the electron, which passes closely by the grating surface, it always experiences an accelerating force. In the sketch above the electric field component of the laser is depicted as arrows (red: accelerating, blue: decelerating). The two sketches correspond to two snapshots in time half an optical cycle apart.



In our proof-of-principle experiment we want to accelerate 27 keV electrons with 70fs-Titanium:Sapphire laser pulses. From numerical simulations we expect accelerating gradients already as high as 26 MeV/m with our proof-of-concept structure. As an electron source we use the electron column of an electron microscope. After passing the grating, the energy of the electrons is analyzed in a home-built retarding-field spectrometer. Right now we have finished setting up the experiment and are hopeful to get our first results in very soon. On the picture you can see the experimental setup with the electron column on the right, the grating mount in the middle (with the blue arrow depicting the electron beam) and the spectrometer on the left. The red wave indicates a laser pulse.

[1] Palmer, Frontiers of Particle Beams, 1988, 607-635
[2] Plettner, Byer, Colby, Cowan, Sears, Spencer, Siemann, Phys. Rev. ST Accel. Beams 8, 121301 (2005)