Particle acceleration on a chip

Researchers are working on various fronts to develop compact, inexpensive particle accelerators for medical imaging, security scanning, and more. One promising approach is dielectric laser acceleration (DLA), in which a laser field drives acceleration across a channel etched in a dielectric material. Now, in an experiment conducted by a team including Joel England (SLAC), Robert Byer (Stanford University), and Byer’s graduate students Edgar Peralta and Ken Soong, DLA has successfully accelerated relativistic (60 MeV) electrons. The figure illustrates the group’s fused silica structure that, at full-size, can fit on a fingertip. Here’s how it works. An 800-nm polarized laser field propagates through the device from top to bottom in the plane of the figure. The arrows in the figure represent the electric field at a particular instant of time; the important point is that in the vertically extended channel regions, the field amplitude is relatively small. The device’s repetitions match the 800-nm laser wavelength. Thus, if a relativistic electron traversing the channel encounters an accelerating field at the starred location, it will encounter another accelerating field one period length and one laser oscillation later. On the way, it passes through a relatively weak decelerating region; on balance, the electron gains energy. In the actual experiment by England and company, some initially relativistic electrons were further accelerated while others were decelerated. But a sizable fraction received an energy boost of 60 keV or more. A team led by Peter Hommelhoff has recently reported that DLA can also accelerate nonrelativistic electrons. (E. A. Peralta et al., Nature 503, 91, 2013, doi:10.1038/nature12664 ; J. Breuer, P. Hommelhoff, Phys. Rev. Lett. 111, 134803, 2013, 10.1103/PhysRevLett.111.134803 .)