Accelerators on a chip: Dielectric laser accelerators
Particle accelerators are exciting research tools that provide energetic high brightness charged particle beams that can be used to probe physical phenomena otherwise inaccessible. However, the enormous cost and user demand of many of the radio-frequency based accelerators limits their availability. To provide an accelerator accessible on the university-lab scale, a novel accelerator design must be developed.
One such design, whose development has been recently funded by the Gordon and Betty Moore Foundation (press release), is the Dielectric Laser Accelerator or DLA for short. DLAs leverage both the GV/m electromagnetic fields of commercially available lasers and the advanced nanofabrication techniques of dielectric materials developed in the semiconductor industry. The large available electromagnetic fields are used to create acceleration gradients that exceed those in radio frequency accelerators by a factor of 100. Even though DLAs are orders of magnitude smaller than their RF brethren, their accelerating gradients allow for DLAs to impart similar energy gains to charged particles. However, instead of imparting these energy gains over meters, DLAs impart these energy gains over millimeters, potentially enabling a University Lab-scale high energy particle accelerator.
An example of a Dielectric Laser Accelerator, which sits on the thin strip in the center of the silicon piece shown here. The silicon piece is upon a cent for scale. (Image: FAU/Joshua McNeur)
A DLA imaged at high magnification, with a strand of hair for scale. (Image: FAU/Joshua McNeur)
The DLAs tested at FAU operate as follows. An incident laser pulse impinges upon a nanofabricated silicon grating, exciting travelling wave modes near the surface of the grating, shown below. Electrons, propagating above the surface of the grating, surf one of the excited modes when their velocity matches the phase velocity of the travelling wave. In this manner the electrons are accelerated (see video produced by our SLAC partners).
A laser impinges upon the silicon grating (with the structural period and grating height d indictated) from above, exciting the travelling wave mode depicted by the blue and red regions in the bottom image that shows a cross section of two periods of the accelerator. An electron, depicted by the green circle and travelling to the right, surfs the travelling wave, also moving to the right. (Image: FAU/Joshua McNeur)
Already there have been repeated confirmations that this principle of acceleration works over a wide range of electron velocities (from 15% to 100% of the speed of light) and lasers [1,2,3,4]. The AChIP project aims to extend the success of DLAs towards the realization of the analogy of an accelerator beamline. Multiple stages of dielectric-laser based acceleration, focusing, and diagnostics will be developed and tested. Additionally, a laser-triggered electron cathode appropriate for operation with DLAs will eventually be incorporated with the multiple stages, resulting in a beamline where electrons are generated via laser-triggered emission, and then alternatingly accelerated, collimated, and diagnosed with sequential DLA-based devices. A hypothetical schematic of such a device is shown below.
A DLA-based Linac, consisting of a single drive laser, a laser-triggered electron source (A), subrelativistic electron accelerating sections (B1-B3), focusing/collimating dielectric laser elements (F1-F3), a speed of light accelerator (C) and a dielectric-laser based undulator (U) capable of generating XUV light. (Image: FAU)
The resulting beam may be used as a high brightness light source via the incorporation of an element that wiggles the electron beam transverse to its direction of motion, creating photons as the beam alternatingly curves upwards and downwards. The compact size of such a beamline and its various components allows for many exciting applications, ranging from handheld MeV electron sources for tumor irradiation to table-top Free Electron Lasers .
As of November 2015, we are happy to announce commencement of an international collaboration aiming to reach these ambitious goals for DLAs. The AChIP (Accelerator on a Chip International Program) is headed by Stanford and our group at FAU and also consists of the following partners:
EPFL (L. Rivkin)
PSI (R. Ischebeck)
Hamburg University (F. Kaertner)
DESY (R. Assmann, I. Hartl)
TU Darmstadt (O. Boine-Frankenheim)
SLAC (J. England, S. Tantawi)
Stanford University (B. Byer, S. Fan, J. Harris, O. Solgaard, J. Vuckovic)
UCLA (P. Musumeci)
Purdue University (M. Qi)
Tech-X (B. Cowan)
The members of AChIP (Image: SLAC National Accelerator Laboratory)
 J. Breuer and P. Hommelhoff, “Laser-Based Acceleration of Nonrelativistic Electrons at a Dielectric Structure,” Physical Review Letters 111, 134803 (2013)
 E. A. Peralta, K. Soong, R. J. England, E. R. Colby, Z. Wu, B. Montazeri, C. McGuinness, J. McNeur, K. J. Leedle, Walz, E. B. Sozer, B. Cowan, B. Schwartz, G. Travish R. L. Byer, “Demonstration of Electron Acceleration in a Laser-Driven Dielectric Micro-Structure,” Nature 503, 7474 (2013).
 K. J. Leedle, A. Ceballos, H. Deng, O. Solgaard, R. Pease, R.L. Byer, J. Harris, “Dielectric Laser Acceleration of sub-100 keV Electrons with Silicon Dual Pillar grating Structures,” Optics Letters 40 18 (2015).
 J. McNeur, M. Kozak, D. Ehberger, N. Schönenberger, A. Tafel, A. Li, P. Hommelhoff, “A Miniaturized Electron Source Based on Dielectric Laser Accelerator Operation at Higher Spatial Harmonics and a Nanotip Photoemitter,” J. Phys. B, accepted.
 R. J. England, R. J. Noble, K. Bane, D.H. Dowell, C. Ng, J.E. Spencer, S. Tantawi, Z. Wu, R. L. Byer, E. Peralta, K. Soong, C. Chang, B. Montazeri, S.J. Wolf, B. Cowan, J. Dawson, W. Gai, P. Hommelhoff, Y. Huang, C. Jing, C. McGuiness, R. B. Palmer, B. Naranjo, J. Rosenzweig, G. Travish, A. Mizrahi, L. Schachter, C. Sears, G. R. Werner, R. B. Yoder. “Dielectric Laser Accelerators,” Rev. Mod. Phys. 86, 1337 (2014).
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