Laser-Based Acceleration of Nonrelativistic Electrons at a Dielectric Structure

Abstract

A proof-of-principle experiment demonstrating dielectric laser acceleration of non-relativistic electrons in the vicinity of a fused-silica grating is reported. The grating structure is utilized to generate an electromagnetic surface wave that travels synchronously with and efficiently imparts momentum on 28 keV electrons. We observe a maximum acceleration gradient of 25 MeV/m. We investigate in detail the parameter dependencies and find excellent agreement with numerical simulations. With the availability of compact and efficient fiber laser technology, these findings may pave the way towards an all-optical compact particle accelerator. This work also represents the demonstration of the inverse Smith-Purcell effect in the optical regime.The acceleration gradients of linear accelerators are limited by breakdown phenomena at the accelerating structures under the influence of large surface fields. Today's accelerators, which are based on metal structures driven by radio frequency fields, operate at acceleration gradients of ∼20-50 MeV/m. The upper limit in future radio frequency accelerators, such as the discussed CLIC and ILC, is ∼100 MeV/m, given by the damage threshold of the metal surfaces [1][2][3]. At optical frequencies dielectric materials withstand roughly two orders of magnitude larger field amplitudes than metals [4]. Together with the large optical field strength attainable with short laser pulses, dielectric laser accelerators (DLAs) hence may support acceleration gradients in the multi-GeV/m range [5]. With this technology lab-size accelerators, providing particle beams with energies currently only available at km-long facilities, seem feasible. Here we demonstrate the efficacy of the concept.Charged particle acceleration with oscillating fields requires an electromagnetic wave with a phase speed equal to the particle's velocity and an electric field component parallel to the particle's trajectory. So far, laserbased particle acceleration schemes employ the longitudinal electric field component of a plasma wave [6][7][8] or of a tightly focused laser beam [9], but in both schemes the accelerating mode has a phase velocity that does not match the speed of light. Therefore, relativistic particles can only be accelerated over short distances and the maximum attainable energies of these devices are limited. Exploiting the near-field of periodic structures, for example of optical gratings, offers the possibility to continuously accelerate non-relativistic as well as relativistic particles. In essence, the effect of the grating is to rectify the oscillating field in the frame co-moving with the electron, conceptually similar to conventional radio frequency devices. Single gratings can only be used to accelerate non-relativistic electrons, an effect also known as the inverse Smith-Purcell effect [10][11][12]. However, double grating structures, in which electrons propagate in a channel between two gratings facing each other, support a longitudinal, accelerating speed-of-light eigenmode that can be used to a...

Recent experimental results and future directions of the DLA single grating project

“…From a series of measurements, this signal was deduced to be an acceleration signal. A maximum acceleration gradient of 25 MeV/m was observed [1], matching well with simulations [2].…”

“…The laser has a central wavelength of 787 nm, a pulse length of 110 fs, a spot size of 9 µm x 9 µm, and a pulse energy of 150 nJ and a repetition rate of 2.7 MHz. The fused silica grating itself is fabricated by a combination of UV lithography and reactive ion etching [1] and is 25 µm long in the direction of electron propagation.…”

“…Acceleration was recently observed in this experiment led by John Breuer in Garching. Although described fully elsewhere [1], the setup and results of this experiment are now briefly summarized.…”

Conceptual layout for a wafer-scale dielectric laser accelerator

“…1, properly phased to match the particle arrival time and to counteract phase slippage in each section. Recent experiments have demonstrated acceleration in DLA devices in both the sub-relativistic and ultra-relativistic regimes with measured gradients of 25 MV/m and 300 MV/m respectively [8,1] . With further optimization of the structure design and materials, gradients exceeding 1 GV/m should be achievable.…”

Towards graphene plasmon-based free-electron infrared to X-ray sources

“…These qualities, coupled with the high confinement factors -and hence small spatial periodicity -of GPs, make graphene a promising platform upon which to realize chip-scale light sources with low-energy electrons, circumventing the use of additional acceleration stages. Interestingly, recent work has already demonstrated large gradient on-chip acceleration [35,45,46], which if successfully scaled up, could create highly-compact few-MeV accelerators, thus making even the hard X-ray graphene-based source accessible on chip. The GP-based free-electron radiation source offers a rich field for further exploration.…”