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...

Dielectric laser acceleration of electrons close to a fused-silica grating has recently been observed [Peralta et al., Nature 503, 91 (2013); Breuer, Hommelhoff, PRL 111, 134803 (2013)]. Here we present the theoretical description of the near-fields close to such a grating that can be utilized to accelerate non-relativistic electrons. We also show simulation results of electrons interacting with such fields in a single and double grating structure geometry and discuss dephasing effects that have to be taken into account when designing a photonic-structure-based accelerator for non-relativistic electrons. We further model the space charge effect using the paraxial ray equation and discuss the resulting expected peak currents for various parameter sets.

Recently, our group has demonstrated dielectric laser acceleration of nonrelativistic electrons at a scalable fused silica grating [J. Breuer and P. Hommelhoff, Phys. Rev. Lett. 111, 134803 (2013)]. This represents a demonstration of the inverse Smith-Purcell effect in the optical regime. The third spatial harmonic of the grating, which is excited by Titanium:sapphire laser pulses, synchronously accelerates 28 keV electrons derived from an electron microscope column. We observe a maximum acceleration gradient of 25 MeV=m. Here we present the experimental setup in detail. We describe grating-related issues such as surface charging and alignment as well as damage threshold measurements. A detailed explanation of the detection scheme is given. Furthermore, extensive numerical simulations are discussed, which agree well with the experimental results.

The enthalpy of formation of the ordered B2 phases in the Fe-Al and Fe-Ni-Al systems was measured with very good accuracy using a special, laboratory-built differential-solution calorimeter. The measurements were performed at 1073 K as a function of composition, with an accuracy of about 1 pct. The enthalpy of formation of B2-FeAl is most negative for the composition Fe 0.50 Al 0.50 (Ϫ36.29 kJ/ mol). Compounds with Al contents less than about 40 at. pct show a deviation from the linear dependence of the enthalpy of formation with composition which prevails for Al contents larger than 40 at. pct. Upon replacing Fe by Ni while maintaining a constant Al content, the enthalpy of formation of B2-(Fe,Ni)Al compounds becomes more negative. With decreasing Al content and for a constant Fe/Ni ratio, the enthalpy of formation of the ternary phase becomes less negative.

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Applying the Bragg-Williams approximation, a pair interaction model has been developed for describing the composition and temperature dependence of thermodynamic properties of ternary intermetallic B2 phases. Taking the activities and the integral enthalpies of formation as input data, values for the enthalpy and the entropy of bonds between atoms have been obtained. An analytical expression has been derived for the dependence of the concentration of point defects (vacancies and antistructure atoms) on composition and temperature. The model has been applied successfully to the available experimental data for the Ni-Al, Co-Al, and Ni-Co-Al systems. It has been shown that the values calculated accordingly for the effective enthalpy (and entropy) of formation of vacancies agree well with those from experimental observations and theoretical ab initio calculations.

Acceleration of charged particles exploiting the large optical field strength of short laser pulses and the proximity of a dielectric structure has been envisioned to revolutionize particle accelerators [1,2]. Direct acceleration by the optical carrier field of the laser can take place in the vicinity of a grating, also known as the inverse Smith-Purcell effect [3], which has been observed at a metal grating with a terahertz radiation source, however, the acceleration gradient was small (keV/m) [4]. Dielectrics allow much larger acceleration gradients and hence much smaller accelerators due to their orders of magnitude higher damage threshold in the optical regime as compared to metals. Using dielectric gratings as an optical accelerator has been proposed by Plettner et al. [5].We observe direct laser acceleration of non-relativistic 28 keV electrons close to a single fused-silica transmission grating that is illuminated by Titanium:sapphire laser pulses from below (see Fig. 1a-c). Our findings represent the first demonstration of realistically scalable laser acceleration and of the inverse Smith-Purcell effect in the optical regime. The observed maximum acceleration gradient of 25 MeV/m (see Fig. 1d) is already comparable to state-of-the-art linear accelerators operating with radio-frequency fields. Our work represents the decisive step towards an all-optical linear accelerator that may allow building table-top free electron lasers [6] and other electron optical devices.

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