The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

We present the demonstration of high-gradient laser acceleration and deflection of electrons with silicon dual-pillar grating structures using both evanescent inverse Smith-Purcell modes and coupled modes. Our devices accelerate subrelativistic 86.5 and 96.3 keV electrons by 2.05 keV over 5.6 μm distance for accelerating gradients of 370 MeV/m with a 3 nJ mode-locked Ti:sapphire laser. We also show that dual pillars can produce uniform accelerating gradients with a coupled-mode field profile. These results represent a significant step toward making practical dielectric laser accelerators for ultrafast, medical, and high-energy applications.

We propose an on-chip optical power delivery system for dielectric laser accelerators based on a fractal 'tree-branch' dielectric waveguide network. This system replaces experimentally demanding free-space manipulations of the driving laser beam with chip-integrated techniques based on precise nano-fabrication, enabling access to orders of magnitude increases in the interaction length and total energy gain for these miniature accelerators. Based on computational modeling, in the relativistic regime, our laser delivery system is estimated to provide 21 keV of energy gain over an acceleration length of 192 µm with a single laser input, corresponding to a 108 MV/m acceleration gradient. The system may achieve 1 MeV of energy gain over a distance less than 1 cm by sequentially illuminating 49 identical structures. These findings are verified by detailed numerical simulation and modeling of the subcomponents and we provide a discussion of the main constraints, challenges, and relevant parameters in regards to on-chip laser coupling for dielectric laser accelerators.

The widespread use of high energy particle beams in basic research 1-3 , medicine 4,5 and coherent Xray generation 6 coupled with the large size of modern radio frequency (RF) accelerator devices and facilities has motivated a strong need for alternative accelerators operating in regimes outside of RF. Working at optical frequencies, dielectric laser accelerators (DLAs) -transparent laser-driven nanoscale dielectric structures whose near fields can synchronously accelerate charged particleshave demonstrated high-gradient acceleration with a variety of laser wavelengths, materials, and electron beam parameters 7-11 , potentially enabling miniaturized accelerators and table-top coherent x-ray sources 9,12 . To realize a useful (i.e. scalable) DLA, crucial developments have remained: concatenation of components including sustained phase synchronicity to reach arbitrary final energies as well as deflection and focusing elements to keep the beam well collimated along the design axis. Here, all of these elements are demonstrated with a subrelativistic electron beam. In particular, by creating two interaction regions via illumination of a nanograting with two spatio-temporally separated pulsed laser beams, we demonstrate a phase-controlled doubling of electron energy gain from 0.7 to 1.4 keV (2.5% to 5% of the initial beam energy) and through use of a chirped grating geometry, we overcome the dephasing limit of 25 keV electrons, increasing their energy gains to a laser power limited 10% of their initial energy. Further, optically-driven transverse focusing of the electron beam with focal lengths below 200 μm is achieved via a parabolic grating geometry. These results lay the cornerstone for future miniaturized phase synchronous vacuum-structure-based accelerators. DLAs are enticing insofar as they can provide high energy particle beams using the well-established principle of phase-synchronous acceleration in vacuum 1,2,13,14 , but with a smaller footprint, higher acceleration gradient and beam properties distinct from those available via microwave acceleration 9 . In dielectric laser acceleration, electrons traverse nanostructured dielectric geometries, gaining energy via interaction with laser-induced accelerating fields 9,15-19 . These fields are generated by imprinting a periodic spatial modulation to the perpendicularly incident laser wavefront that matches the periodicity of the structure and leads to optical near-field modes travelling along the structure surface (see Figure 1). Electrons with a velocity matching the phase velocity of one of the surface modes are accelerated if injected at an appropriate phase. Notably, DLAs are based on a vacuum scheme, similar to RF accelerators, and the imparted energy gain scales linearly with the incident optical field strength, presenting clear advantages over nonlinear acceleration schemes requiring matter 20-21 . Due to the linear interaction of the laser-induced fields with the accelerated electrons, the imparted energy gain can be extended by adding sequential interactio...

We present the demonstration of phase-dependent laser acceleration and deflection of electrons using a symmetrically driven silicon dual pillar grating structure. We show that exciting an evanescent inverse Smith-Purcell mode on each side of a dual pillar grating can produce hyperbolic cosine acceleration and hyperbolic sine deflection modes, depending on the relative excitation phase of each side. Our devices accelerate sub-relativistic 99.0 keV kinetic energy electrons by 3.0 keV over a 15 μm distance with accelerating gradients of 200 MeV/m with 40 nJ, 300 fs, 1940 nm pulses from an optical parametric amplifier. These results represent a significant step towards making practical dielectric laser accelerators for ultrafast, medical, and high-energy applications.