Subrelativistic Alternating Phase Focusing Dielectric Laser Accelerators.

We demonstrate a silicon-based electron accelerator that uses laser optical near fields to both accelerate and confine electrons over extended distances. Two dielectric laser accelerator (DLA) designs were tested, each consisting of two arrays of silicon pillars pumped symmetrically by pulse front tilted laser beams, designed for average acceleration gradients 35 and 50 MeV/m, respectively. The DLAs are designed to act as alternating phase focusing (APF) lattices, where electrons, depending on the electron-laser interaction phase, will alternate between opposing longitudinal and transverse focusing and defocusing forces. By incorporating fractional period drift sections that alter the synchronous phase between ±60° off crest, electrons captured in the designed acceleration bucket experience half the peak gradient as average gradient while also experiencing strong confinement forces that enable long interaction lengths. We demonstrate APF accelerators with interaction lengths up to 708 µm and energy gains up to 23.7±1.07 keV FWHM, a 25% increase from starting energy, demonstrating the ability to achieve substantial energy gains with subrelativistic DLA.

Three Dimensional Alternating-Phase Focusing for Dielectric-Laser Electron Accelerators.

The concept of dielectric-laser acceleration (DLA) provides the highest gradients among breakdown-limited (nonplasma) particle accelerators and thus the potential of miniaturization. The implementation of a fully scalable electron accelerator on a microchip by two-dimensional alternating phase focusing (APF), which relies on homogeneous laser fields and external magnetic focusing in the third direction, was recently proposed. In this Letter, we generalize the APF for DLA scheme to 3D, such that stable beam transport and acceleration is attained without any external equipment, while the structures can still be fabricated by entirely two-dimensional lithographic techniques. In the new scheme, we obtain significantly higher accelerating gradients at given incident laser field by additionally exploiting the new horizontal edge. This enables ultralow injection energies of about 2.5 keV (ß=0.1) and bulky high voltage equipment as used in previous DLA experiments can be omitted. DLAs have applications in ultrafast time-resolved electron microscopy and diffraction. Our findings are crucial for the miniaturization of the entire setup and pave the way towards integration of DLAs in optical fiber driven endoscopes, e.g., for medical purposes.

Laser-Driven Modulation of Electron Beams in a Dielectric Micro-Structure for X-Ray Free-Electron Lasers.

We describe an application of laser-driven modulation in a dielectric micro-structure for the electron beam in a free-electron laser (FEL). The energy modulation is transferred into longitudinal bunching via compression in a magnetic chicane before entering the undulator section of the FEL. The bunched electron beam comprises a series of enhanced current spikes separated by the wavelength of the modulating laser. For beam parameters of SwissFEL at a total bunch charge of 30 pC, the individual spikes are expected to be as short as 140 as (FWHM) with peak currents exceeding 4 kA. The proposed modulation scheme requires the electron beam to be focused into the micrometer scale aperture of the dielectric structure, which imposes strict emittance and charge limitations, but, due to the small interaction region, the scheme is expected to require ten times less laser power as compared to laser modulation in a wiggler magnet, which is the conventional approach to create a pulse train in FELs.

Laser-Driven Electron Lensing in Silicon Microstructures.

We demonstrate a laser-driven, tunable electron lens fabricated in monolithic silicon. The lens consists of an array of silicon pillars pumped symmetrically by two 300 fs, 1.95 µm wavelength, nJ-class laser pulses from an optical parametric amplifier. The optical near field of the pillar structure focuses electrons in the plane perpendicular to the pillar axes. With 100±10 MV/m incident laser fields, the lens focal length is measured to be 50±4 µm, which corresponds to an equivalent quadrupole focusing gradient B^{'} of 1.4±0.1 MT/m. By varying the incident laser field strength, the lens can be tuned from a 21±2 µm focal length (B^{'}>3.3 MT/m) to focal lengths on the centimeter scale.

Dielectric laser electron acceleration in a dual pillar grating with a distributed Bragg reflector.

We report on the efficacy of a novel design for dielectric laser accelerators by adding a distributed Bragg reflector (DBR) to a dual pillar grating accelerating structure. This mimics a double-sided laser illumination, resulting in an enhanced longitudinal electric field while reducing the deflecting transverse effects when compared to single-sided illumination. We improve the coupling efficiency of the incident electric field into the accelerating mode by 57%. The 12 µm long structures accelerate sub-relativistic 28 keV electrons with gradients of up to 200 MeV/m in theory and 133 MeV/m in practice. This Letter shows how lithographically produced nano-structures help to make novel laser accelerators more efficient.

Net Acceleration and Direct Measurement of Attosecond Electron Pulses in a Silicon Dielectric Laser Accelerator.

Net acceleration of attosecond-scale electron pulses is critical to the development of on-chip accelerators. We demonstrate a silicon-based laser-driven two-stage accelerator as an injector stage prototype for a Dielectric Laser Accelerator (DLA). The first stage converts a 57-keV (500±100)-fs (FWHM) electron pulse into a pulse train of 700±200 as (FWHM) microbunches. The second stage harnesses the tunability of dual-drive DLA to perform both a net acceleration and a streaking measurement. In the acceleration mode, the second stage increases the net energy of the electron pulse by 200 eV over 12.25 µm. In the deflection mode, the microbunch temporal profile is analyzed by a direct streaking measurement with 200 as resolution. This work provides a demonstration of a novel, on-chip method to access the attosecond regime, opening new paths towards attosecond science using DLA.

Generation and Characterization of Attosecond Microbunched Electron Pulse Trains via Dielectric Laser Acceleration.

Dielectric laser acceleration is a versatile scheme to accelerate and control electrons with the help of femtosecond laser pulses in nanophotonic structures. We demonstrate here the generation of a train of electron pulses with individual pulse durations as short as 270±80 attoseconds (FWHM), measured in an indirect fashion, based on two subsequent dielectric laser interaction regions connected by a free-space electron drift section, all on a single photonic chip. In the first interaction region (the modulator), an energy modulation is imprinted on the electron pulse. During free propagation, this energy modulation evolves into a charge density modulation, which we probe in the second interaction region (the analyzer). These results will lead to new ways of probing ultrafast dynamics in matter and are essential for future laser-based particle accelerators on a photonic chip.

Alternating-Phase Focusing for Dielectric-Laser Acceleration.

The concept of dielectric-laser acceleration provides the highest gradients among breakdown-limited (nonplasma) particle accelerators. However, stable beam transport and staging have not been shown experimentally yet. We present a scheme that confines the beam longitudinally and in one transverse direction. Confinement in the other direction is obtained by a single conventional quadrupole magnet. Within the small aperture of 420 nm we find the matched distributions, which allow an optimized injection into pure transport, bunching, and accelerating structures. The combination of these resembles the photonics analogue of the radio frequency quadrupole, but since our setup is entirely two dimensional, it can be manufactured on a microchip by lithographic techniques. This is a crucial step towards relativistic electrons in the MeV range from low-cost, handheld devices and connects the two fields of attosecond physics and accelerator physics.