The enormous size and cost of current state-of-the-art accelerators based on conventional radio-frequency technology has spawned great interest in the development of new acceleration concepts that are more compact and economical. Micro-fabricated dielectric laser accelerators (DLAs) are an attractive approach, because such dielectric microstructures can support accelerating fields one to two orders of magnitude higher than can radio-frequency cavity-based accelerators. DLAs use commercial lasers as a power source, which are smaller and less expensive than the radio-frequency klystrons that power today's accelerators. In addition, DLAs are fabricated via low-cost, lithographic techniques that can be used for mass production. However, despite several DLA structures having been proposed recently, no successful demonstration of acceleration in these structures has so far been shown. Here we report high-gradient (beyond 250 MeV m(-1)) acceleration of electrons in a DLA. Relativistic (60-MeV) electrons are energy-modulated over 563 ± 104 optical periods of a fused silica grating structure, powered by a 800-nm-wavelength mode-locked Ti:sapphire laser. The observed results are in agreement with analytical models and electrodynamic simulations. By comparison, conventional modern linear accelerators operate at gradients of 10-30 MeV m(-1), and the first linear radio-frequency cavity accelerator was ten radio-frequency periods (one metre) long with a gradient of approximately 1.6 MeV m(-1) (ref. 5). Our results set the stage for the development of future multi-staged DLA devices composed of integrated on-chip systems. This would enable compact table-top accelerators on the MeV-GeV (10(6)-10(9) eV) scale for security scanners and medical therapy, university-scale X-ray light sources for biological and materials research, and portable medical imaging devices, and would substantially reduce the size and cost of a future collider on the multi-TeV (10(12) eV) scale.

The use of infrared lasers to power optical-scale lithographically fabricated particle accelerators is a developing area of research that has garnered increasing interest in recent years. We review the physics and technology of this approach, which we refer to as dielectric laser acceleration (DLA). In the DLA scheme operating at typical laser pulse lengths of 0.1 to 1 ps, the laser damage fluences for robust dielectric materials correspond to peak surface electric fields in the GV/m regime. The corresponding accelerating field enhancement represents a potential reduction in active length of the accelerator between 1 and 2 orders of magnitude. Power sources for DLA-based accelerators (lasers) are less costly than microwave sources (klystrons) for equivalent average power levels due to wider availability and private sector investment. Due to the high laser-to-particle coupling efficiency, required pulse energies are consistent with tabletop microJoule class lasers. Combined with the very high (MHz) repetition rates these lasers can provide, the DLA approach appears promising for a variety of applications, including future high energy physics colliders, compact light sources, and portable medical scanners and radiative therapy machines.

We report the first experimental demonstration of the echo-enabled harmonic generation (EEHG) technique which holds great promise for generation of high power, fully coherent short-wavelength radiation. In this experiment, coherent radiation at the 3rd and 4th harmonic of the second seed laser is generated from the so-called beam echo effect. The experiment confirms the physics behind this technique and paves the way for applying the EEHG technique for seeded x-ray free electron lasers.PACS numbers: 41.60.CrFree electron lasers (FELs) can provide high-intensity coherent short-wavelength radiation which is essential for the studies of molecular and atomic dynamics. In the xray wavelength, the two leading concepts are the selfamplified spontaneous emission (SASE) configuration [1,2] and the high-gain harmonic generation (HGHG) scheme [3,4]. One FEL working in the SASE mode has been successfully operated at hard x-ray wavelengths [5]. While the radiation from a SASE FEL has excellent transverse coherence, it typically has rather limited temporal coherence because a SASE FEL starts from electron beam shot noise. Alternatively, the HGHG scheme allows the generation of fully coherent radiation by upconverting the frequency of a high-power seed laser. However, due to the relatively low up-frequency conversion efficiency, multiple stages of HGHG FELs are needed [6] in order to generate coherent x-rays from a UV laser.The up-frequency conversion efficiency can be greatly improved with the recently proposed echo-enabled harmonic generation (EEHG) technique [7,8]. In the EEHG scheme, the beam is energy modulated by a laser with wave number k 1 in the first modulator and then sent through a chicane with strong dispersion after which the modulation obtained in the first modulator is macroscopically smeared. Simultaneously, complicated fine structures are introduced into the phase space of the beam. A second laser with wave number k 2 is used to further modulate the beam energy in the second modulator. After passing through a second chicane the echo signal then occurs at the wave number k E = nk 1 + mk 2 as a recoherence effect, where n and m are integers. The main advantage of EEHG is that the bunching factor is a very slowly decaying function of the harmonic number, thus allowing the generation of coherent soft x-ray radiation directly from a UV seed laser in a single stage.The remarkable up-frequency conversion efficiency of the EEHG technique has stimulated world-wide interest in using EEHG to achieve fully coherent radiation in the x-ray wavelength from UV seed lasers [9][10][11]. While significantly relaxing the requirements on laser power and beam slice energy spread as compared to the HGHG scheme, EEHG requires more challenging control of the beam dynamics as the beam goes through the undulators and chicanes, because it involves a long-term memory of the beam phase space correlations.In this Letter we report the first experimental demonstration of the EEHG technique at the Next Linear Collider Test Accelerator (NLCTA)...

Acceleration of electrons using laser-driven dielectric microstructures is a promising technology for the miniaturization of particle accelerators. Achieving the desired GV m^-1 accelerating gradients is possible only with laser pulse durations shorter than ∼1  ps. In this Letter, we present, to the best of our knowledge, the first demonstration of acceleration of relativistic electrons at a dielectric microstructure driven by femtosecond duration laser pulses. Using this technique, an electron accelerating gradient of 690±100  MV m^-1 was measured-a record for dielectric laser accelerators.

Echo-enabled harmonic generation (EEHG) free electron lasers (FELs) hold great promise in generation of fully coherent radiation in x-ray wavelengths. Here we report the first evidence of high harmonics from the EEHG technique in the realistic scenario where the laser energy modulation is comparable to the beam slice energy spread. In this experiment, coherent radiation at the 7th harmonic of the second seed laser is generated when the energy modulation amplitude is about 2 ∼ 3 times the slice energy spread. The experiment confirms the underlying physics of EEHG and may have a strong impact on emerging seeded x-ray FELs that are capable of generating laser-like x-rays which will advance many areas of science.PACS numbers: 41.60.CrFree electron lasers (FELs) can provide tunable highpower coherent radiation which is enabling forefront science in various areas. At x-ray wavelengths, most of the FELs operate in the self-amplified spontaneous emission (SASE) mode [1,2]. One FEL working in the SASE mode has been operated at hard x-ray wavelengths [3], which marks the beginning of a new era of x-ray science [4][5][6]. However, since SASE FEL radiation starts from beam shot noise, the FEL output has limited temporal coherence (i.e. noisy in both temporal profile and spectrum). FELs with improved temporal coherence (i.e. a well-controlled pulse shape and a bandwidth close to transform limit) should benefit many applications and enable new science in spectroscopic studies of correlated electron materials.Various techniques [7][8][9][10][11][12][13][14] have been proposed to improve the FEL temporal coherence. In the self-seeding scheme, a monochromator is used to purify the spectrum of a SASE FEL and an additional undulator is employed to amplify the purified radiation to the GW level. However, suffering from the intrinsic chaotic properties of the SASE radiation in the first undulator, the monochromated radiation has large intensity fluctuations which may affect the stability of the final output. Furthermore, it is difficult to synchronize the FEL output from the self-seeding scheme to external lasers, which is generally required in pump-probe experiments. Alternatively, seeding with an external source generated from an external laser may provide a fully coherent output having well-defined timing with respect to the laser. One way to directly seed an FEL is to use the high harmonic generation (HHG) source generated when a high power laser is injected to a noble gas. While seeding at 160 nm [8] and 61 nm [9] from a HHG source have been demonstrated, seeding with HHG source in the x-ray wavelength still requires major progress in laser technology.To circumvent the need for a high power laser at short wavelength, several frequency up-conversion techniques [10][11][12][13][14] have been envisioned to convert the external seed to shorter wavelengths. In the classic high-gain harmonic generation (HGHG) scheme, a single modulatorchicane system is used to bunch the beam at the harmonic frequency of the seed laser [10]. While bei...