Research at modern light sources continues to improve our knowledge of the natural world, from the subtle workings of life to matter under extreme conditions. Free-electron lasers, for instance, have enabled the characterization of biomolecular structures with sub-ångström spatial resolution, and paved the way to controlling the molecular functions. On the other hand, attosecond temporal resolution is necessary to broaden our scope of the ultrafast world. Here we discuss attosecond pulse generation beyond present capabilities. Furthermore, we review three recently proposed methods of generating attosecond x-ray pulses. These novel methods exploit the coherent radiation of microbunched electrons in undulators and the tailoring of the emitted wavefronts. The computed pulse energy outperforms pre-existing technologies by three orders of magnitude. Specifically, our simulations of the proposed Soft X-ray Laser at MAX IV (Lund, Sweden) show that a pulse duration of 50–100 as and a pulse energy up to 5 J is feasible with the novel methods. In addition, the methods feature pulse shape control, enable the incorporation of orbital angular momentum, and can be used in combination with modern compact free-electron laser setups.

A THz-pulse-driven compact, < 150 mm in total length, two-stage electron accelerator setup was designed. It uses 2 × 2 pairs of nearly counter-propagating focused THz pulses. The effects of the initial bunch charge and the propagation direction of the THz pulses on the energy of the accelerated electrons were investigated by numerical simulations. Generation of 8 fC electron bunches with up to 340 keV energy; only 2.0% energy spread and compressed on-target duration of 200 fs is predicted using single-cycle low-frequency THz pulses with less than 4.5 mJ total energy.

A relativistic electron source is proposed, driven by the wakefield of an intense terahertz (THz) pulse in low-density gas plasma. In contrast to the optical and near-infrared regimes, the low (3.5 THz) frequency and the long ( λ _T = 85.6 μ m) wavelength of the THz pulse offers distinct advantages, such as the λ T 2 -scaling of the electron ponderomotive energy. Two-dimension-in-space and three-dimension-in-velocity particle-in-cell simulation results show that relativistic electrons of ∼1 MeV energy and high charge can be generated by an intense THz pulse at kilohertz repetition rate from a gas plasma target. These results may lead to a new regime of applications, such as ultrafast electron diffraction or high-repetition-rate gamma ray sources for materials characterization or medical radiography, which would benefit from lower energy (1–10 MeV) but higher repetition rate (∼1 kHz) sources of relativistic electrons.

In recent years several microstructured lithium niobate THz pulse source were suggested for high-energy applications. Two types of those, the reflective and the transmissive nonlinear slab are adopted here for semiconductors. These new sources are scalable both in THz energy and size. Furthermore, they can outperform the already demonstrated contact grating source in diffraction and THz generation efficiency. Compared to the lithium niobate sources, they are more feasible, thanks to the easier manufacturing and the longer pump wavelength. They can produce intense, nearly single-cycle THz pulses at higher frequencies. With 20 mJ pumping at 1.8 µm wavelength, 45 µJ THz energy, and 17 MV/cm focused peak electric field can be expected at 3 THz phase matching frequency from the transmissive nonlinear echelon slab setup consisting of a 4 mm thick structured plan-parallel gallium phosphide crystal.

Practical aspects of the robust method we proposed for producing few-cycle attosecond pulses with arbitrary waveform in the extreme ultraviolet spectral range are studied numerically. It is based on the undulator radiation of relativistic ultrathin electron layers produced by laser-driven energy modulation. By using realistic specifications, we show that isolated waveform-controlled extreme ultraviolet attosecond pulses at 5 nm with 10 nJ energy and 20 as pulse duration, at 20 nm with 90 nJ energy and 80 as pulse duration, and at 60 nm with 200 nJ energy and 240 as duration can be generated, respectively. PACS numbers: 41.60.Cr, 41.50.+h, 41.75.Ht