Zoltán Tibai

Organic crystals offer promising potential for THz generation, but face limitations in wavelength tunability and damage threshold. By applying tilted pulse‐front pumping to organic crystals an additional degree of freedom is introduced into the pumping conditions enabling a wider range of pumping wavelengths without compromising phase matching. Additionally, the lifespan of organic materials can be extended by using longer pumping wavelength and eliminate lower‐order multi‐photon absorption, allowing for higher pumping intensity without significant free‐carrier absorption, thus increasing the damage threshold. Simulations predict significant improvement for four out of six investigated crystals when tilted pulse‐front pumping is applied. By using volume phase holographic grating one can achieve pulse‐front tilt in organic crystals in collinear geometry with high diffraction efficiency. Design parameters are also presented.

We report an unusual regime of emittance self-compensation in an electron bunch generated in the blow-out mode by a radio-frequency photocathode gun. This regime is observed for a strong space-charge field on the cathode reaching around 30%–35% of the accelerating field. Simulations clearly show an initial growth and a subsequent self-compensation of projected emittance in a divergent electron bunch originating from the effects of: (a) strong space-charge forces of mirror charges on the cathode, (b) an energy chirp in the bunch and (c) substantial re-shaping of the electron bunch. Furthermore, we show analytically and numerically how a complex interplay between these effects leads to emittance self-compensation at the gun exit—the effect that is normally observed only in the presence of focusing fields.

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.

We have developed a waveguide structure for electron acceleration using a few µJ energy THz pulse. The metallic device focuses the incoming linearly polarized nearly single-cycle THz pulse, hence increasing the peak electric field strength. We experimentally verified the gain and the temporal profile of the electric field in the structure using electro-optic sampling technique. The acceleration of the electron bunch from rest up to 8 keV was predicted using single-cycle THz pulses with µJ-energy level.

We report an unusual regime of emittance self-compensation in an electron bunch generated in blow-out mode by a radio-frequency photocathode gun. Simulations clearly show an initial growth and a subsequent self-compensation of projected emittance in a divergent electron bunch originating from the effects of: (i) strong space-charge forces of mirror charges on the cathode, (ii) an energy chirp in the bunch and (iii) substantial re-shaping of the electron bunch. Furthermore, we show analytically and numerically how a complex interplay between these effects leads to emittance self-compensation in free space -- the effect that is normally observed only in a focusing magnetic field.

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.

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.