High-flux table-top source for femtosecond hard X-ray pulses

Elementary processes in physics, chemistry, and biology are connected with changes of the atomic or molecular structure on a femtosecond time scale (1 femtosecond (fs) = 10^-15 seconds). Ultrafast X-ray methods hold strong potential for following structure changes in space and time and generate 'movies' of the motions of electrons, atoms and molecules. This perspective has resulted in a strong demand for femtosecond hard X-ray pulses to be applied in X-ray scattering and spectroscopy.

There are two main approaches to generate ultrashort hard X-ray pulses. The first are sources based on large-scale electron accelerators and undulators in which femtosecond electron bunches radiate bright X-ray pulses. The second are small-frame laboratory sources driven by intense femtosecond optical lasers. Here, electron acceleration occurs in the strong electric field of an optical pulse and X-ray pulses are generated by collisional interaction of such electrons with atoms of a metal target, similar to a conventional X-ray tube.

Researchers at the Max Born Institute (MBI) in Berlin have now accomplished a breakthrough in table-top generation of femtosecond X-ray pulses by demonstrating a stable pulse train at kilohertz repetition rate with a total flux of some 10^12 X-ray photons per second. As they report in Optics Letters, the combination of a novel optical driver providing femtosecond mid-infrared pulses around a 5 µm (5000 nm) wavelength with a metallic tape target in a transmission geometry allows for generating hard X-ray pulses at a wavelength of 0.154 nm with very high efficiency.

The optical driver is based on optical parametric chirped pulse amplification (OPCPA) and provides 80-fs pulses at a central wavelength of 5 µm with an energy of 3 mJ and a repetition rate of 1 kHz. To generate X-ray pulses, the mid-infrared pulses are tightly focused onto a thin copper target. In an optical cycle of the optical field, electrons are extracted from the copper tape, accelerated in vacuum and steered back to the target. Electrons with a kinetic energy of up to 100 keV reenter the target and generate bright copper Kα pulses at a wavelength of 0.154 nm, accompanied by spectrally broad bremsstrahlung. The longer optical cycle of the mid-infrared pulses compared to pulses at shorter optical wavelengths results in longer acceleration times of the electrons, higher kinetic energies, and eventually higher efficiency in X-ray generation.

The new table-top X-ray source reaches an average number of Cu-Kα photons up to 1.5x10^9 photons per pulse in the full solid angle or 1.5x10^12 photons per second. This photon flux is 30 times higher than from commonly used table-top X-ray sources driven by Ti:sapphire lasers at the central wavelength of 0.8 µm. Such source parameters open exciting perspectives for investigating ultrafast structure changes in condensed matter by time-resolved X-ray scattering.