The longer the better: Long-wave light pulses produce brilliant ultra-short, hard X-ray flashes

Scientists from the Max Born Institute and the Vienna University of Technology present a new, compact laboratory source for ultra-short, hard X-ray pulses with unprecedented photon flux.

X-rays are a key tool for mapping the structure of materials and analyzing their composition - at the physician, in the chemistry lab, and in the materials sciences. If one radiates so-called hard X-radiation, which has a wavelength comparable to the distance between atoms, to a material, one can determine the spatial arrangement of the atoms from the pattern of the scattered X-radiation. This standard method has so far deciphered equilibrium structures of increasing complexity, ranging from simple inorganic crystals to highly complex biomolecules such as the DNA or large protein molecules.

Fig. 1 Left: Generation of X-rays in a conventional X-ray tube. Electrons are emitted from the heated cathode (-) and then accelerated to the anode (+) by a constant electric field. Within the metal anode (e.g., copper), inelastic collisions of the electrons with the metal atoms result in the generation of both characteristic line emission of X-rays (narrow lines in the lower spectrum) and bremsstrahlung. Right: femtosecond light pulses in the middle infrared (λ = 3900nm) from an OPCPA system are focused on a copper band target. The electrons are first pulled out of the surface by the strong electric field of the light pulse, then accelerated in a vacuum and finally shot back into the copper strip. During deceleration in the metal band, the energetic electrons produce characteristic line emission and Bremsstrahlung, which is measured by an X-ray detector.

Today, many scientists are anxious to watch the atoms "working", i. they want to directly observe the movement of the atoms in a vibration, chemical reaction or material modification. Atomic motions typically occur in a time range of femtoseconds (1 femtosecond = 10-15 seconds). Therefore, one needs for such an "X-ray film" an extremely short exposure time with correspondingly short X-ray flashes. There are two complementary approaches worldwide to produce ultra-short, hard X-ray pulses. On the one hand there are the large machines, e.g. Electron accelerators such as the Free Electron Laser (FEL) in Stanford USA (LCLS at SLAC) or at SACLA in Japan. On the other hand, compact laboratory sources can be built for ultrashort, hard x-ray pulses driven by femtosecond laser systems. Although the X-ray flux from the accelerator sources is significantly higher than in the laboratory sources, the latter have been found to be suitable cameras for the femtosecond "X-ray films". The quality of such films is ultimately determined by the number of X-ray photons scattered from the sample being examined. A joint team from the Max Born Institute (MBI) in Berlin and the Vienna University of Technology has now achieved a breakthrough for compact laboratory sources and increased the flux of hard X-rays by a factor of 25. In the latest issue of the journal Nature Photonics. describe a combination of a new optical driver laser providing femtosecond light pulses at wavelengths around 4000 nm (4 μm) in the mid-infrared, and a metal band target in a vacuum chamber containing ultrashort pulses of hard X-radiation at a wavelength of 0.154 nanometers Generated unprecedented efficiency.

Fig. 2 Analogy of electron acceleration in vacuum with the acceleration of a diver in the gravity of the earth as it jumps from different platforms of the tower. Longer wavelengths correspond to longer oscillation periods of the optical field and lead to a time-prolonged acceleration of the electrons in a vacuum. The time interval Δt between leaving the platform and immersion in the water surface increases with the jump height and the kinetic energy of the jumper on the water surface is proportional to Δt2. Similarly, electrons in a longer acceleration phase Δt also get a significantly higher kinetic energy before they hit the metal band. This in turn leads to a more efficient generation of X-rays.

X-ray generation is performed in 3 steps (Fig. 1), (i) electron extraction from the metal band by the electric field of the driving light pulse, (ii) acceleration of the electrons in vacuum through the strong optical field and return to the band with a tremendous gain and kinetic energy, and (iii) the generation of X-ray flashes by inelastic collisions of the energetic electrons with the metal atoms in the ribbon. Longer optical wavelengths correspond to a longer oscillation period of the optical field and therefore lead to a longer acceleration time of the electrons in a vacuum. A consequence of the longer acceleration time is the significantly higher kinetic energy with which the electrons strike the metal strip, which leads to a significantly higher efficiency in the X-ray generation. The situation is very similar to that of dive-jumpers who jump into the water from different platforms of a diving tower (Fig. 2). Again, the time lapse .DELTA.t of the free fall determines the kinetic energy of the jumper when submerged in the water. The energy is proportional to Δt2. The field-driven excursions of the electrons in vacuo were analyzed in detail in theoretical calculations and are shown in the accompanying animation (Fig. 3).

Fig. 3 Animation of the acceleration of electrons (blue spheres) in vacuum above the metal surface, which are exposed to the strong, oscillating, electric field of the laser pulse (black curve).

The experiments were carried out at the Vienna University of Technology, where the researchers combined a new driver laser system based on the concept of Optical Parametric Chirped Pulse Amplification (OPCPA) with an MBI x-ray generation chamber. Light pulses of 80 fs duration and energy up to 18 mJ at a center wavelength of 3900nm (3.9μm) were focused on a 20μm thick copper tape. This concept allows for the unprecedented generation of one billion hard X-ray photons per laser shot at a wavelength of 0.154 nm. Compared to previous experiments with a drive wavelength of 800nm, an exaggeration around the factor 25 is seen, which is approximately the square of the wavelength ratio (3900nm / 800nm)2 corresponds. This behavior is in quantitative agreement with the theoretical analysis based on the 3-step concept in Figure 1. The groundbreaking results pave the way for new, compact laboratory sources capable of producing up to 1010 x-ray X-ray photons per laser shot at a repetition rate of 1000 Hz will generate.

Original publication

High-brightness table-top hard X-ray source driven by sub-100-femtosecond mid-infrared pulses

J. Weisshaupt, V. Juvé, M. Holtz, S.A. Ku, M. Woerner, T. Elsaesser, S. Ališauskas, A. Pugžlys, A. Baltuška

Nature Photonics 8 (2014) 927-930