The longer the better: Optical long-wavelength pulses generate brilliant ultrashort hard x-ray flashes

Researchers from the Max-Born-Institut and the Technical University of Vienna present a novel table-top source of ultrashort hard x-ray pulses with an unprecedented photon flux.

X-rays are a key tool for imaging materials and analyzing their composition - at the doctor, in the chemistry lab and in materials research. Shining so-called hard x-rays of a wavelength comparable to the distance between atoms on the material, one can determine the atomic arrangement in space by analyzing the pattern of scattered x-rays. This method has unraveled equilibrium time-averaged structures of increasing complexity, from simple inorganic crystals to highly complex biomolecules such as DNA or proteins.

Today, there is a strong quest for mapping atoms ‘on the fly’, that is, for following atomic motions in space, during a vibration, a chemical reaction, or a change of the material’s structure. Atomic  motions typically occur in the time range of femtoseconds (1 femtosecond = 10-15s), requiring an exposure by extremely short x-ray flashes to take snapshots. There are essentially two complementary approaches to generate ultrashort hard x-ray pulses, large scale facilities based on electron accelerators such as the free electron lasers in Stanford (LCLS at SLAC) or at SACLA in Japan, or highly compact table-top sources driven by intense ultrashort optical pulses. While the overall x-ray flux from accelerator sources is much higher than from table-top sources, the latter are versatile tools for making femtosecond x-ray ‘movies’ with a quality that is eventually set by the number of x-ray photons scattered from the sample. A joint research team from the Max-Born-Institut (MBI) in Berlin and the Technical University in Vienna has now accomplished a breakthrough in table-top x-ray generation, allowing for an enhancement of the generated hard x-ray flux by a factor of 25. As they report in the current issue of Nature Photonics. the combination of a novel optical driver providing femtosecond mid-infrared pulses around a 4000 nm (4µm) wavelength with a metallic tape target allows for generating hard x-ray pulses at a wavelength of 0.154 nm with unprecedented efficiency.
Left: x-ray generation in a conventional x-ray tube. Electrons which were emitted from the heated cathode (-) are accelerated by a constant electric field towards the anode (+). Within the metal target (e.g. copper) inelastic collisions of the accelerated electrons with atoms lead to the generation of both characteristic line emission of x-rays (sharp lines in the spectrum at the bottom) and Bremsstrahlung. Right: Femtosecond mid-infrared pulses (λ = 3900 nm) from a OPCPA system are focused onto a copper band target. Electrons are extracted from the surface, accelerated into the vacuum and smashed back into the target by the strong electric field of the light. During deceleration in the metal target the energetic electrons produce characteristic line emission and Bremsstrahlung which can be measured by an x-ray detector.

The x-ray generation process consists of 3 steps (Fig. 1), (i) electron extraction from the metal target induced by the electric field of the driving pulse, (ii) electron acceleration in vacuum by the strong optical field and return into the target with an increased kinetic energy, and (iii) generation of x-rays in the target by inelastic collisions of electrons with atoms. Longer optical wavelengths are equivalent to a longer oscillation period of the optical field and, thus, to a longer period of electron acceleration in vacuum. As a result, the accelerated electrons acquire a higher kinetic energy before they re-enter the target and generate x-rays with a higher efficiency. A simple analogy of the acceleration process is the mechanical acceleration when jumping from platforms at different height into water (Fig. 2). Here, the time interval Δt between leaving the platform and reaching the water surface increases with height and the kinetic energy at the water surface is proportional to Δt2. The electron pathways in vacuum were analyzed in detail by theoretical calculations and are shown in the movie attached (Fig. 3).

Fig. 2: Analogy of electron acceleration in the vacuum to the acceleration by gravity when jumping from platforms at different height into water. Longer optical wavelengths are equivalent to a longer oscillation period of the optical field and, thus, to a longer period of electron acceleration in vacuum. The time interval Δt between leaving the platform and reaching the water surface increases with height and the kinetic energy at the water surface is proportional to Δt2. As a result, electrons accelerated in longer time interval Δt acquire a higher kinetic energy before they re-enter the target and generate x-rays with a higher efficiency.

Movie of the acceleration of electrons (blue balls) in

the vacuum above a metal surface when applying the strong oscillating

electric field of the laser pulse (black line).

The experiments were performed at the TU Vienna combining a novel driver system based on Optical Parametric Chirped Pulse Amplification (OPCPA) with an x-ray target chamber from MBI. Pulses of 80 fs duration and up to 18 mJ energy at a center wavelength of 3900 nm (3.9 µm) were focused down onto a 20 µm thick copper tape. This scheme allows for generating an unprecedented number of 109 hard x-ray photons at a 0.154 nm wavelength per driving pulse. A comparison with previous experiments performed with 800 nm driver pulses shows that the enhancement of the x-ray flux in the new scheme scales with the square of the wavelength ratio, i.e., (3900 nm/800 nm)2 ≈ 25. This behavior is in quantitative agreement with a theoretical analysis of the 3-step generation scheme of Fig. 1. The results pave the way for a new generation of table-top hard x-ray sources, providing up to 1010 x-ray photons per pulse at elevated, e.g., kilohertz repetition rates.

Original publication:
Jannick Weisshaupt, Vincent Juvé, Marcel Holtz, ShinAn Ku, Michael

Woerner, Thomas Elsaesser, Skirmantas Ališauskas, Audrius Pugzlys and

Andrius Baltuška
High-brightness table-top hard X-ray source driven by sub-100 femtosecond mid-infrared pulses
Nature Photonics doi:10.1038/nphoton.2014.256.

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