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

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.

The x-ray generation process consists of 3 steps:

(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. 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.

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. 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.