Ludwig-Maximilians University and Max Planck Institute of Quantum Optics Munich, Germany

A team of physicists at Ludwig-Maximilians University Munich and the Max Planck Institute of Quantum Optics in Munich, Germany, reports that they have validated a novel laser-driven means of generating bright and highly energetic X-ray beams. This method, they say, opens up new ways of imaging the fine structure of matter.

Fig. 1 – The ATLAS Lasersystem serves as a light source for the highly energetic X-ray radiation. (Credit: Thorsten Naeser)
X-rays produced in a specialized type of vacuum tube have been used for medical imaging for more than 100 years. But, this method emits radiation from a large spot into all directions and over a broad energy range. And this mode of imaging results in a relatively modest resolution attainable. X-rays generated in synchrotrons provide much higher resolution, but their dimensions and cost preclude their routine use in clinical settings.

The new approach provides an alternative. Two laser pulses can generate X-rays of similar quality to synchrotron radiation in devices with a far smaller footprint, the Munich physicists explain. One pulse accelerates electrons to very high energy and the other forces them into an undulating motion. Under these conditions, they say, electrons emit X-radiation that is both highly energetic (hard) and highly intense, making it ideal to probe the microscopic structure of matter.

How It Works

Using two laser pulses, the researchers were able to generate ultrashort bursts of X-rays with defined wavelengths tailored for different applications. The new source can image structures of varying composition with a resolution of less than 10 micrometers, which opens up a range of promising uses in materials science, biology, and medicine.

Imaging microscopic structures in any sample of matter requires the use of a very brilliant beam of light with a very short wavelength. Brilliant radiation is able to concentrate a maximum amount of light quanta or photons of a single defined wavelength within the smallest area and shortest duration. Hard X-radiation is, therefore, ideal for this purpose, because it penetrates matter and exhibits wavelengths of a few hundredths of a nanometer.

Unfortunately, the only sources of high-intensity beams of hard X-rays so far available are particle accelerators, which are typically huge and highly expensive. But, in principle, there may be a far more economical and compact way of doing the job—with optical light. Led by Prof. Stefan Karsch and Dr. Laszlo Veisz, the Munich scientists succeeded in generating bright beams of hard X-radiation by purely optical means. Moreover, the wavelength of the emitted radiation can be readily adjusted for different applications, they say.

The physicists focused a laser pulse, lasting 25 femtoseconds and packing 60 terawatts of power, onto a fine jet of hydrogen gas. The strong electric field associated with each pulse knocks negatively charged electrons out of the gas, giving rise to a cloud of ionized plasma. The wavefront cuts through the plasma like a snow-plow, sweeping the electrons aside and leaving behind the positively charged atoms. The separation of oppositely charged particles generates very strong electrical fields, which cause the displaced electrons to whip back and forth. This in turn creates a wave-like pattern within the plasma, which propagates in the wake of the laser pulse. A fraction of the free electrons are caught up in this wave and can effectively “surf” directly behind the advancing laser pulse. These surfing electrons can be rapidly accelerated to velocities very near the speed of light.

When the electrons reached maximum speed, they collide head-on with a counter-propagating laser pulse, creating a so-called optical undulator whose oscillating electric field causes the free electrons to oscillate along a direction perpendicular to their direction of propagation. Highly energetic electrons that are forced to oscillate in this way emit radiation in the form of X-ray photons with wavelengths as short as 0.03 nm. In addition, in these experiments, the higher harmonics entrained on the electron motions by the light field could be detected directly in the X-ray spectrum, which has been attempted many times on conventional particle accelerators without success.

One of the great advantages of the new system in comparison with conventional X-ray sources is that the wavelength of the emitted light can be tuned over a wide range. This ability to alter the wavelength allows radiologists to analyze different types of tissue, for instance. By fine-tuning the incident beam, one can gain the maximum information about the sample one wishes to characterize. Not only is the laser-driven radiation tunable and extremely bright, it is produced in pulsed form. Each 25-fs laser pulse gives rise to X-ray flashes of a few fs duration. The new optically generated radiation can also be combined with phase-contrast X-ray tomography. This technique extracts information from the light that is scattered by an object. Using this method, the physicists say they can image structures as small as 10 micrometers in diameter in opaque materials and obtain even more detailed information from living tissues and other materials.