Nanostructures in extreme light

Plasmas reaching high densities and high temperatures of more than 100 million kelvins may occur when high-intensity laser pulses interact with solids. At extremely high laser intensities, the electrons in the solid may then be accelerated up to velocities close to the speed of light. To achieve this, the incident light pulse must interact with the maximum volume of the plasma generated. With increasing wavelength, the laser intensity it takes to reach the relativistic regime may decrease, but at the same time, the plasma᾽s state also changes more quickly, making it opaque to the incident light pulse. The light can then be absorbed only by a small volume near the surface and only there can it contribute to accelerating the electrons.

In cooperation with institutes from Jena, Düsseldorf, Vienna, Darmstadt and Frankfurt/Main, the LENA Junior Research Group for Metrology of Functional Nanosystems, which is a cooperation project of PTB and Braunschweig Technical University, have succeeded in solving this problem for the first time by combining femtosecond laser pulses with an average wavelength of 3.9 μm and nanostructured targets of crystalline silicon. Based on the X-ray emissions spectra measured and with the aid of numerical simulations, it has been demonstrated that the nanostructures change the properties of the thus created plasma in such a way that 80 % of the incident light are absorbed. At lab level, this allows longlived plasmas with temperatures in the range of 300 million kelvins with highly charged ions and electron densities of 6 · 10²³/cm³ to be generated. The densities achieved are approximately a thousand time higher than those obtained with a conventional, unstructured target.

Instead of laser pulses in the UV spectral range like those previously used, which had energies in the range of a few joules, using laser systems with wavelengths in the medium IR spectral range only requires energies of a few tens of millijoules, which is very promising for laser-based nuclear physics applications. In addition, femtosecond laser pulses in the X-ray spectral range were generated during the experiments carried out. These laser pulses can be very interesting for the time-resolved characterization of plasmas.