We have demonstrated a new way to turn initially transparent nanoparticles suddenly into strong absorbers for intense laser light and let them explode.

Intense laser pulses can transform transparent material into a plasma that captures energy of the incoming light very efficiently. At the MBI we have discovered a trick to start and control this process in a way that is so efficient that it could advance methods in nanofabrication and medicine. We studied the interaction of intense near-infrared (NIR) laser pulses with atomic nanoclusters. The visible NIR light pulse alone can only generate a plasma if its electromagnetic waves are so strong that they rip individual atoms apart into electrons and ions. We could outsmart this so-called ignition threshold by illuminating the clusters with an additional weak extreme-ultraviolet (XUV) laser pulse that is invisible to the human eye and lasts only a few femtoseconds (a femtosecond is a millionth of a billionth of a second). With this trick we could “switch on” the energy transfer from the near-infrared light to the particle at unexpectedly low NIR intensities and created nano-fireworks, during which electrons, ions and colourful fluorescence light are sent out from the clusters in different directions (Figure 1). The results open unprecedented opportunities for both fundamental laser-matter research and applications and was published in the latest issue of Physical Review Letters.
The experiments were carried out at the Max Born Institute at a 12 meter long high-harmonic generation (HHG) beamline. Our collaborators Mathias Arbeiter and Thomas Fennel from the University of Rostock modelled the light-matter processes with numerical simulations and uncovered the origin of the observed synergy of the two laser pulses. They found that only a few seed electrons created by the ionizing radiation of the XUV pulse are sufficient to start a process similar to a snow avalanche in the mountains. The seed electrons are heated in the NIR laser light and kick out even more electrons.
The novel concept of starting ionization avalanching with XUV light makes it possible to spatially and temporally control the strong-field ionization of nanoparticles and solids. Using HHG pulses paves the way for monitoring and controlling the ionization of nanoparticles on attosecond time scales, which is incredibly fast. One attosecond compares to a second as one second to the age of the universe. Moreover, the ignition method is expected to be applicable also to dielectric solids. This makes the concept very interesting for applications, in which intense laser pulses are used for the fabrication of nanostructures. By applying XUV pulses, a smaller focus size and therefore a higher precision could be achieved. At the same time, the overall efficiency can be improved, as NIR pulses with a much lower intensity compared to current methods could be used. In this way, novel nanolithography and nanosurgery applications may become possible in the future.

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