A team of scientists from the Max Born Institute in Berlin and the University of Rostock has found a new way to make transparent nanoparticles abruptly opaque and to heat them up with lightning fast. Their results could open up unprecedented opportunities for medicine and technology.
Intense light pulses can transform transparent material into a plasma, which then captures the light energy very efficiently. The scientists from Berlin and Rostock were now able to control this process extremely precisely. They used a trick that could greatly simplify medical methods and the production of nanomaterials. The meeting of light and matter was researched by a team of physicists from the Max Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) in Berlin and the Institute of Physics of the University of Rostock.
The scientists studied the interaction of intense near-infrared (NIR) laser flashes with tiny particles of just a few nanometers from a few thousand argon atoms called atomic clusters. The visible NIR light alone can only produce a plasma if its electromagnetic light waves are so strong that it tears individual atoms into electrons and ions (ionizes). The researchers were able to outsmart this firing threshold by irradiating the clusters with a second, much weaker femtosecond light flash in the extreme ultraviolet spectral range, invisible to the human eye (one femtosecond is one millionth of a billionth of a second). Using this trick, researchers were able to "turn on" energy capture even for unexpectedly weak visible laser light, observing a nano fireworks display in which electrons, ions, and colored fluorescent light were emitted from the clusters in different directions (Figure 1). Their findings open up new possibilities for basic research and application and have been published in the current issue of the renowned journal Physical Review Letters.
The experiments were carried out at the Max Born Institute on a 12 m long apparatus for the production of high harmonic (HH). "The observation that argon clusters are strongly ionized even at moderate light intensity was very surprising" (fig. Bernd Schütte from the MBI, who designed and carried out the experiment. "Although the extra XUV flash is very weak, its presence is critical: without the XUV firing pulse, the nanoparticles remained unchanged and transparent to the visible light." Scientists led by Prof. Thomas Fennel from the University of Rostock were able to reveal the secret of the synergy of the two flashes of light through numerical computer simulations. They found that providing just a few electrons was enough to start a process similar to a snow avalanche in the mountains. These "seed electrons" are generated by the ionizing XUV radiation, then heated by the visible light and knock out more electrons from neighboring atoms. "In this avalanche, the number of free electrons in the nanoparticle grows exponentially," explains Prof. Fennel. "Ultimately, the particles heat up so much that highly charged ions can be generated."
The novel concept of the ionization avalanche ignited by XUV light makes it possible to control the strong field ionization of nanoparticles and possibly also solids extremely spatially and temporally. This should make it possible to observe the ionization of nanoparticles over the time span of attoseconds - an unimaginably short time. An attosecond is one second, like a second to the age of the universe. The scientists expect that the ignition method can be used on many transparent materials such as glass or plastic. This makes this concept particularly interesting for the production of nanostructures. The advantage derives from the characteristics of the XUV flashes, which can be focused on a much smaller area and thus allow a higher precision. At the same time, the efficiency increases compared to current methods, since visible NIR pulses with much lower intensity are sufficient to heat up the material considerably. This could lead to new methods for nanolithography and nanomedicine in the future.
