June 2, 2011 Even tiny gold nanoparticles, with a diameter of only 40 millionths of a millimeter, have something like a heartbeat. When focusing a short laser pulse on the particles they heat up very briefly and start to vibrate. But even the best microscopes can not resolve these nanoparticles, which are therefore very difficult to study.
Now, assistant professor Markus Lippitz from the Max Planck Institute for Solid State Research, together with his Ph.D. student Thorsten Schumacher have achieved a breakthrough in this area. Reporting in the journal Nature Communications, the researchers used a nanoantenna, which has already been applied successfully as a nanosensor by Prof. Harald Giessen from the 4th Physics Institute of the University of Stuttgart.
As in a mobile phone
Lippitz' goal is to investigate the mechanical properties of the smallest nanoparticles. "The surface to volume ratio would then be huge, and we would expect new nanomechanical properties," he explains. To get one step closer to this dream, he placed a small antenna near the tiny particle. This nanoantenna focuses the laser light very tightly on the nanoparticle under examination. Consequently, the light modulation due to the nanomechanical vibrations are very efficiently coupled back into the laser beam. "This is the first time that someone uses nanoantennas to investigate ultrafast nonlinear optical effects. The whole thing works like a mobile phone, in which the antenna makes that the electromagnetic waves are effectively coupled into the small electronic circuits of the phone" Lippitz explains.
Lippitz sees a huge potential for his new method: "In the future, we will be able to put the smallest nano-objects of a few nanometers in diameter in the focal point of a nanoantenna and study them using non-linear optical processes of only a few femtoseconds in duration (1 femtosecond = 1 millionth of a billionth of a second). Then we can make movies on the nanoscale, using the most extreme slow motion. Not only can we investigate nanoobjects such as semiconductor quantum dots, but also chemical and biological objects, such as molecules and viruses."
The work of Lippitz was funded by the state of Baden-Württemberg within the junior professors initiative and by the Deutsche Forschungsgemeinschaft and the Federal Ministry of Education and Research.
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- Thorsten Schumacher, Kai Kratzer, David Molnar, Mario Hentschel, Harald Giessen, Markus Lippitz. Nanoantenna-enhanced ultrafast nonlinear spectroscopy of a single gold nanoparticle. Nature Communications, 2011; 2: 333 DOI: 10.1038/ncomms1334
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